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Message-ID: <CACePvbV9Zg5EzeXRee0bS2FooKmbNp=k5-=pZHq9YFRGH3mePg@mail.gmail.com>
Date: Fri, 29 Aug 2025 20:40:24 -0700
From: Chris Li <chrisl@...nel.org>
To: Baoquan He <bhe@...hat.com>
Cc: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>,
Hugh Dickins <hughd@...gle.com>, Barry Song <baohua@...nel.org>, Nhat Pham <nphamcs@...il.com>,
Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>,
David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] mm, swap: use the swap table for the swap cache and
switch API
On Fri, Aug 29, 2025 at 6:55 PM Baoquan He <bhe@...hat.com> wrote:
>
> On 08/23/25 at 03:20am, Kairui Song wrote:
> > From: Kairui Song <kasong@...cent.com>
> ......snip...
> > diff --git a/mm/swap.h b/mm/swap.h
> > index 7b3efaa51624..4af42bc2cd72 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> ......snip...
> > +/*
> > + * All swap cache helpers below require the caller to ensure the swap entries
> > + * are valid and pin the device. This can be guaranteed by:
> > + * - get_swap_device: this ensures a single entry is valid and increases the
> > + * swap device's refcount.
> > + * - Locking a folio in the swap cache: this ensures the folio won't be freed
> > + * from the swap cache, stabilizes its entries, and the swap device.
> > + * - Locking anything referencing the swap entry: e.g. locking the PTL that
> > + * protects swap entries in the page table, so they won't be freed.
> > + */
> > +extern struct folio *swap_cache_get_folio(swp_entry_t entry);
> > +extern void *swap_cache_get_shadow(swp_entry_t entry);
> > +extern int swap_cache_add_folio(swp_entry_t entry,
> > + struct folio *folio, void **shadow);
> > +extern void swap_cache_del_folio(struct folio *folio);
> > +/* Below helpers also require the caller to lock the swap cluster. */
> > +extern void __swap_cache_del_folio(swp_entry_t entry,
> > + struct folio *folio, void *shadow);
> > +extern void __swap_cache_replace_folio(struct swap_cluster_info *ci,
> > + swp_entry_t entry, struct folio *old,
> > + struct folio *new);
> > +extern void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents);
> > +
> > void show_swap_cache_info(void);
> > -void *get_shadow_from_swap_cache(swp_entry_t entry);
> > -int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> > - gfp_t gfp, void **shadowp);
> > -void __delete_from_swap_cache(struct folio *folio,
> > - swp_entry_t entry, void *shadow);
> > -void delete_from_swap_cache(struct folio *folio);
> > -void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > - unsigned long end);
> > void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
> > -struct folio *swap_cache_get_folio(swp_entry_t entry);
> > struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > struct vm_area_struct *vma, unsigned long addr,
> > struct swap_iocb **plug);
>
> I would put this function renaming change to another standalone patch,
> then let this key patch focus on swap table introducing.
>
> > @@ -235,6 +283,33 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
> >
> > #else /* CONFIG_SWAP */
> > struct swap_iocb;
> > +
> > +static inline struct swap_cluster_info *swap_cluster_lock(
> > + struct swap_info_struct *si, pgoff_t offset, bool irq)
> > +{
> > + return NULL;
> > +}
> > +
> > +static inline struct swap_cluster_info *swap_cluster_lock_by_folio(
> > + struct folio *folio)
> > +{
> > + return NULL;
> > +}
> > +
> > +static inline struct swap_cluster_info *swap_cluster_lock_by_folio_irq(
> > + struct folio *folio)
> > +{
> > + return NULL;
> > +}
> > +
> > +static inline void swap_cluster_unlock(struct swap_cluster_info *ci)
> > +{
> > +}
> > +
> > +static inline void swap_cluster_unlock_irq(struct swap_cluster_info *ci)
> > +{
> > +}
> > +
> > static inline struct swap_info_struct *swp_info(swp_entry_t entry)
> > {
> > return NULL;
> > @@ -252,11 +327,6 @@ static inline struct address_space *swap_address_space(swp_entry_t entry)
> > return NULL;
> > }
> >
> > -static inline pgoff_t swap_cache_index(swp_entry_t entry)
> > -{
> > - return 0;
> > -}
> > -
> > static inline bool folio_contains_swap(struct folio *folio, swp_entry_t entry)
> > {
> > return false;
> > @@ -298,28 +368,27 @@ static inline struct folio *swap_cache_get_folio(swp_entry_t entry)
> > return NULL;
