Message-ID: <CAGsJ_4wpu2Nq0y3z7LYjPLJ1ZBi+BNRNKkyUPsXKDAg1POLjLA@mail.gmail.com>
Date: Tue, 9 Apr 2024 21:22:20 +1200
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>, Gao Xiang <xiang@...nel.org>, 
	Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Chris Li <chrisl@...nel.org>, 
	Lance Yang <ioworker0@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

On Tue, Apr 9, 2024 at 8:51 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Tue, Apr 9, 2024 at 6:40 AM Ryan Roberts <ryan.roberts@....com> wrote:
> >
> > Now that we no longer have a convenient flag in the cluster to determine
> > if a folio is large, free_swap_and_cache() will take a reference and
> > lock a large folio much more often, which could lead to contention and
> > (e.g.) failure to split large folios, etc.
> >
> > Let's solve that problem by batch freeing swap and cache with a new
> > function, free_swap_and_cache_nr(), to free a contiguous range of swap
> > entries together. This allows us to first drop a reference to each swap
> > slot before we try to release the cache folio. This means we only try to
> > release the folio once, only taking the reference and lock once - much
> > better than the previous 512 times for the 2M THP case.
> >
> > Contiguous swap entries are gathered in zap_pte_range() and
> > madvise_free_pte_range() in a similar way to how present ptes are
> > already gathered in zap_pte_range().
> >
> > While we are at it, let's simplify by converting the return type of both
> > functions to void. The return value was used only by zap_pte_range() to
> > print a bad pte, and was ignored by everyone else, so the extra
> > reporting wasn't exactly guaranteed. We will still get the warning with
> > most of the information from get_swap_device(). With the batch version,
> > we wouldn't know which pte was bad anyway so could print the wrong one.
> >
> > Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> > ---
> >  include/linux/pgtable.h | 29 ++++++++++++
> >  include/linux/swap.h    | 12 +++--
> >  mm/internal.h           | 63 ++++++++++++++++++++++++++
> >  mm/madvise.c            | 12 +++--
> >  mm/memory.c             | 13 +++---
> >  mm/swapfile.c           | 97 +++++++++++++++++++++++++++++++++--------
> >  6 files changed, 195 insertions(+), 31 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index a3fc8150b047..75096025fe52 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -708,6 +708,35 @@ static inline void pte_clear_not_present_full(struct mm_struct *mm,
> >  }
> >  #endif
> >
> > +#ifndef clear_not_present_full_ptes
> > +/**
> > + * clear_not_present_full_ptes - Clear multiple not present PTEs which are
> > + *                              consecutive in the pgtable.
> > + * @mm: Address space the ptes represent.
> > + * @addr: Address of the first pte.
> > + * @ptep: Page table pointer for the first entry.
> > + * @nr: Number of entries to clear.
> > + * @full: Whether we are clearing a full mm.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented as a simple
> > + * loop over pte_clear_not_present_full().
> > + *
> > + * Context: The caller holds the page table lock.  The PTEs are all not present.
> > + * The PTEs are all in the same PMD.
