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Message-ID: <871q39kq0e.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Thu, 01 Aug 2024 09:09:37 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com,
hannes@...xchg.org, hughd@...gle.com, kaleshsingh@...gle.com,
kasong@...cent.com, linux-kernel@...r.kernel.org, mhocko@...e.com,
minchan@...nel.org, nphamcs@...il.com, ryan.roberts@....com,
senozhatsky@...omium.org, shakeel.butt@...ux.dev, shy828301@...il.com,
surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org,
xiang@...nel.org, yosryahmed@...gle.com
Subject: Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and
swapcache_clear to support large folios
Barry Song <21cnbao@...il.com> writes:
> On Wed, Jul 31, 2024 at 4:28 PM Huang, Ying <ying.huang@...el.com> wrote:
>>
>> Barry Song <21cnbao@...il.com> writes:
>>
>> > On Wed, Jul 31, 2024 at 4:14 PM Huang, Ying <ying.huang@...el.com> wrote:
>> >>
>> >> Hi, Barry,
>> >>
>> >> Barry Song <21cnbao@...il.com> writes:
>> >>
>> >> > From: Barry Song <v-songbaohua@...o.com>
>> >> >
>> >> > Right now, swapcache_prepare() and swapcache_clear() supports one entry
>> >> > only, to support large folios, we need to handle multiple swap entries.
>> >> >
>> >> > To optimize stack usage, we iterate twice in __swap_duplicate(): the
>> >> > first time to verify that all entries are valid, and the second time
>> >> > to apply the modifications to the entries.
>> >> >
>> >> > Currently, we're using nr=1 for the existing users.
>> >> >
>> >> > Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> >> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
>> >> > ---
>> >> > include/linux/swap.h | 4 +-
>> >> > mm/memory.c | 6 +--
>> >> > mm/swap.h | 5 ++-
>> >> > mm/swap_state.c | 2 +-
>> >> > mm/swapfile.c | 101 +++++++++++++++++++++++++------------------
>> >> > 5 files changed, 68 insertions(+), 50 deletions(-)
>> >> >
>> >> > diff --git a/include/linux/swap.h b/include/linux/swap.h
>> >> > index ba7ea95d1c57..5b920fa2315b 100644
>> >> > --- a/include/linux/swap.h
>> >> > +++ b/include/linux/swap.h
>> >> > @@ -480,7 +480,7 @@ extern int get_swap_pages(int n, swp_entry_t swp_entries[], int order);
>> >> > extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>> >> > extern void swap_shmem_alloc(swp_entry_t);
>> >> > extern int swap_duplicate(swp_entry_t);
>> >> > -extern int swapcache_prepare(swp_entry_t);
>> >> > +extern int swapcache_prepare(swp_entry_t entry, int nr);
>> >> > extern void swap_free_nr(swp_entry_t entry, int nr_pages);
>> >> > extern void swapcache_free_entries(swp_entry_t *entries, int n);
>> >> > extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>> >> > @@ -554,7 +554,7 @@ static inline int swap_duplicate(swp_entry_t swp)
>> >> > return 0;
>> >> > }
>> >> >
>> >> > -static inline int swapcache_prepare(swp_entry_t swp)
>> >> > +static inline int swapcache_prepare(swp_entry_t swp, int nr)
>> >> > {
>> >> > return 0;
>> >> > }
>> >> > diff --git a/mm/memory.c b/mm/memory.c
>> >> > index 833d2cad6eb2..b8675617a5e3 100644
>> >> > --- a/mm/memory.c
>> >> > +++ b/mm/memory.c
>> >> > @@ -4081,7 +4081,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> > * reusing the same entry. It's undetectable as
>> >> > * pte_same() returns true due to entry reuse.
