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References: <20240408183946.2991168-1-ryan.roberts@arm.com> <20240408183946.2991168-3-ryan.roberts@arm.com>
In-Reply-To: <20240408183946.2991168-3-ryan.roberts@arm.com>
From: Barry Song <21cnbao@gmail.com>
Date: Tue, 9 Apr 2024 20:51:40 +1200
Message-ID: <CAGsJ_4xMaO8AWMGc4Od-FLWBhhT-u8f7QbR11VsqD0uqH3Kp6g@mail.gmail.com>
Subject: Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, David Hildenbrand <david@redhat.com>, 
	Matthew Wilcox <willy@infradead.org>, Huang Ying <ying.huang@intel.com>, Gao Xiang <xiang@kernel.org>, 
	Yu Zhao <yuzhao@google.com>, Yang Shi <shy828301@gmail.com>, Michal Hocko <mhocko@suse.com>, 
	Kefeng Wang <wangkefeng.wang@huawei.com>, Chris Li <chrisl@kernel.org>, 
	Lance Yang <ioworker0@gmail.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Tue, Apr 9, 2024 at 6:40=E2=80=AFAM Ryan Roberts <ryan.roberts@arm.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@arm.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 a=
re
> + *                              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 si=
mple
> + * 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 ful=
l)
> +{
> +       for (;;) {
> +               pte_clear_not_present_full(mm, addr, ptep, full);
> +               if (--nr =3D=3D 0)
> +                       break;
> +               ptep++;
> +               addr +=3D 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_s=
truct *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=3D0 */
> -#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_s=
truct *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 *foli=
o, 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, inclu=
ding
> + * 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 =3D pte_to_swp_entry(pte);
> +       pte_t new =3D __swp_entry_to_pte(__swp_entry(swp_type(entry),
> +                                                  swp_offset(entry) + 1)=
);
> +
> +       if (pte_swp_soft_dirty(pte))
> +               new =3D pte_swp_mksoft_dirty(new);
> +       if (pte_swp_exclusive(pte))
> +               new =3D pte_swp_mkexclusive(new);
> +       if (pte_swp_uffd_wp(pte))
> +               new =3D 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 =3D folio_test_anon(folio) &&
                                 PageAnonExclusive(subpage);

same questions also for diry, wp etc.