> > }
> >
> > -static inline void *get_shadow_from_swap_cache(swp_entry_t entry)
> > +static inline void *swap_cache_get_shadow(swp_entry_t end)
> > {
> > return NULL;
> > }
> >
> > -static inline int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> > - gfp_t gfp_mask, void **shadowp)
> > +static inline int swap_cache_add_folio(swp_entry_t end, struct folio *folio, void **shadow)
> > {
> > - return -1;
> > + return -EINVAL;
> > }
> >
> > -static inline void __delete_from_swap_cache(struct folio *folio,
> > - swp_entry_t entry, void *shadow)
> > +static inline void swap_cache_del_folio(struct folio *folio)
> > {
> > }
> >
> > -static inline void delete_from_swap_cache(struct folio *folio)
> > +static inline void __swap_cache_del_folio(swp_entry_t entry, struct folio *folio, void *shadow)
> > {
> > }
> >
> > -static inline void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > - unsigned long end)
> > +static inline void __swap_cache_replace_folio(
> > + struct swap_cluster_info *ci, swp_entry_t entry,
> > + struct folio *old, struct folio *new)
> > {
> > }
> >
> > @@ -354,7 +423,7 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
> > static inline pgoff_t folio_index(struct folio *folio)
> > {
> > if (unlikely(folio_test_swapcache(folio)))
> > - return swap_cache_index(folio->swap);
> > + return swp_offset(folio->swap);
> > return folio->index;
> > }
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 721ff1a5e73a..c0342024b4a8 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -23,6 +23,7 @@
> > #include <linux/huge_mm.h>
> > #include <linux/shmem_fs.h>
> > #include "internal.h"
> > +#include "swap_table.h"
> > #include "swap.h"
> >
> > /*
> > @@ -36,8 +37,11 @@ static const struct address_space_operations swap_aops = {
> > #endif
> > };
> >
> > -struct address_space *swapper_spaces[MAX_SWAPFILES] __read_mostly;
> > -static unsigned int nr_swapper_spaces[MAX_SWAPFILES] __read_mostly;
> > +/* Set swap_space is read only as swap cache is handled by swap table */
> > +struct address_space swap_space __ro_after_init = {
> > + .a_ops = &swap_aops,
> > +};
> > +
> > static bool enable_vma_readahead __read_mostly = true;
> >
> > #define SWAP_RA_ORDER_CEILING 5
> > @@ -69,7 +73,7 @@ void show_swap_cache_info(void)
> > printk("Total swap = %lukB\n", K(total_swap_pages));
> > }
> >
> > -/*
> > +/**
> > * swap_cache_get_folio - Lookup a swap entry in the swap cache.
> > *
> > * A found folio will be returned unlocked and with its refcount increased.
> > @@ -79,155 +83,179 @@ void show_swap_cache_info(void)
> > */
> > struct folio *swap_cache_get_folio(swp_entry_t entry)
> > {
> > - struct folio *folio = filemap_get_folio(swap_address_space(entry),
> > - swap_cache_index(entry));
> > - if (!IS_ERR(folio))
> > - return folio;
> > + unsigned long swp_tb;
> > + struct folio *folio;
> > +
> > + for (;;) {
> > + swp_tb = __swap_table_get(swp_cluster(entry), swp_cluster_offset(entry));
> > + if (!swp_tb_is_folio(swp_tb))
> > + return NULL;
> > + folio = swp_tb_to_folio(swp_tb);
> > + if (folio_try_get(folio))
> > + return folio;
> > + }
> > +
> > return NULL;
> > }
> >
> > -void *get_shadow_from_swap_cache(swp_entry_t entry)
> > +/**
> > + * swap_cache_get_shadow - Lookup a shadow in the swap cache.
> > + *
> > + * Context: Caller must ensure @entry is valid and pin the swap device.
> > + */
> > +void *swap_cache_get_shadow(swp_entry_t entry)
> > {
> > - struct address_space *address_space = swap_address_space(entry);
> > - pgoff_t idx = swap_cache_index(entry);
> > - void *shadow;
> > + unsigned long swp_tb;
> > +
> > + swp_tb = __swap_table_get(swp_cluster(entry), swp_cluster_offset(entry));
> > + if (swp_tb_is_shadow(swp_tb))
> > + return swp_tb_to_shadow(swp_tb);
> >
> > - shadow = xa_load(&address_space->i_pages, idx);
> > - if (xa_is_value(shadow))
> > - return shadow;
> > return NULL;
> > }
> >
> > -/*
> > - * add_to_swap_cache resembles filemap_add_folio on swapper_space,
> > - * but sets SwapCache flag and 'swap' instead of mapping and index.
> > +/**
> > + * swap_cache_add_folio - add a folio into the swap cache.
> > + *
> > + * The folio will be used for swapin or swapout of swap entries
> > + * starting with @entry. May fail due to race.
> > + *
> > + * Context: Caller must ensure @entry is valid and pin the swap device.