>> >> > */
>> >> > - if (swapcache_prepare(entry)) {
>> >> > + if (swapcache_prepare(entry, 1)) {
>> >> > /* Relax a bit to prevent rapid repeated page faults */
>> >> > schedule_timeout_uninterruptible(1);
>> >> > goto out;
>> >> > @@ -4387,7 +4387,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> > out:
>> >> > /* Clear the swap cache pin for direct swapin after PTL unlock */
>> >> > if (need_clear_cache)
>> >> > - swapcache_clear(si, entry);
>> >> > + swapcache_clear(si, entry, 1);
>> >> > if (si)
>> >> > put_swap_device(si);
>> >> > return ret;
>> >> > @@ -4403,7 +4403,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> > folio_put(swapcache);
>> >> > }
>> >> > if (need_clear_cache)
>> >> > - swapcache_clear(si, entry);
>> >> > + swapcache_clear(si, entry, 1);
>> >> > if (si)
>> >> > put_swap_device(si);
>> >> > return ret;
>> >> > diff --git a/mm/swap.h b/mm/swap.h
>> >> > index baa1fa946b34..7c6330561d84 100644
>> >> > --- a/mm/swap.h
>> >> > +++ b/mm/swap.h
>> >> > @@ -59,7 +59,7 @@ void __delete_from_swap_cache(struct folio *folio,
>> >> > 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);
>> >> > +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 vm_area_struct *vma, unsigned long addr);
>> >> > struct folio *filemap_get_incore_folio(struct address_space *mapping,
>> >> > @@ -120,7 +120,7 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
>> >> > return 0;
>> >> > }
>> >> >
>> >> > -static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
>> >> > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
>> >> > {
>> >> > }
>> >> >
>> >> > @@ -172,4 +172,5 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
>> >> > return 0;
>> >> > }
>> >> > #endif /* CONFIG_SWAP */
>> >> > +
>> >>
>> >> NITPICK: Is it necessary to add a blank line here? But I don't think a
>> >> new version is necessary if this is the only change needed.
>> >
>> > No need to add a blank line; it was probably a mistake I made in Vim.
>> >
>> >>
>> >> > #endif /* _MM_SWAP_H */
>> >> > diff --git a/mm/swap_state.c b/mm/swap_state.c
>> >> > index a1726e49a5eb..b06f2a054f5a 100644
>> >> > --- a/mm/swap_state.c
>> >> > +++ b/mm/swap_state.c
>> >> > @@ -477,7 +477,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>> >> > /*
>> >> > * Swap entry may have been freed since our caller observed it.
>> >> > */
>> >> > - err = swapcache_prepare(entry);
>> >> > + err = swapcache_prepare(entry, 1);
>> >> > if (!err)
>> >> > break;
>> >> >
>> >> > diff --git a/mm/swapfile.c b/mm/swapfile.c
>> >> > index 5f73a8553371..757d38a86f56 100644
>> >> > --- a/mm/swapfile.c
>> >> > +++ b/mm/swapfile.c
>> >> > @@ -3363,7 +3363,7 @@ void si_swapinfo(struct sysinfo *val)
>> >> > }
>> >> >
>> >> > /*
>> >> > - * Verify that a swap entry is valid and increment its swap map count.
>> >> > + * Verify that nr swap entries are valid and increment their swap map counts.
>> >> > *
>> >> > * Returns error code in following case.
>> >> > * - success -> 0
>> >> > @@ -3373,60 +3373,77 @@ void si_swapinfo(struct sysinfo *val)
>> >> > * - swap-cache reference is requested but the entry is not used. -> ENOENT
>> >> > * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
>> >> > */
>> >> > -static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>> >> > +static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>> >> > {
>> >> > struct swap_info_struct *p;
>> >> > struct swap_cluster_info *ci;
>> >> > unsigned long offset;
>> >> > unsigned char count;
>> >> > unsigned char has_cache;
>> >> > - int err;
>> >> > + int err, i;
>> >> >
>> >> > p = swp_swap_info(entry);
>> >> >
>> >> > offset = swp_offset(entry);
>> >> > + VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
>> >> > ci = lock_cluster_or_swap_info(p, offset);
>> >> >
>> >> > - count = p->swap_map[offset];
>> >> > + err = 0;
>> >> > + for (i = 0; i < nr; i++) {
>> >> > + count = p->swap_map[offset + i];
>> >> >
>> >> > - /*
>> >> > - * swapin_readahead() doesn't check if a swap entry is valid, so the
>> >> > - * swap entry could be SWAP_MAP_BAD. Check here with lock held.
