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Message-ID: <CANeU7QnYOx-=xoeoLWotdQWOs2KMvw0E7LuRq27LO4RDA_ManQ@mail.gmail.com>
Date: Wed, 3 Apr 2024 15:12:34 -0700
From: Chris Li <chrisl@...nel.org>
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>, Barry Song <21cnbao@...il.com>,
Lance Yang <ioworker0@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
Hi Ryan,
Sorry for the late reply. I want to review this series but don't have
the chance to do it sooner.
On Wed, Apr 3, 2024 at 4:40 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> As preparation for supporting small-sized THP in the swap-out path,
> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
> which, when present, always implies PMD-sized THP, which is the same as
> the cluster size.
>
> The only use of the flag was to determine whether a swap entry refers to
> a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
> Instead of relying on the flag, we now pass in nr_pages, which
> originates from the folio's number of pages. This allows the logic to
> work for folios of any order.
>
> The one snag is that one of the swap_page_trans_huge_swapped() call
> sites does not have the folio. But it was only being called there to
> shortcut a call __try_to_reclaim_swap() in some cases.
> __try_to_reclaim_swap() gets the folio and (via some other functions)
> calls swap_page_trans_huge_swapped(). So I've removed the problematic
> call site and believe the new logic should be functionally equivalent.
>
> That said, removing the fast path means that we will take a reference
> and trylock a large folio much more often, which we would like to avoid.
> The next patch will solve this.
>
> Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster()
> which used to be called during folio splitting, since
> split_swap_cluster()'s only job was to remove the flag.
Seems necessary to remove the assumption of large folio be PMD size.
Acked-by: Chris Li <chrisl@...nel.org>
>
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
> include/linux/swap.h | 10 ----------
> mm/huge_memory.c | 3 ---
> mm/swapfile.c | 47 ++++++++------------------------------------
> 3 files changed, 8 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a211a0383425..f6f78198f000 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -259,7 +259,6 @@ struct swap_cluster_info {
> };
> #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
> #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
> -#define CLUSTER_FLAG_HUGE 4 /* This cluster is backing a transparent huge page */
>
> /*
> * We assign a cluster to each CPU, so each CPU can allocate swap entry from
> @@ -590,15 +589,6 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
> }
> #endif /* CONFIG_SWAP */
>
> -#ifdef CONFIG_THP_SWAP
> -extern int split_swap_cluster(swp_entry_t entry);
> -#else
> -static inline int split_swap_cluster(swp_entry_t entry)
> -{
> - return 0;
> -}
> -#endif
> -
> #ifdef CONFIG_MEMCG
> static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ea6d1f09a0b9..3ca9282a0dc9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2844,9 +2844,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> shmem_uncharge(folio->mapping->host, nr_dropped);
> remap_page(folio, nr);
>
> - if (folio_test_swapcache(folio))
> - split_swap_cluster(folio->swap);
> -
> /*
> * set page to its compound_head when split to non order-0 pages, so
> * we can skip unlocking it below, since PG_locked is transferred to
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5e6d2304a2a4..0d44ee2b4f9c 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -343,18 +343,6 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
> info->data = 0;
> }
>
> -static inline bool cluster_is_huge(struct swap_cluster_info *info)
> -{
> - if (IS_ENABLED(CONFIG_THP_SWAP))
> - return info->flags & CLUSTER_FLAG_HUGE;