> > */
> > -int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> > - gfp_t gfp, void **shadowp)
> > +int swap_cache_add_folio(swp_entry_t entry, struct folio *folio, void **shadowp)
> > {
> > - struct address_space *address_space = swap_address_space(entry);
> > - pgoff_t idx = swap_cache_index(entry);
> > - XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));
> > - unsigned long i, nr = folio_nr_pages(folio);
> > - void *old;
> > -
> > - xas_set_update(&xas, workingset_update_node);
> > -
> > - VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> > - VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
> > - VM_BUG_ON_FOLIO(!folio_test_swapbacked(folio), folio);
> > + unsigned long exist;
> > + void *shadow = NULL;
> > + struct swap_cluster_info *ci;
> > + unsigned int ci_start, ci_off, ci_end;
> > + unsigned long nr_pages = folio_nr_pages(folio);
> > +
> > + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> > + VM_WARN_ON_ONCE_FOLIO(folio_test_swapcache(folio), folio);
> > + VM_WARN_ON_ONCE_FOLIO(!folio_test_swapbacked(folio), folio);
> > +
> > + ci = swap_cluster_lock(swp_info(entry), swp_offset(entry));
> > + ci_start = swp_cluster_offset(entry);
> > + ci_end = ci_start + nr_pages;
> > + ci_off = ci_start;
> > + do {
> > + exist = __swap_table_get(ci, ci_off);
> > + if (unlikely(swp_tb_is_folio(exist)))
> > + goto fail;
> > + if (swp_tb_is_shadow(exist))
> > + shadow = swp_tb_to_shadow(exist);
> > + } while (++ci_off < ci_end);
> > +
> > + ci_off = ci_start;
> > + do {
> > + __swap_table_set_folio(ci, ci_off, folio);
> > + } while (++ci_off < ci_end);
> >
> > - folio_ref_add(folio, nr);
> > + folio_ref_add(folio, nr_pages);
> > folio_set_swapcache(folio);
> > folio->swap = entry;
> > + swap_cluster_unlock(ci);
> >
> > - do {
> > - xas_lock_irq(&xas);
> > - xas_create_range(&xas);
> > - if (xas_error(&xas))
> > - goto unlock;
> > - for (i = 0; i < nr; i++) {
> > - VM_BUG_ON_FOLIO(xas.xa_index != idx + i, folio);
> > - if (shadowp) {
> > - old = xas_load(&xas);
> > - if (xa_is_value(old))
> > - *shadowp = old;
> > - }
> > - xas_store(&xas, folio);
> > - xas_next(&xas);
> > - }
> > - address_space->nrpages += nr;
> > - __node_stat_mod_folio(folio, NR_FILE_PAGES, nr);
> > - __lruvec_stat_mod_folio(folio, NR_SWAPCACHE, nr);
> > -unlock:
> > - xas_unlock_irq(&xas);
> > - } while (xas_nomem(&xas, gfp));
> > -
> > - if (!xas_error(&xas))
> > - return 0;
> > + node_stat_mod_folio(folio, NR_FILE_PAGES, nr_pages);
> > + lruvec_stat_mod_folio(folio, NR_SWAPCACHE, nr_pages);
> >
> > - folio_clear_swapcache(folio);
> > - folio_ref_sub(folio, nr);
> > - return xas_error(&xas);
> > + if (shadowp)
> > + *shadowp = shadow;
> > + return 0;
> > +fail:
> > + swap_cluster_unlock(ci);
> > + return -EEXIST;
> > }
> >
> > /*
> > - * This must be called only on folios that have
> > - * been verified to be in the swap cache.
> > + * Caller must ensure the folio is in the swap cache and locked,
> > + * also lock the swap cluster.
> > */
> > -void __delete_from_swap_cache(struct folio *folio,
> > - swp_entry_t entry, void *shadow)
> > +void __swap_cache_del_folio(swp_entry_t entry, struct folio *folio,
> > + void *shadow)
> > {
> > - struct address_space *address_space = swap_address_space(entry);
> > - int i;
> > - long nr = folio_nr_pages(folio);
> > - pgoff_t idx = swap_cache_index(entry);
> > - XA_STATE(xas, &address_space->i_pages, idx);
> > -
> > - xas_set_update(&xas, workingset_update_node);
> > -
> > - VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> > - VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
> > - VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
> > -
> > - for (i = 0; i < nr; i++) {
> > - void *entry = xas_store(&xas, shadow);
> > - VM_BUG_ON_PAGE(entry != folio, entry);
> > - xas_next(&xas);
> > - }
> > + unsigned long exist;
> > + struct swap_cluster_info *ci;
> > + unsigned int ci_start, ci_off, ci_end;
> > + unsigned long nr_pages = folio_nr_pages(folio);
> > +
> > + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> > + VM_WARN_ON_ONCE_FOLIO(!folio_test_swapcache(folio), folio);
> > + VM_WARN_ON_ONCE_FOLIO(folio_test_writeback(folio), folio);
> > +
> > + ci = swp_offset_cluster(swp_info(entry), swp_offset(entry));
> > + ci_start = swp_cluster_offset(entry);
> > + ci_end = ci_start + nr_pages;
> > + ci_off = ci_start;
> > + do {
> > + exist = __swap_table_get(ci, ci_off);
> > + VM_WARN_ON_ONCE(swp_tb_to_folio(exist) != folio);
> > + /* If shadow is NULL, we sets an empty shadow */
> > + __swap_table_set_shadow(ci, ci_off, shadow);
> > + } while (++ci_off < ci_end);
> > +
> > folio->swap.val = 0;
> > folio_clear_swapcache(folio);
> > - address_space->nrpages -= nr;
> > - __node_stat_mod_folio(folio, NR_FILE_PAGES, -nr);
> > - __lruvec_stat_mod_folio(folio, NR_SWAPCACHE, -nr);
> > + node_stat_mod_folio(folio, NR_FILE_PAGES, -nr_pages);
> > + lruvec_stat_mod_folio(folio, NR_SWAPCACHE, -nr_pages);
> > }
> >
> > /*
> > - * This must be called only on folios that have
> > - * been verified to be in the swap cache and locked.