>> >> > - */
>> >> > - if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
>> >> > - err = -ENOENT;
>> >> > - goto unlock_out;
>> >> > - }
>> >> > + /*
>> >> > + * swapin_readahead() doesn't check if a swap entry is valid, so the
>> >> > + * swap entry could be SWAP_MAP_BAD. Check here with lock held.
>> >> > + */
>> >> > + if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
>> >> > + err = -ENOENT;
>> >> > + goto unlock_out;
>> >> > + }
>> >> >
>> >> > - has_cache = count & SWAP_HAS_CACHE;
>> >> > - count &= ~SWAP_HAS_CACHE;
>> >> > - err = 0;
>> >> > + has_cache = count & SWAP_HAS_CACHE;
>> >> > + count &= ~SWAP_HAS_CACHE;
>> >> >
>> >> > - if (usage == SWAP_HAS_CACHE) {
>> >> > + if (usage == SWAP_HAS_CACHE) {
>> >> > + /* set SWAP_HAS_CACHE if there is no cache and entry is used */
>> >> > + if (!has_cache && count)
>> >> > + continue;
>> >> > + else if (has_cache) /* someone else added cache */
>> >> > + err = -EEXIST;
>> >> > + else /* no users remaining */
>> >> > + err = -ENOENT;
>> >> >
>> >> > - /* set SWAP_HAS_CACHE if there is no cache and entry is used */
>> >> > - if (!has_cache && count)
>> >> > - has_cache = SWAP_HAS_CACHE;
>> >> > - else if (has_cache) /* someone else added cache */
>> >> > - err = -EEXIST;
>> >> > - else /* no users remaining */
>> >> > - err = -ENOENT;
>> >> > + } else if (count || has_cache) {
>> >> >
>> >> > - } else if (count || has_cache) {
>> >> > + if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> > + continue;
>> >> > + else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
>> >> > + err = -EINVAL;
>> >> > + else if (swap_count_continued(p, offset + i, count))
>> >> > + continue;
>> >>
>> >> IIUC, this will make the change to swap map directly instead of
>> >> verification. If the verification failed for some entry later, the
>> >> count will be wrong? Or I missed something?
>> >
>> > To avoid using a bitmap or a larger stack, we actually verify during
>> > the first iteration.
>> > This ensures that by the second iteration, we can safely commit the
>> > modification.
>> >
>> > I actually put some words in the changelog :-)
>> >
>> > To optimize stack usage, we iterate twice in __swap_duplicate(): the
>> > first time to verify that all entries are valid, and the second time
>> > to apply the modifications to the entries.
>>
>> Yes, I have seen it and I think that it is a good strategy.
>>
>> But, IIUC, swap_count_continued() will change the higher bits of the
>> swap_map instead of verifying. Or, my understanding is wrong?
>>
>
> Ying, your understanding is 100% correct. but the code also has nothing
> broken. we didn't extend swap_duplicate() to have argument nr,
> so all users which can set usage=1 will definitely have nr=1.
>
> int swap_duplicate(swp_entry_t entry)
> {
> int err = 0;
>
> while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
> err = add_swap_count_continuation(entry, GFP_ATOMIC);
> return err;
> }
I understand that we don't have requirements to support "usage == 1 &&
nr > 1" case for __swap_duplicate() at least for now.
> Maybe I can add a VM_WARN_ON to warn those people who might
> want to extend swap_duplicate()? in that case, things could be quite
> tricky.
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3386,6 +3386,7 @@ static int __swap_duplicate(swp_entry_t entry,
> unsigned char usage, int nr)
>
> offset = swp_offset(entry);
> VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
> + VM_WARN_ON(usage == 1 && nr > 1);
> ci = lock_cluster_or_swap_info(p, offset);
>
> err = 0;
Please add this. And, I think that we need to make it explicit in patch
description and comments to avoid potential confusing.
