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Message-ID: <CAGsJ_4zqFXr4DUHH9JAnx2qP8Y8OzAw+MzR1ERdiqNtBQzoY7Q@mail.gmail.com>
Date: Tue, 19 Mar 2024 11:35:40 +1300
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chengming.zhou@...ux.dev,
chrisl@...nel.org, david@...hat.com, hannes@...xchg.org, kasong@...cent.com,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
mhocko@...e.com, nphamcs@...il.com, shy828301@...il.com, steven.price@....com,
surenb@...gle.com, wangkefeng.wang@...wei.com, willy@...radead.org,
xiang@...nel.org, ying.huang@...el.com, yosryahmed@...gle.com,
yuzhao@...gle.com, Barry Song <v-songbaohua@...o.com>, Hugh Dickins <hughd@...gle.com>,
Minchan Kim <minchan@...nel.org>, SeongJae Park <sj@...nel.org>
Subject: Re: [RFC PATCH v3 4/5] mm: swap: introduce swapcache_prepare_nr and
swapcache_clear_nr for large folios swap-in
On Wed, Mar 13, 2024 at 4:35 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 04/03/2024 08:13, Barry Song wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") supports
> > one entry only, to support large folio swap-in, we need to handle multiple
> > swap entries.
> >
> > Cc: Kairui Song <kasong@...cent.com>
> > Cc: "Huang, Ying" <ying.huang@...el.com>
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: Chris Li <chrisl@...nel.org>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Yosry Ahmed <yosryahmed@...gle.com>
> > Cc: Yu Zhao <yuzhao@...gle.com>
> > Cc: SeongJae Park <sj@...nel.org>
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > ---
> > include/linux/swap.h | 1 +
> > mm/swap.h | 1 +
> > mm/swapfile.c | 118 ++++++++++++++++++++++++++-----------------
> > 3 files changed, 74 insertions(+), 46 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index d6ab27929458..22105f0fe2d4 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -480,6 +480,7 @@ 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_nr(swp_entry_t entry, int nr);
> > extern void swap_free(swp_entry_t);
> > extern void swap_nr_free(swp_entry_t entry, int nr_pages);
> > extern void swapcache_free_entries(swp_entry_t *entries, int n);
> > diff --git a/mm/swap.h b/mm/swap.h
> > index fc2f6ade7f80..1cec991efcda 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -42,6 +42,7 @@ 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_nr(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,
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 244106998a69..bae1b8165b11 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3309,7 +3309,7 @@ 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 count.
> > *
> > * Returns error code in following case.
> > * - success -> 0
> > @@ -3319,66 +3319,76 @@ 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_nr(swp_entry_t entry, int nr, unsigned char usage)
>
> perhaps its better to pass order instead of nr to all these functions to make it
> clearer that entry should be naturally aligned and be a power-of-2 number of
> pages, no bigger than SWAPFILE_CLUSTER?
>
> > {
> > struct swap_info_struct *p;
> > struct swap_cluster_info *ci;
> > unsigned long offset;
> > - unsigned char count;
> > - unsigned char has_cache;
> > - int err;
> > + unsigned char count[SWAPFILE_CLUSTER];
> > + unsigned char has_cache[SWAPFILE_CLUSTER];
>
> I'm not sure this 1K stack buffer is a good idea?
>
> Could you split it slightly differently so that loop 1 just does error checking
> and bails out if an error is found, and loop 2 does the new value calculation
> and writeback? Then you don't need these arrays.
right. we can totally remove thoe arrays by re-reading swap_map.