> > - * It will never put the folio into the free list,
> > - * the caller has a reference on the folio.
> > + * Replace an old folio in the swap cache with a new one. The caller must
> > + * hold the cluster lock and set the new folio's entry and flags.
> > */
> > -void delete_from_swap_cache(struct folio *folio)
> > +void __swap_cache_replace_folio(struct swap_cluster_info *ci, swp_entry_t entry,
> > + struct folio *old, struct folio *new)
> > +{
> > + unsigned int ci_off = swp_cluster_offset(entry);
> > + unsigned long nr_pages = folio_nr_pages(new);
> > + unsigned int ci_end = ci_off + nr_pages;
> > +
> > + VM_WARN_ON_ONCE(entry.val != new->swap.val);
> > + VM_WARN_ON_ONCE(!folio_test_locked(old) || !folio_test_locked(new));
> > + VM_WARN_ON_ONCE(!folio_test_swapcache(old) || !folio_test_swapcache(new));
> > + do {
> > + WARN_ON_ONCE(swp_tb_to_folio(__swap_table_get(ci, ci_off)) != old);
> > + __swap_table_set_folio(ci, ci_off, new);
> > + } while (++ci_off < ci_end);
> > +
> > + /*
> > + * If the old folio is partially replaced (e.g., splitting a large
> > + * folio, the old folio is shrunk in place, and new split sub folios
> > + * are added to cache), ensure the new folio doesn't overlap it.
> > + */
> > + if (IS_ENABLED(CONFIG_DEBUG_VM) &&
> > + folio_order(old) != folio_order(new)) {
> > + ci_off = swp_cluster_offset(old->swap);
> > + ci_end = ci_off + folio_nr_pages(old);
> > + while (ci_off++ < ci_end)
> > + WARN_ON_ONCE(swp_tb_to_folio(__swap_table_get(ci, ci_off)) != old);
> > + }
> > +}
> > +
> > +void swap_cache_del_folio(struct folio *folio)
> > {
> > + struct swap_cluster_info *ci;
> > swp_entry_t entry = folio->swap;
> > - struct address_space *address_space = swap_address_space(entry);
> >
> > - xa_lock_irq(&address_space->i_pages);
> > - __delete_from_swap_cache(folio, entry, NULL);
> > - xa_unlock_irq(&address_space->i_pages);
> > + ci = swap_cluster_lock(swp_info(entry), swp_offset(entry));
> > + __swap_cache_del_folio(entry, folio, NULL);
> > + swap_cluster_unlock(ci);
> >
> > put_swap_folio(folio, entry);
> > folio_ref_sub(folio, folio_nr_pages(folio));
> > }
> >
> > -void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > - unsigned long end)
> > +void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents)
> > {
> > - unsigned long curr = begin;
> > - void *old;
> > -
> > - for (;;) {
> > - swp_entry_t entry = swp_entry(type, curr);
> > - unsigned long index = curr & SWAP_ADDRESS_SPACE_MASK;
> > - struct address_space *address_space = swap_address_space(entry);
> > - XA_STATE(xas, &address_space->i_pages, index);
> > -
> > - xas_set_update(&xas, workingset_update_node);
> > -
> > - xa_lock_irq(&address_space->i_pages);
> > - xas_for_each(&xas, old, min(index + (end - curr), SWAP_ADDRESS_SPACE_PAGES)) {
> > - if (!xa_is_value(old))
> > - continue;
> > - xas_store(&xas, NULL);
> > - }
> > - xa_unlock_irq(&address_space->i_pages);
> > + struct swap_cluster_info *ci = swp_cluster(entry);
> > + unsigned int ci_off = swp_cluster_offset(entry), ci_end;
> >
> > - /* search the next swapcache until we meet end */
> > - curr = ALIGN((curr + 1), SWAP_ADDRESS_SPACE_PAGES);
> > - if (curr > end)
> > - break;
> > - }
> > + ci_end = ci_off + nr_ents;
> > + do {
> > + WARN_ON_ONCE(swp_tb_is_folio(__swap_table_get(ci, ci_off)));
> > + __swap_table_init_null(ci, ci_off);
> > + } while (++ci_off < ci_end);
> > }
> >
> > /*
> > @@ -292,8 +320,7 @@ static inline bool swap_use_vma_readahead(void)
> > /*
> > * Update the readahead statistics of a vma or globally.