> > + */
> > +static inline void clear_not_present_full_ptes(struct mm_struct *mm,
> > +               unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> > +{
> > +       for (;;) {
> > +               pte_clear_not_present_full(mm, addr, ptep, full);
> > +               if (--nr == 0)
> > +                       break;
> > +               ptep++;
> > +               addr += PAGE_SIZE;
> > +       }
> > +}
> > +#endif
> > +
> >  #ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
> >  extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
> >                               unsigned long address,
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index f6f78198f000..5737236dc3ce 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -471,7 +471,7 @@ extern int swap_duplicate(swp_entry_t);
> >  extern int swapcache_prepare(swp_entry_t);
> >  extern void swap_free(swp_entry_t);
> >  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> > -extern int free_swap_and_cache(swp_entry_t);
> > +extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
> >  int swap_type_of(dev_t device, sector_t offset);
> >  int find_first_swap(dev_t *device);
> >  extern unsigned int count_swap_pages(int, int);
> > @@ -520,8 +520,9 @@ static inline void put_swap_device(struct swap_info_struct *si)
> >  #define free_pages_and_swap_cache(pages, nr) \
> >         release_pages((pages), (nr));
> >
> > -/* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
> > -#define free_swap_and_cache(e) is_pfn_swap_entry(e)
> > +static inline void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> > +{
> > +}
> >
> >  static inline void free_swap_cache(struct folio *folio)
> >  {
> > @@ -589,6 +590,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
> >  }
> >  #endif /* CONFIG_SWAP */
> >
> > +static inline void free_swap_and_cache(swp_entry_t entry)
> > +{
> > +       free_swap_and_cache_nr(entry, 1);
> > +}
> > +
> >  #ifdef CONFIG_MEMCG
> >  static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> >  {
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 3bdc8693b54f..de68705624b0 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -11,6 +11,8 @@
> >  #include <linux/mm.h>
> >  #include <linux/pagemap.h>
> >  #include <linux/rmap.h>
> > +#include <linux/swap.h>
> > +#include <linux/swapops.h>
> >  #include <linux/tracepoint-defs.h>
> >
> >  struct folio_batch;
> > @@ -189,6 +191,67 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >
> >         return min(ptep - start_ptep, max_nr);
> >  }
> > +
> > +/**
> > + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> > + * @pte: The initial pte state; is_swap_pte(pte) must be true.
> > + *
> > + * Increments the swap offset, while maintaining all other fields, including
> > + * swap type, and any swp pte bits. The resulting pte is returned.
> > + */
> > +static inline pte_t pte_next_swp_offset(pte_t pte)
> > +{
> > +       swp_entry_t entry = pte_to_swp_entry(pte);
> > +       pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry),
> > +                                                  swp_offset(entry) + 1));
> > +
> > +       if (pte_swp_soft_dirty(pte))
> > +               new = pte_swp_mksoft_dirty(new);
> > +       if (pte_swp_exclusive(pte))
> > +               new = pte_swp_mkexclusive(new);
> > +       if (pte_swp_uffd_wp(pte))
> > +               new = pte_swp_mkuffd_wp(new);
>
> I don't quite understand this. If this page table entry is exclusive,
> will its subsequent page table entry also be exclusive without
> question?
> in try_to_unmap_one, exclusive is per-subpage but not per-folio:
>
>                 anon_exclusive = folio_test_anon(folio) &&
>                                  PageAnonExclusive(subpage);
>
> same questions also for diry, wp etc.

Sorry for the noise. you are right. based on your new version, I think I should
entirely drop:

[PATCH v2 3/5] mm: swap_pte_batch: add an output argument to reture if
all swap entries are exclusive

https://lore.kernel.org/linux-mm/20240409082631.187483-4-21cnbao@gmail.com/

>
> > +
> > +       return new;
> > +}
> > +
> > +/**
> > + * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
> > + * @start_ptep: Page table pointer for the first entry.
> > + * @max_nr: The maximum number of table entries to consider.
> > + * @pte: Page table entry for the first entry.
> > + *
> > + * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs
> > + * containing swap entries all with consecutive offsets and targeting the same
> > + * swap type, all with matching swp pte bits.
> > + *
> > + * max_nr must be at least one and must be limited by the caller so scanning
> > + * cannot exceed a single page table.
> > + *
> > + * Return: the number of table entries in the batch.