>
> > + int err, i;
> >
> > p = swp_swap_info(entry);
> >
> > offset = swp_offset(entry);
> > ci = lock_cluster_or_swap_info(p, offset);
> >
> > - count = p->swap_map[offset];
> > -
> > - /*
> > - * 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;
> > -
> > - if (usage == SWAP_HAS_CACHE) {
> > + for (i = 0; i < nr; i++) {
> > + count[i] = p->swap_map[offset + i];
> >
> > - /* 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 */
> > + /*
> > + * 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[i]) == SWAP_MAP_BAD)) {
> > err = -ENOENT;
> > + goto unlock_out;
> > + }
> >
> > - } else if (count || has_cache) {
> > -
> > - 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 */
> > + has_cache[i] = count[i] & SWAP_HAS_CACHE;
> > + count[i] &= ~SWAP_HAS_CACHE;
> > + err = 0;
> > +
> > + if (usage == SWAP_HAS_CACHE) {
> > +
> > + /* set SWAP_HAS_CACHE if there is no cache and entry is used */
> > + if (!has_cache[i] && count[i])
> > + has_cache[i] = SWAP_HAS_CACHE;
> > + else if (has_cache[i]) /* someone else added cache */
> > + err = -EEXIST;
> > + else /* no users remaining */
> > + err = -ENOENT;
> > + } else if (count[i] || has_cache[i]) {
> > +
> > + if ((count[i] & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> > + count[i] += usage;
> > + else if ((count[i] & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> > + err = -EINVAL;
> > + else if (swap_count_continued(p, offset + i, count[i]))
> > + count[i] = COUNT_CONTINUED;
> > + else
> > + err = -ENOMEM;
> > + } else
> > + err = -ENOENT; /* unused swap entry */
> >
> > - if (!err)
> > - WRITE_ONCE(p->swap_map[offset], count | has_cache);
> > + if (err)
> > + break;
> > + }
> >
> > + if (!err) {
> > + for (i = 0; i < nr; i++)
> > + WRITE_ONCE(p->swap_map[offset + i], count[i] | has_cache[i]);
> > + }
> > unlock_out:
> > unlock_cluster_or_swap_info(p, ci);
> > return err;
> > }
> >
> > +static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> > +{
> > + return __swap_duplicate_nr(entry, 1, usage);
> > +}
> > +
> > /*
> > * Help swapoff by noting that swap entry belongs to shmem/tmpfs
> > * (in which case its reference count is never incremented).
> > @@ -3417,17 +3427,33 @@ int swapcache_prepare(swp_entry_t entry)
> > return __swap_duplicate(entry, SWAP_HAS_CACHE);
> > }
> >
> > -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +int swapcache_prepare_nr(swp_entry_t entry, int nr)
> > +{
> > + return __swap_duplicate_nr(entry, nr, SWAP_HAS_CACHE);
> > +}
> > +
> > +void swapcache_clear_nr(struct swap_info_struct *si, swp_entry_t entry, int nr)
> > {
> > struct swap_cluster_info *ci;
> > unsigned long offset = swp_offset(entry);
> > - unsigned char usage;
> > + unsigned char usage[SWAPFILE_CLUSTER];
> > + int i;
> >
> > ci = lock_cluster_or_swap_info(si, offset);
> > - usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE);
> > + for (i = 0; i < nr; i++)
> > + usage[i] = __swap_entry_free_locked(si, offset + i, SWAP_HAS_CACHE);
> > unlock_cluster_or_swap_info(si, ci);
> > - if (!usage)
> > - free_swap_slot(entry);
> > + for (i = 0; i < nr; i++) {
> > + if (!usage[i]) {
> > + free_swap_slot(entry);
> > + entry.val++;
> > + }
> > + }
> > +}
>
> This is pretty similar to swap_nr_free() which you added in patch 2. Except
> swap_nr_free() passes 1 as last param to __swap_entry_free_locked() and this
> passes SWAP_HAS_CACHE. Perhaps their should be a common helper? I think
> swap_nr_free()'s usage bitmap is preferable to this version's char array too.
right.
>
> Thanks,
> Ryan
>
> > +
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > + swapcache_clear_nr(si, entry, 1);
> > }
> >
> > struct swap_info_struct *swp_swap_info(swp_entry_t entry)
Thanks
Barry
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