> > */
> > -void swap_update_readahead(struct folio *folio,
> > - struct vm_area_struct *vma,
> > +void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
> > unsigned long addr)
> > {
> > bool readahead, vma_ra = swap_use_vma_readahead();
> > @@ -387,7 +414,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > goto put_and_return;
> >
> > /*
> > - * We might race against __delete_from_swap_cache(), and
> > + * We might race against __swap_cache_del_folio(), and
> > * stumble across a swap_map entry whose SWAP_HAS_CACHE
> > * has not yet been cleared. Or race against another
> > * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
> > @@ -405,8 +432,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > if (mem_cgroup_swapin_charge_folio(new_folio, NULL, gfp_mask, entry))
> > goto fail_unlock;
> >
> > - /* May fail (-ENOMEM) if XArray node allocation failed. */
> > - if (add_to_swap_cache(new_folio, entry, gfp_mask & GFP_RECLAIM_MASK, &shadow))
> > + if (swap_cache_add_folio(entry, new_folio, &shadow))
> > goto fail_unlock;
> >
> > memcg1_swapin(entry, 1);
> > @@ -572,11 +598,11 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > end_offset = si->max - 1;
> >
> > blk_start_plug(&plug);
> > - for (offset = start_offset; offset <= end_offset ; offset++) {
> > + for (offset = start_offset; offset <= end_offset; offset++) {
> > /* Ok, do the async read-ahead now */
> > folio = __read_swap_cache_async(
> > - swp_entry(swp_type(entry), offset),
> > - gfp_mask, mpol, ilx, &page_allocated, false);
> > + swp_entry(swp_type(entry), offset), gfp_mask, mpol, ilx,
> > + &page_allocated, false);
> > if (!folio)
> > continue;
> > if (page_allocated) {
> > @@ -600,41 +626,6 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > return folio;
> > }
> >
> > -int init_swap_address_space(unsigned int type, unsigned long nr_pages)
> > -{
> > - struct address_space *spaces, *space;
> > - unsigned int i, nr;
> > -
> > - nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);
> > - spaces = kvcalloc(nr, sizeof(struct address_space), GFP_KERNEL);
> > - if (!spaces)
> > - return -ENOMEM;
> > - for (i = 0; i < nr; i++) {
> > - space = spaces + i;
> > - xa_init_flags(&space->i_pages, XA_FLAGS_LOCK_IRQ);
> > - atomic_set(&space->i_mmap_writable, 0);
> > - space->a_ops = &swap_aops;
> > - /* swap cache doesn't use writeback related tags */
> > - mapping_set_no_writeback_tags(space);
> > - }
> > - nr_swapper_spaces[type] = nr;
> > - swapper_spaces[type] = spaces;
> > -
> > - return 0;
> > -}
> > -
> > -void exit_swap_address_space(unsigned int type)
> > -{
> > - int i;
> > - struct address_space *spaces = swapper_spaces[type];
> > -
> > - for (i = 0; i < nr_swapper_spaces[type]; i++)
> > - VM_WARN_ON_ONCE(!mapping_empty(&spaces[i]));
> > - kvfree(spaces);
> > - nr_swapper_spaces[type] = 0;
> > - swapper_spaces[type] = NULL;
> > -}
> > -
> > static int swap_vma_ra_win(struct vm_fault *vmf, unsigned long *start,
> > unsigned long *end)
> > {
> > @@ -807,7 +798,7 @@ static const struct attribute_group swap_attr_group = {
> > .attrs = swap_attrs,
> > };
> >
> > -static int __init swap_init_sysfs(void)
> > +static int __init swap_init(void)
> > {
> > int err;
> > struct kobject *swap_kobj;
> > @@ -822,11 +813,13 @@ static int __init swap_init_sysfs(void)
> > pr_err("failed to register swap group\n");
> > goto delete_obj;
> > }
> > + /* swap_space is set RO after init, so do it here before init ends. */
> > + mapping_set_no_writeback_tags(&swap_space);
> > return 0;
> >
> > delete_obj:
> > kobject_put(swap_kobj);
> > return err;
> > }
> > -subsys_initcall(swap_init_sysfs);
> > +subsys_initcall(swap_init);
> > #endif
> > diff --git a/mm/swap_table.h b/mm/swap_table.h
> > new file mode 100644
> > index 000000000000..ed9676547071
> > --- /dev/null
> > +++ b/mm/swap_table.h
> > @@ -0,0 +1,106 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _MM_SWAP_TABLE_H
> > +#define _MM_SWAP_TABLE_H
> > +
> > +#include "swap.h"
> > +
> > +/*
> > + * A swap table entry represents the status of a swap slot on a swap
> > + * (physical or virtual) device. The swap table in each cluster is a
> > + * 1:1 map of the swap slots in this cluster.
> > + *
> > + * Each swap table entry could be a pointer (folio), a XA_VALUE
> > + * (shadow), or NULL.
> > + */
> > +
> > +/*
> > + * Helpers for casting one type of info into a swap table entry.