> > + */
> > +static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
> > +{
> > +       pte_t expected_pte = pte_next_swp_offset(pte);
> > +       const pte_t *end_ptep = start_ptep + max_nr;
> > +       pte_t *ptep = start_ptep + 1;
> > +
> > +       VM_WARN_ON(max_nr < 1);
> > +       VM_WARN_ON(!is_swap_pte(pte));
> > +       VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
> > +
> > +       while (ptep < end_ptep) {
> > +               pte = ptep_get(ptep);
> > +
> > +               if (!pte_same(pte, expected_pte))
> > +                       break;
> > +
> > +               expected_pte = pte_next_swp_offset(expected_pte);
> > +               ptep++;
> > +       }
> > +
> > +       return ptep - start_ptep;
> > +}
> >  #endif /* CONFIG_MMU */
> >
> >  void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 1f77a51baaac..5011ecb24344 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -628,6 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >         struct folio *folio;
> >         int nr_swap = 0;
> >         unsigned long next;
> > +       int nr, max_nr;
> >
> >         next = pmd_addr_end(addr, end);
> >         if (pmd_trans_huge(*pmd))
> > @@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >                 return 0;
> >         flush_tlb_batched_pending(mm);
> >         arch_enter_lazy_mmu_mode();
> > -       for (; addr != end; pte++, addr += PAGE_SIZE) {
> > +       for (; addr != end; pte += nr, addr += PAGE_SIZE * nr) {
> > +               nr = 1;
> >                 ptent = ptep_get(pte);
> >
> >                 if (pte_none(ptent))
> > @@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >
> >                         entry = pte_to_swp_entry(ptent);
> >                         if (!non_swap_entry(entry)) {
> > -                               nr_swap--;
> > -                               free_swap_and_cache(entry);
> > -                               pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > +                               max_nr = (end - addr) / PAGE_SIZE;
> > +                               nr = swap_pte_batch(pte, max_nr, ptent);
> > +                               nr_swap -= nr;
> > +                               free_swap_and_cache_nr(entry, nr);
> > +                               clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
> >                         } else if (is_hwpoison_entry(entry) ||
> >                                    is_poisoned_swp_entry(entry)) {
> >                                 pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b98e4d907a14..0db2aa066a5a 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1637,12 +1637,13 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >                                 folio_remove_rmap_pte(folio, page, vma);
> >                         folio_put(folio);
> >                 } else if (!non_swap_entry(entry)) {
> > -                       /* Genuine swap entry, hence a private anon page */
> > +                       max_nr = (end - addr) / PAGE_SIZE;
> > +                       nr = swap_pte_batch(pte, max_nr, ptent);
> > +                       /* Genuine swap entries, hence a private anon pages */
> >                         if (!should_zap_cows(details))
> >                                 continue;
> > -                       rss[MM_SWAPENTS]--;
> > -                       if (unlikely(!free_swap_and_cache(entry)))
> > -                               print_bad_pte(vma, addr, ptent, NULL);
> > +                       rss[MM_SWAPENTS] -= nr;
> > +                       free_swap_and_cache_nr(entry, nr);
> >                 } else if (is_migration_entry(entry)) {
> >                         folio = pfn_swap_entry_folio(entry);
> >                         if (!should_zap_folio(details, folio))
> > @@ -1665,8 +1666,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >                         pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
> >                         WARN_ON_ONCE(1);
> >                 }
> > -               pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > -               zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent);
> > +               clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
> > +               zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details, ptent);
> >         } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
> >
> >         add_mm_rss_vec(mm, rss);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 1ded6d1dcab4..20c45757f2b2 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -130,7 +130,11 @@ static inline unsigned char swap_count(unsigned char ent)
> >  /* Reclaim the swap entry if swap is getting full*/
> >  #define TTRS_FULL              0x4
> >
> > -/* returns 1 if swap entry is freed */
> > +/*
> > + * returns number of pages in the folio that backs the swap entry. If positive,
> > + * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
> > + * folio was associated with the swap entry.
> > + */
> >  static int __try_to_reclaim_swap(struct swap_info_struct *si,
> >                                  unsigned long offset, unsigned long flags)
> >  {
> > @@ -155,6 +159,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> >                         ret = folio_free_swap(folio);
> >                 folio_unlock(folio);
> >         }
> > +       ret = ret ? folio_nr_pages(folio) : -folio_nr_pages(folio);
> >         folio_put(folio);
> >         return ret;
> >  }
> > @@ -895,7 +900,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> >                 swap_was_freed = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
> >                 spin_lock(&si->lock);
> >                 /* entry was freed successfully, try to use this again */
> > -               if (swap_was_freed)
> > +               if (swap_was_freed > 0)
> >                         goto checks;
> >                 goto scan; /* check next one */
> >         }
> > @@ -1572,32 +1577,88 @@ bool folio_free_swap(struct folio *folio)
> >         return true;
> >  }
> >
> > -/*
> > - * Free the swap entry like above, but also try to
> > - * free the page cache entry if it is the last user.
