Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and swapcache_clear to support large folios

From: Huang, Ying
Date: Wed Jul 31 2024 - 04:29:03 EST

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Barry Song <21cnbao@xxxxxxxxx> writes:

> On Wed, Jul 31, 2024 at 4:14 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Hi, Barry,
>>
>> Barry Song <21cnbao@xxxxxxxxx> writes:
>>
>> > From: Barry Song <v-songbaohua@xxxxxxxx>
>> >
>> > 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@xxxxxxxxxxxxxxxxx>
>> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>> > ---
>> > 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?

--
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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In reply to: Barry Song: "Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and swapcache_clear to support large folios"
Next in thread: Barry Song: "Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and swapcache_clear to support large folios"
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