> > + */
> > +static inline unsigned long null_to_swp_tb(void)
> > +{
> > + BUILD_BUG_ON(sizeof(unsigned long) != sizeof(atomic_long_t));
> > + return 0;
> > +}
> > +
> > +static inline unsigned long folio_to_swp_tb(struct folio *folio)
> > +{
> > + BUILD_BUG_ON(sizeof(unsigned long) != sizeof(void *));
> > + return (unsigned long)folio;
> > +}
> > +
> > +static inline unsigned long shadow_swp_to_tb(void *shadow)
> > +{
> > + BUILD_BUG_ON((BITS_PER_XA_VALUE + 1) !=
> > + BITS_PER_BYTE * sizeof(unsigned long));
> > + VM_WARN_ON_ONCE(shadow && !xa_is_value(shadow));
> > + return (unsigned long)shadow;
> > +}
> > +
> > +/*
> > + * Helpers for swap table entry type checking.
> > + */
> > +static inline bool swp_tb_is_null(unsigned long swp_tb)
> > +{
> > + return !swp_tb;
> > +}
> > +
> > +static inline bool swp_tb_is_folio(unsigned long swp_tb)
> > +{
> > + return !xa_is_value((void *)swp_tb) && !swp_tb_is_null(swp_tb);
> > +}
> > +
> > +static inline bool swp_tb_is_shadow(unsigned long swp_tb)
> > +{
> > + return xa_is_value((void *)swp_tb);
> > +}
> > +
> > +/*
> > + * Helpers for retrieving info from swap table.
> > + */
> > +static inline struct folio *swp_tb_to_folio(unsigned long swp_tb)
> > +{
> > + VM_WARN_ON(!swp_tb_is_folio(swp_tb));
> > + return (void *)swp_tb;
> > +}
> > +
> > +static inline void *swp_tb_to_shadow(unsigned long swp_tb)
> > +{
> > + VM_WARN_ON(!swp_tb_is_shadow(swp_tb));
> > + return (void *)swp_tb;
> > +}
> > +
> > +/*
> > + * Helpers for accessing or modifying the swap table of a cluster,
> > + * the swap cluster must be locked.
> > + */
> > +static inline void __swap_table_set(struct swap_cluster_info *ci,
> > + unsigned int off, unsigned long swp_tb)
> > +{
> > + VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
> > + atomic_long_set(&ci->table[off], swp_tb);
> > +}
> > +
> > +static inline unsigned long __swap_table_get(struct swap_cluster_info *ci,
> > + unsigned int off)
> > +{
> > + VM_WARN_ON_ONCE(off >= SWAPFILE_CLUSTER);
> > + return atomic_long_read(&ci->table[off]);
> > +}
> > +
> > +static inline void __swap_table_set_folio(struct swap_cluster_info *ci,
> > + unsigned int off, struct folio *folio)
> > +{
> > + __swap_table_set(ci, off, folio_to_swp_tb(folio));
> > +}
> > +
> > +static inline void __swap_table_set_shadow(struct swap_cluster_info *ci,
> > + unsigned int off, void *shadow)
> > +{
> > + __swap_table_set(ci, off, shadow_swp_to_tb(shadow));
> > +}
> > +
> > +static inline void __swap_table_init_null(struct swap_cluster_info *ci, unsigned int off)
> > +{
> > + __swap_table_set(ci, off, null_to_swp_tb());
> > +}
> > +#endif
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 85606fbebf0f..df68b5e242a6 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -46,6 +46,7 @@
> > #include <asm/tlbflush.h>
> > #include <linux/swapops.h>
> > #include <linux/swap_cgroup.h>
> > +#include "swap_table.h"
> > #include "internal.h"
> > #include "swap.h"
> >
> > @@ -268,7 +269,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > if (!need_reclaim)
> > goto out_unlock;
> >
> > - delete_from_swap_cache(folio);
> > + swap_cache_del_folio(folio);
> > folio_set_dirty(folio);
> > ret = nr_pages;
> > out_unlock:
> > @@ -422,6 +423,34 @@ static inline unsigned int cluster_offset(struct swap_info_struct *si,
> > return cluster_index(si, ci) * SWAPFILE_CLUSTER;
> > }
> >
> > +static int swap_table_alloc_table(struct swap_cluster_info *ci)
> > +{
> > + WARN_ON(ci->table);
> > + ci->table = kzalloc(sizeof(unsigned long) * SWAPFILE_CLUSTER, GFP_KERNEL);
> > + if (!ci->table)
> > + return -ENOMEM;
> > + return 0;
> > +}
> > +
> > +static void swap_cluster_free_table(struct swap_cluster_info *ci)
> > +{
> > + unsigned int ci_off;
> > + unsigned long swp_tb;
> > +
> > + if (!ci->table)
> > + return;
> > +
> > + for (ci_off = 0; ci_off < SWAPFILE_CLUSTER; ci_off++) {
> > + swp_tb = __swap_table_get(ci, ci_off);
> > + if (!swp_tb_is_null(swp_tb))
> > + pr_err_once("swap: unclean swap space on swapoff: 0x%lx",
> > + swp_tb);
> > + }
> > +
> > + kfree(ci->table);
> > + ci->table = NULL;
> > +}
> > +
> > static void move_cluster(struct swap_info_struct *si,
> > struct swap_cluster_info *ci, struct list_head *list,
> > enum swap_cluster_flags new_flags)
> > @@ -704,6 +733,25 @@ static bool cluster_scan_range(struct swap_info_struct *si,
> > return true;
> > }
> >
> > +/*
> > + * Currently, the swap table is not used for count tracking,
> > + * just do a sanity check to ensure nothing went wrong.