> +
> +       return new;
> +}
> +
> +/**
> + * swap_pte_batch - detect a PTE batch for a set of contiguous swap entr=
ies
> + * @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 th=
e same
> + * swap type, all with matching swp pte bits.
> + *
> + * max_nr must be at least one and must be limited by the caller so scan=
ning
> + * 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 pt=
e)
> +{
> +       pte_t expected_pte =3D pte_next_swp_offset(pte);
> +       const pte_t *end_ptep =3D start_ptep + max_nr;
> +       pte_t *ptep =3D 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 =3D ptep_get(ptep);
> +
> +               if (!pte_same(pte, expected_pte))
> +                       break;
> +
> +               expected_pte =3D 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, unsigne=
d long addr,
>         struct folio *folio;
>         int nr_swap =3D 0;
>         unsigned long next;
> +       int nr, max_nr;
>
>         next =3D pmd_addr_end(addr, end);
>         if (pmd_trans_huge(*pmd))
> @@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigne=
d long addr,
>                 return 0;
>         flush_tlb_batched_pending(mm);
>         arch_enter_lazy_mmu_mode();
> -       for (; addr !=3D end; pte++, addr +=3D PAGE_SIZE) {
> +       for (; addr !=3D end; pte +=3D nr, addr +=3D PAGE_SIZE * nr) {
> +               nr =3D 1;
>                 ptent =3D ptep_get(pte);
>
>                 if (pte_none(ptent))
> @@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsign=
ed long addr,
>
>                         entry =3D 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 =3D (end - addr) / PAGE_SIZE;
> +                               nr =3D swap_pte_batch(pte, max_nr, ptent)=
;
> +                               nr_swap -=3D 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_gat=
her *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 =3D (end - addr) / PAGE_SIZE;
> +                       nr =3D swap_pte_batch(pte, max_nr, ptent);
> +                       /* Genuine swap entries, hence a private anon pag=
es */
>                         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] -=3D nr;
> +                       free_swap_and_cache_nr(entry, nr);
>                 } else if (is_migration_entry(entry)) {
>                         folio =3D pfn_swap_entry_folio(entry);
>                         if (!should_zap_folio(details, folio))
> @@ -1665,8 +1666,8 @@ static unsigned long zap_pte_range(struct mmu_gathe=
r *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->fullm=
m);
> +               zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details=
, ptent);
>         } while (pte +=3D nr, addr +=3D PAGE_SIZE * nr, addr !=3D 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 po=
sitive,
> + * 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 flag=
s)
>  {
> @@ -155,6 +159,7 @@ static int __try_to_reclaim_swap(struct swap_info_str=
uct *si,
>                         ret =3D folio_free_swap(folio);
>                 folio_unlock(folio);
>         }
> +       ret =3D 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_struc=
t *si,
>                 swap_was_freed =3D __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 prese=
nt. 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 =3D swp_offset(entry);
> +       const unsigned long end_offset =3D start_offset + nr;
> +       unsigned int type =3D swp_type(entry);
> +       struct swap_info_struct *si;
> +       bool any_only_cache =3D false;
> +       unsigned long offset;
>         unsigned char count;
>
>         if (non_swap_entry(entry))
> -               return 1;
> +               return;
>
> -       p =3D get_swap_device(entry);
> -       if (p) {
> -               if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) =
{
> -                       put_swap_device(p);
> -                       return 0;
> +       si =3D 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 =3D start_offset; offset < end_offset; offset++) {
> +               if (data_race(si->swap_map[offset])) {
> +                       count =3D __swap_entry_free(si, swp_entry(type, o=
ffset));
> +                       if (count =3D=3D SWAP_HAS_CACHE)
> +                               any_only_cache =3D 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 =3D __swap_entry_free(p, entry);
> -               if (count =3D=3D SWAP_HAS_CACHE)
> -                       __try_to_reclaim_swap(p, swp_offset(entry),
> +       /*
> +        * Now go back over the range trying to reclaim the swap cache. T=
his is
> +        * more efficient for large folios because we will only try to re=
claim
> +        * the swap once per folio in the common case. If we do
> +        * __swap_entry_free() and __try_to_reclaim_swap() in the same lo=
op, the
> +        * latter will get a reference and lock the folio for every indiv=
idual
> +        * page but will only succeed once the swap slot for every subpag=
e is
> +        * zero.
> +        */
> +       for (offset =3D start_offset; offset < end_offset; offset +=3D nr=
) {
> +               nr =3D 1;
> +               if (READ_ONCE(si->swap_map[offset]) =3D=3D SWAP_HAS_CACHE=
) {
> +                       /*
> +                        * Folios are always naturally aligned in swap so
> +                        * advance forward to the next boundary. Zero mea=
ns no
> +                        * folio was found for the swap entry, so advance=
 by 1
> +                        * in this case. Negative value means folio was f=
ound
> +                        * but could not be reclaimed. Here we can still =
advance
> +                        * to the next boundary.
> +                        */
> +                       nr =3D __try_to_reclaim_swap(si, offset,
>                                               TTRS_UNMAPPED | TTRS_FULL);
> -               put_swap_device(p);
> +                       if (nr =3D=3D 0)
> +                               nr =3D 1;
> +                       else if (nr < 0)
> +                               nr =3D -nr;
> +                       nr =3D ALIGN(offset + 1, nr) - offset;
> +               }
>         }
> -       return p !=3D NULL;
> +
> +out:
> +       put_swap_device(si);
>  }
>
>  #ifdef CONFIG_HIBERNATION
> --
> 2.25.1
>

Thanks
Barry