And, because it's hard to implement the verify and change strategy if
"usage == 1". Can we only use that strategy for "usage ==
SWAP_HAS_CACHE"?
--
Best Regards,
Huang, Ying
>> >>
>> >> > + else
>> >> > + err = -ENOMEM;
>> >> > + } else
>> >> > + err = -ENOENT; /* unused swap entry */
>> >> >
>> >> > - if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> > + if (err)
>> >> > + goto unlock_out;
>> >> > + }
>> >> > +
>> >> > + for (i = 0; i < nr; i++) {
>> >> > + count = p->swap_map[offset + i];
>> >> > + has_cache = count & SWAP_HAS_CACHE;
>> >> > + count &= ~SWAP_HAS_CACHE;
>> >> > +
>> >> > + if (usage == SWAP_HAS_CACHE)
>> >> > + has_cache = SWAP_HAS_CACHE;
>> >> > + else if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> > count += usage;
>> >> > - else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
>> >> > - err = -EINVAL;
>> >> > - else if (swap_count_continued(p, offset, count))
>> >> > - count = COUNT_CONTINUED;
>> >> > else
>> >> > - err = -ENOMEM;
>> >> > - } else
>> >> > - err = -ENOENT; /* unused swap entry */
>> >> > + count = COUNT_CONTINUED;
>> >> >
>> >> > - if (!err)
>> >> > - WRITE_ONCE(p->swap_map[offset], count | has_cache);
>> >> > + WRITE_ONCE(p->swap_map[offset + i], count | has_cache);
>> >> > + }
>> >> >
>> >> > unlock_out:
>> >> > unlock_cluster_or_swap_info(p, ci);
>> >> > @@ -3439,7 +3456,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>> >> > */
>> >> > void swap_shmem_alloc(swp_entry_t entry)
>> >> > {
>> >> > - __swap_duplicate(entry, SWAP_MAP_SHMEM);
>> >> > + __swap_duplicate(entry, SWAP_MAP_SHMEM, 1);
>> >> > }
>> >> >
>> >> > /*
>> >> > @@ -3453,29 +3470,29 @@ int swap_duplicate(swp_entry_t entry)
>> >> > {
>> >> > int err = 0;
>> >> >
>> >> > - while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
>> >> > + while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
>> >> > err = add_swap_count_continuation(entry, GFP_ATOMIC);
>> >> > return err;
>> >> > }
>> >> >
>> >> > /*
>> >> > - * @entry: swap entry for which we allocate swap cache.
>> >> > + * @entry: first swap entry from which we allocate nr swap cache.
>> >> > *
>> >> > - * Called when allocating swap cache for existing swap entry,
>> >> > + * Called when allocating swap cache for existing swap entries,
>> >> > * This can return error codes. Returns 0 at success.
>> >> > * -EEXIST means there is a swap cache.
>> >> > * Note: return code is different from swap_duplicate().
>> >> > */
>> >> > -int swapcache_prepare(swp_entry_t entry)
>> >> > +int swapcache_prepare(swp_entry_t entry, int nr)
>> >> > {
>> >> > - return __swap_duplicate(entry, SWAP_HAS_CACHE);
>> >> > + return __swap_duplicate(entry, SWAP_HAS_CACHE, nr);
>> >> > }
>> >> >
>> >> > -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
>> >> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
>> >> > {
>> >> > unsigned long offset = swp_offset(entry);
>> >> >
>> >> > - cluster_swap_free_nr(si, offset, 1, SWAP_HAS_CACHE);
>> >> > + cluster_swap_free_nr(si, offset, nr, SWAP_HAS_CACHE);
>> >> > }
>> >> >
>> >> > struct swap_info_struct *swp_swap_info(swp_entry_t entry)
>> >>
>> >> --
>> >> Best Regards,
>> >> Huang, Ying
>> >
>> > Thanks
>> > Barry
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