> > + */
> > +static void cluster_table_check(struct swap_cluster_info *ci,
> > + unsigned int start, unsigned int nr)
> > +{
> > + unsigned int ci_off = start % SWAPFILE_CLUSTER;
> > + unsigned int ci_end = ci_off + nr;
> > + unsigned long swp_tb;
> > +
> > + if (IS_ENABLED(CONFIG_DEBUG_VM)) {
> > + do {
> > + swp_tb = __swap_table_get(ci, ci_off);
> > + VM_WARN_ON_ONCE(!swp_tb_is_null(swp_tb));
> > + } while (++ci_off < ci_end);
> > + }
> > +}
> > +
> > static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster_info *ci,
> > unsigned int start, unsigned char usage,
> > unsigned int order)
> > @@ -723,6 +771,7 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
> > ci->order = order;
> >
> > memset(si->swap_map + start, usage, nr_pages);
> > + cluster_table_check(ci, start, nr_pages);
> > swap_range_alloc(si, nr_pages);
> > ci->count += nr_pages;
> >
> > @@ -1100,8 +1149,7 @@ static void swap_range_alloc(struct swap_info_struct *si,
> > static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> > unsigned int nr_entries)
> > {
> > - unsigned long begin = offset;
> > - unsigned long end = offset + nr_entries - 1;
> > + unsigned long start = offset, end = offset + nr_entries - 1;
>
> And this kind of clean up or code style adjustment, adding them here will
> distract people from focusing on swap table introducing.
+1. The "begin" to "start" change is not necessary. The other reason
to nitpick is that I might be the one writing the original "begin". It
sounds like a word I would use to pair with "end". I do recall I wrote
"begin" and "end" somewhere before, I forget if this function or not.
Pure nitpick anyway. Renaming itself is trivial. Just try to not do
unnecessary rename.
> > void (*swap_slot_free_notify)(struct block_device *, unsigned long);
> > unsigned int i;
> >
> > @@ -1125,7 +1173,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> > swap_slot_free_notify(si->bdev, offset);
> > offset++;
> > }
> > - clear_shadow_from_swap_cache(si->type, begin, end);
> > + __swap_cache_clear_shadow(swp_entry(si->type, start), nr_entries);
> >
> > /*
> > * Make sure that try_to_unuse() observes si->inuse_pages reaching 0
> > @@ -1282,15 +1330,7 @@ int folio_alloc_swap(struct folio *folio, gfp_t gfp)
> > if (!entry.val)
> > return -ENOMEM;
> >
> > - /*
> > - * XArray node allocations from PF_MEMALLOC contexts could
> > - * completely exhaust the page allocator. __GFP_NOMEMALLOC
> > - * stops emergency reserves from being allocated.
> > - *
> > - * TODO: this could cause a theoretical memory reclaim
> > - * deadlock in the swap out path.
> > - */
> > - if (add_to_swap_cache(folio, entry, gfp | __GFP_NOMEMALLOC, NULL))
> > + if (swap_cache_add_folio(entry, folio, NULL))
> > goto out_free;
> >
> > return 0;
> > @@ -1557,6 +1597,7 @@ static void swap_entries_free(struct swap_info_struct *si,
> >
> > mem_cgroup_uncharge_swap(entry, nr_pages);
> > swap_range_free(si, offset, nr_pages);
> > + cluster_table_check(ci, offset, nr_pages);
> >
> > if (!ci->count)
> > free_cluster(si, ci);
> > @@ -1760,7 +1801,7 @@ bool folio_free_swap(struct folio *folio)
> > if (folio_swapped(folio))
> > return false;
> >
> > - delete_from_swap_cache(folio);
> > + swap_cache_del_folio(folio);
> > folio_set_dirty(folio);
> > return true;
> > }
> > @@ -2634,6 +2675,18 @@ static void wait_for_allocation(struct swap_info_struct *si)
> > }
> > }
> >
> > +static void free_cluster_info(struct swap_cluster_info *cluster_info,
> > + unsigned long maxpages)
> > +{
> > + int i, nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
> > +
> > + if (!cluster_info)
> > + return;
> > + for (i = 0; i < nr_clusters; i++)
> > + swap_cluster_free_table(&cluster_info[i]);
> > + kvfree(cluster_info);
> > +}
> > +
> > /*
> > * Called after swap device's reference count is dead, so
> > * neither scan nor allocation will use it.
