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Message-ID: <CAMgjq7CXcZ4DnPUfPBw8OXYp-zP+Pb_p4KqO0O6kkYvyGLZDYg@mail.gmail.com>
Date: Sat, 3 Aug 2024 20:18:57 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <baohua@...nel.org>
Cc: chrisl@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
"Huang, Ying" <ying.huang@...el.com>, Kalesh Singh <kaleshsingh@...gle.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 6/9] mm: swap: allow cache reclaim to skip slot cache
On Sat, Aug 3, 2024 at 6:39 PM Barry Song <baohua@...nel.org> wrote:
>
> On Wed, Jul 31, 2024 at 2:49 PM <chrisl@...nel.org> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > Currently we free the reclaimed slots through slot cache even
> > if the slot is required to be empty immediately. As a result
> > the reclaim caller will see the slot still occupied even after a
> > successful reclaim, and need to keep reclaiming until slot cache
> > get flushed. This caused ineffective or over reclaim when SWAP is
> > under stress.
> >
> > So introduce a new flag allowing the slot to be emptied bypassing
> > the slot cache.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> > mm/swapfile.c | 152 +++++++++++++++++++++++++++++++++++++++++-----------------
> > 1 file changed, 109 insertions(+), 43 deletions(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 9b63b2262cc2..4c0fc0409d3c 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -53,8 +53,15 @@
> > static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
> > unsigned char);
> > static void free_swap_count_continuations(struct swap_info_struct *);
> > +static void swap_entry_range_free(struct swap_info_struct *si, swp_entry_t entry,
> > + unsigned int nr_pages);
> > static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
> > unsigned int nr_entries);
> > +static bool folio_swapcache_freeable(struct folio *folio);
> > +static struct swap_cluster_info *lock_cluster_or_swap_info(
> > + struct swap_info_struct *si, unsigned long offset);
> > +static void unlock_cluster_or_swap_info(struct swap_info_struct *si,
> > + struct swap_cluster_info *ci);
> >
> > static DEFINE_SPINLOCK(swap_lock);
> > static unsigned int nr_swapfiles;
> > @@ -129,8 +136,25 @@ static inline unsigned char swap_count(unsigned char ent)
> > * corresponding page
> > */
> > #define TTRS_UNMAPPED 0x2
> > -/* Reclaim the swap entry if swap is getting full*/
> > +/* Reclaim the swap entry if swap is getting full */
> > #define TTRS_FULL 0x4
> > +/* Reclaim directly, bypass the slot cache and don't touch device lock */
> > +#define TTRS_DIRECT 0x8
> > +
> > +static bool swap_is_has_cache(struct swap_info_struct *si,
> > + unsigned long offset, int nr_pages)
> > +{
> > + unsigned char *map = si->swap_map + offset;
> > + unsigned char *map_end = map + nr_pages;
> > +
> > + do {
> > + VM_BUG_ON(!(*map & SWAP_HAS_CACHE));
> > + if (*map != SWAP_HAS_CACHE)
> > + return false;
> > + } while (++map < map_end);
> > +
> > + return true;
> > +}
> >
> > /*
> > * returns number of pages in the folio that backs the swap entry. If positive,
> > @@ -141,12 +165,22 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > unsigned long offset, unsigned long flags)
> > {
> > swp_entry_t entry = swp_entry(si->type, offset);
> > + struct address_space *address_space = swap_address_space(entry);
> > + struct swap_cluster_info *ci;
> > struct folio *folio;
> > - int ret = 0;
> > + int ret, nr_pages;
> > + bool need_reclaim;
> >
> > - folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
> > + folio = filemap_get_folio(address_space, swap_cache_index(entry));
> > if (IS_ERR(folio))
> > return 0;
> > +
> > + /* offset could point to the middle of a large folio */
> > + entry = folio->swap;
> > + offset = swp_offset(entry);
> > + nr_pages = folio_nr_pages(folio);
> > + ret = -nr_pages;
> > +
> > /*
> > * When this function is called from scan_swap_map_slots() and it's
> > * called by vmscan.c at reclaiming folios. So we hold a folio lock
> > @@ -154,14 +188,50 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > * case and you should use folio_free_swap() with explicit folio_lock()
> > * in usual operations.
> > */
> > - if (folio_trylock(folio)) {
> > - if ((flags & TTRS_ANYWAY) ||
> > - ((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
> > - ((flags & TTRS_FULL) && mem_cgroup_swap_full(folio)))
> > - ret = folio_free_swap(folio);
> > - folio_unlock(folio);
> > + if (!folio_trylock(folio))
> > + goto out;
> > +
> > + need_reclaim = ((flags & TTRS_ANYWAY) ||
> > + ((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
> > + ((flags & TTRS_FULL) && mem_cgroup_swap_full(folio)));
> > + if (!need_reclaim || !folio_swapcache_freeable(folio))
> > + goto out_unlock;
> > +
> > + /*
> > + * It's safe to delete the folio from swap cache only if the folio's
> > + * swap_map is HAS_CACHE only, which means the slots have no page table
> > + * reference or pending writeback, and can't be allocated to others.
> > + */
> > + ci = lock_cluster_or_swap_info(si, offset);
> > + need_reclaim = swap_is_has_cache(si, offset, nr_pages);
> > + unlock_cluster_or_swap_info(si, ci);
> > + if (!need_reclaim)
> > + goto out_unlock;
> > +
> > + if (!(flags & TTRS_DIRECT)) {
> > + /* Free through slot cache */
> > + delete_from_swap_cache(folio);
> > + folio_set_dirty(folio);
> > + ret = nr_pages;
> > + goto out_unlock;
> > }
> > - ret = ret ? folio_nr_pages(folio) : -folio_nr_pages(folio);
> > +
> > + xa_lock_irq(&address_space->i_pages);
> > + __delete_from_swap_cache(folio, entry, NULL);
> > + xa_unlock_irq(&address_space->i_pages);
> > + folio_ref_sub(folio, nr_pages);
> > + folio_set_dirty(folio);
> > +
> > + spin_lock(&si->lock);
> > + /* Only sinple page folio can be backed by zswap */
> > + if (!nr_pages)
> > + zswap_invalidate(entry);
>
> I am trying to figure out if I am mad :-) Does nr_pages == 0 means single
> page folio?
>
Hi Barry
I'm sorry, this should be nr_pages == 1, I messed up order and nr, as
zswap only works for single page folios.
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