Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and swapcache_clear to support large folios
From: Barry Song
Date: Wed Jul 31 2024 - 04:20:52 EST
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.
>
> > + 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