> > +/**
> > + * free_swap_and_cache_nr() - Release reference on range of swap entries and
> > + *                            reclaim their cache if no more references remain.
> > + * @entry: First entry of range.
> > + * @nr: Number of entries in range.
> > + *
> > + * For each swap entry in the contiguous range, release a reference. If any swap
> > + * entries become free, try to reclaim their underlying folios, if present. The
> > + * offset range is defined by [entry.offset, entry.offset + nr).
> >   */
> > -int free_swap_and_cache(swp_entry_t entry)
> > +void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> >  {
> > -       struct swap_info_struct *p;
> > +       const unsigned long start_offset = swp_offset(entry);
> > +       const unsigned long end_offset = start_offset + nr;
> > +       unsigned int type = swp_type(entry);
> > +       struct swap_info_struct *si;
> > +       bool any_only_cache = false;
> > +       unsigned long offset;
> >         unsigned char count;
> >
> >         if (non_swap_entry(entry))
> > -               return 1;
> > +               return;
> >
> > -       p = get_swap_device(entry);
> > -       if (p) {
> > -               if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
> > -                       put_swap_device(p);
> > -                       return 0;
> > +       si = get_swap_device(entry);
> > +       if (!si)
> > +               return;
> > +
> > +       if (WARN_ON(end_offset > si->max))
> > +               goto out;
> > +
> > +       /*
> > +        * First free all entries in the range.
> > +        */
> > +       for (offset = start_offset; offset < end_offset; offset++) {
> > +               if (data_race(si->swap_map[offset])) {
> > +                       count = __swap_entry_free(si, swp_entry(type, offset));
> > +                       if (count == SWAP_HAS_CACHE)
> > +                               any_only_cache = true;
> > +               } else {
> > +                       WARN_ON_ONCE(1);
> >                 }
> > +       }
> > +
> > +       /*
> > +        * Short-circuit the below loop if none of the entries had their
> > +        * reference drop to zero.
> > +        */
> > +       if (!any_only_cache)
> > +               goto out;
> >
> > -               count = __swap_entry_free(p, entry);
> > -               if (count == SWAP_HAS_CACHE)
> > -                       __try_to_reclaim_swap(p, swp_offset(entry),
> > +       /*
> > +        * Now go back over the range trying to reclaim the swap cache. This is
> > +        * more efficient for large folios because we will only try to reclaim
> > +        * the swap once per folio in the common case. If we do
> > +        * __swap_entry_free() and __try_to_reclaim_swap() in the same loop, the
> > +        * latter will get a reference and lock the folio for every individual
> > +        * page but will only succeed once the swap slot for every subpage is
> > +        * zero.
> > +        */
> > +       for (offset = start_offset; offset < end_offset; offset += nr) {
> > +               nr = 1;
> > +               if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
> > +                       /*
> > +                        * Folios are always naturally aligned in swap so
> > +                        * advance forward to the next boundary. Zero means no
> > +                        * folio was found for the swap entry, so advance by 1
> > +                        * in this case. Negative value means folio was found
> > +                        * but could not be reclaimed. Here we can still advance
> > +                        * to the next boundary.
> > +                        */
> > +                       nr = __try_to_reclaim_swap(si, offset,
> >                                               TTRS_UNMAPPED | TTRS_FULL);
> > -               put_swap_device(p);
> > +                       if (nr == 0)
> > +                               nr = 1;
> > +                       else if (nr < 0)
> > +                               nr = -nr;
> > +                       nr = ALIGN(offset + 1, nr) - offset;
> > +               }
> >         }
> > -       return p != NULL;
> > +
> > +out:
> > +       put_swap_device(si);
> >  }
> >
> >  #ifdef CONFIG_HIBERNATION
> > --
> > 2.25.1
> >
>
> Thanks
> Barry

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