> - return false;
> -}
> -
> -static inline void cluster_clear_huge(struct swap_cluster_info *info)
> -{
> - info->flags &= ~CLUSTER_FLAG_HUGE;
> -}
> -
> static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
> unsigned long offset)
> {
> @@ -1027,7 +1015,7 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
> offset = idx * SWAPFILE_CLUSTER;
> ci = lock_cluster(si, offset);
> alloc_cluster(si, idx);
> - cluster_set_count_flag(ci, SWAPFILE_CLUSTER, CLUSTER_FLAG_HUGE);
> + cluster_set_count(ci, SWAPFILE_CLUSTER);
>
> memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
> unlock_cluster(ci);
> @@ -1365,7 +1353,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>
> ci = lock_cluster_or_swap_info(si, offset);
> if (size == SWAPFILE_CLUSTER) {
> - VM_BUG_ON(!cluster_is_huge(ci));
> map = si->swap_map + offset;
> for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> val = map[i];
> @@ -1373,7 +1360,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
> if (val == SWAP_HAS_CACHE)
> free_entries++;
> }
> - cluster_clear_huge(ci);
> if (free_entries == SWAPFILE_CLUSTER) {
> unlock_cluster_or_swap_info(si, ci);
> spin_lock(&si->lock);
> @@ -1395,23 +1381,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
> unlock_cluster_or_swap_info(si, ci);
> }
>
> -#ifdef CONFIG_THP_SWAP
> -int split_swap_cluster(swp_entry_t entry)
> -{
> - struct swap_info_struct *si;
> - struct swap_cluster_info *ci;
> - unsigned long offset = swp_offset(entry);
> -
> - si = _swap_info_get(entry);
> - if (!si)
> - return -EBUSY;
> - ci = lock_cluster(si, offset);
> - cluster_clear_huge(ci);
> - unlock_cluster(ci);
> - return 0;
> -}
> -#endif
> -
> static int swp_entry_cmp(const void *ent1, const void *ent2)
> {
> const swp_entry_t *e1 = ent1, *e2 = ent2;
> @@ -1519,22 +1488,23 @@ int swp_swapcount(swp_entry_t entry)
> }
>
> static bool swap_page_trans_huge_swapped(struct swap_info_struct *si,
> - swp_entry_t entry)
> + swp_entry_t entry,
> + unsigned int nr_pages)
> {
> struct swap_cluster_info *ci;
> unsigned char *map = si->swap_map;
> unsigned long roffset = swp_offset(entry);
> - unsigned long offset = round_down(roffset, SWAPFILE_CLUSTER);
> + unsigned long offset = round_down(roffset, nr_pages);
It is obvious this code only works for powers two nr_pages. The
SWAPFILE_CLSTER is a power of two. If we switch to an API for
nr_pages, we might want to warn/ban passing in the non-power of two
nr_pages.
> int i;
> bool ret = false;
>
> ci = lock_cluster_or_swap_info(si, offset);
> - if (!ci || !cluster_is_huge(ci)) {
> + if (!ci || nr_pages == 1) {
> if (swap_count(map[roffset]))
> ret = true;
> goto unlock_out;
> }
> - for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> + for (i = 0; i < nr_pages; i++) {
Here we assume the swap entry offset is contiguous. That is beyond
your patch's scope. If in the future we want to have non-contiguous
swap entries to swap out large pages, we will need to find out and
change all the places that have the assumption of contiguous swap
entries.
Chris
> if (swap_count(map[offset + i])) {
> ret = true;
> break;
> @@ -1556,7 +1526,7 @@ static bool folio_swapped(struct folio *folio)
> if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!folio_test_large(folio)))
> return swap_swapcount(si, entry) != 0;
>
> - return swap_page_trans_huge_swapped(si, entry);
> + return swap_page_trans_huge_swapped(si, entry, folio_nr_pages(folio));
> }
>
> /**
> @@ -1622,8 +1592,7 @@ int free_swap_and_cache(swp_entry_t entry)
> }
>
> count = __swap_entry_free(p, entry);
> - if (count == SWAP_HAS_CACHE &&
> - !swap_page_trans_huge_swapped(p, entry))
> + if (count == SWAP_HAS_CACHE)
> __try_to_reclaim_swap(p, swp_offset(entry),
> TTRS_UNMAPPED | TTRS_FULL);
> put_swap_device(p);
> --
> 2.25.1
>
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