> > @@ -2768,12 +2821,13 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> >
> > swap_file = p->swap_file;
> > p->swap_file = NULL;
> > - p->max = 0;
> > swap_map = p->swap_map;
> > p->swap_map = NULL;
> > zeromap = p->zeromap;
> > p->zeromap = NULL;
> > cluster_info = p->cluster_info;
> > + free_cluster_info(cluster_info, p->max);
> > + p->max = 0;
> > p->cluster_info = NULL;
> > spin_unlock(&p->lock);
> > spin_unlock(&swap_lock);
> > @@ -2784,10 +2838,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> > p->global_cluster = NULL;
> > vfree(swap_map);
> > kvfree(zeromap);
> > - kvfree(cluster_info);
> > /* Destroy swap account information */
> > swap_cgroup_swapoff(p->type);
> > - exit_swap_address_space(p->type);
> >
> > inode = mapping->host;
> >
> > @@ -3171,8 +3223,11 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
> > if (!cluster_info)
> > goto err;
> >
> > - for (i = 0; i < nr_clusters; i++)
> > + for (i = 0; i < nr_clusters; i++) {
> > spin_lock_init(&cluster_info[i].lock);
> > + if (swap_table_alloc_table(&cluster_info[i]))
> > + goto err_free;
> > + }
> >
> > if (!(si->flags & SWP_SOLIDSTATE)) {
> > si->global_cluster = kmalloc(sizeof(*si->global_cluster),
> > @@ -3233,9 +3288,8 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
> > }
> >
> > return cluster_info;
> > -
> > err_free:
> > - kvfree(cluster_info);
> > + free_cluster_info(cluster_info, maxpages);
> > err:
> > return ERR_PTR(err);
> > }
> > @@ -3429,13 +3483,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> > }
> > }
> >
> > - error = init_swap_address_space(si->type, maxpages);
> > - if (error)
> > - goto bad_swap_unlock_inode;
> > -
> > error = zswap_swapon(si->type, maxpages);
> > if (error)
> > - goto free_swap_address_space;
> > + goto bad_swap_unlock_inode;
> >
> > /*
> > * Flush any pending IO and dirty mappings before we start using this
> > @@ -3470,8 +3520,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> > goto out;
> > free_swap_zswap:
> > zswap_swapoff(si->type);
> > -free_swap_address_space:
> > - exit_swap_address_space(si->type);
> > bad_swap_unlock_inode:
> > inode_unlock(inode);
> > bad_swap:
> > @@ -3486,7 +3534,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> > spin_unlock(&swap_lock);
> > vfree(swap_map);
> > kvfree(zeromap);
> > - kvfree(cluster_info);
> > + if (cluster_info)
> > + free_cluster_info(cluster_info, maxpages);
> > if (inced_nr_rotate_swap)
> > atomic_dec(&nr_rotate_swap);
> > if (swap_file)
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index b0afd7f41a22..1ed3cf9dac4e 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -730,13 +730,18 @@ static int __remove_mapping(struct address_space *mapping, struct folio *folio,
> > {
> > int refcount;
> > void *shadow = NULL;
> > + struct swap_cluster_info *ci;
> >
> > BUG_ON(!folio_test_locked(folio));
> > BUG_ON(mapping != folio_mapping(folio));
> >
> > - if (!folio_test_swapcache(folio))
> > + if (folio_test_swapcache(folio)) {
> > + ci = swap_cluster_lock_by_folio_irq(folio);
> > + } else {
> > spin_lock(&mapping->host->i_lock);
> > - xa_lock_irq(&mapping->i_pages);
> > + xa_lock_irq(&mapping->i_pages);
> > + }
> > +
> > /*
> > * The non racy check for a busy folio.
> > *
> > @@ -776,9 +781,9 @@ static int __remove_mapping(struct address_space *mapping, struct folio *folio,
> >
> > if (reclaimed && !mapping_exiting(mapping))
> > shadow = workingset_eviction(folio, target_memcg);
> > - __delete_from_swap_cache(folio, swap, shadow);
> > + __swap_cache_del_folio(swap, folio, shadow);
> > memcg1_swapout(folio, swap);
> > - xa_unlock_irq(&mapping->i_pages);
> > + swap_cluster_unlock_irq(ci);
> > put_swap_folio(folio, swap);
> > } else {
> > void (*free_folio)(struct folio *);
> > @@ -816,9 +821,12 @@ static int __remove_mapping(struct address_space *mapping, struct folio *folio,
> > return 1;
> >
> > cannot_free:
> > - xa_unlock_irq(&mapping->i_pages);
> > - if (!folio_test_swapcache(folio))
> > + if (folio_test_swapcache(folio)) {
> > + swap_cluster_unlock_irq(ci);
> > + } else {
> > + xa_unlock_irq(&mapping->i_pages);
> > spin_unlock(&mapping->host->i_lock);
> > + }
> > return 0;
> > }
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index ee443b317ac7..c869859eec77 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1166,7 +1166,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
> >
> > out:
> > if (ret && ret != -EEXIST) {
> > - delete_from_swap_cache(folio);
> > + swap_cache_del_folio(folio);
> > folio_unlock(folio);
> > }
> > folio_put(folio);
> > --
> > 2.51.0
> >
>
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