The patch titled Subject: cachestat: do not flush stats in recency check has been added to the -mm mm-hotfixes-unstable branch. Its filename is cachestat-do-not-flush-stats-in-recency-check.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Nhat Pham nphamcs@gmail.com Subject: cachestat: do not flush stats in recency check Date: Thu, 27 Jun 2024 13:17:37 -0700

syzbot detects that cachestat() is flushing stats, which can sleep, in its RCU read section (see [1]). This is done in the workingset_test_recent() step (which checks if the folio's eviction is recent).

Move the stat flushing step to before the RCU read section of cachestat, and skip stat flushing during the recency check.

[1]: https://lore.kernel.org/cgroups/000000000000f71227061bdf97e0@google.com/

Link: https://lkml.kernel.org/r/20240627201737.3506959-1-nphamcs@gmail.com Fixes: b00684722262 ("mm: workingset: move the stats flush into workingset_test_recent()") Signed-off-by: Nhat Pham nphamcs@gmail.com Reported-by: syzbot+b7f13b2d0cc156edf61a@syzkaller.appspotmail.com Closes: https://lore.kernel.org/cgroups/000000000000f71227061bdf97e0@google.com/ Debugged-by: Johannes Weiner hannes@cmpxchg.org Suggested-by: Johannes Weiner hannes@cmpxchg.org Cc: Al Viro viro@zeniv.linux.org.uk Cc: David Hildenbrand david@redhat.com Cc: "Huang, Ying" ying.huang@intel.com Cc: Kairui Song kasong@tencent.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Ryan Roberts ryan.roberts@arm.com Cc: Shakeel Butt shakeel.butt@linux.dev Cc: Yosry Ahmed yosryahmed@google.com Cc: stable@vger.kernel.org [6.8+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/swap.h | 3 ++- mm/filemap.c | 5 ++++- mm/workingset.c | 14 +++++++++++--- 3 files changed, 17 insertions(+), 5 deletions(-)

--- a/include/linux/swap.h~cachestat-do-not-flush-stats-in-recency-check +++ a/include/linux/swap.h @@ -354,7 +354,8 @@ static inline swp_entry_t page_swap_entr }

/* linux/mm/workingset.c */ -bool workingset_test_recent(void *shadow, bool file, bool *workingset); +bool workingset_test_recent(void *shadow, bool file, bool *workingset, + bool flush); void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages); void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg); void workingset_refault(struct folio *folio, void *shadow); --- a/mm/filemap.c~cachestat-do-not-flush-stats-in-recency-check +++ a/mm/filemap.c @@ -4248,6 +4248,9 @@ static void filemap_cachestat(struct add XA_STATE(xas, &mapping->i_pages, first_index); struct folio *folio;

+ /* Flush stats (and potentially sleep) outside the RCU read section. */ + mem_cgroup_flush_stats_ratelimited(NULL); + rcu_read_lock(); xas_for_each(&xas, folio, last_index) { int order; @@ -4311,7 +4314,7 @@ static void filemap_cachestat(struct add goto resched; } #endif - if (workingset_test_recent(shadow, true, &workingset)) + if (workingset_test_recent(shadow, true, &workingset, false)) cs->nr_recently_evicted += nr_pages;

goto resched; --- a/mm/workingset.c~cachestat-do-not-flush-stats-in-recency-check +++ a/mm/workingset.c @@ -412,10 +412,12 @@ void *workingset_eviction(struct folio * * @file: whether the corresponding folio is from the file lru. * @workingset: where the workingset value unpacked from shadow should * be stored. + * @flush: whether to flush cgroup rstat. * * Return: true if the shadow is for a recently evicted folio; false otherwise. */ -bool workingset_test_recent(void *shadow, bool file, bool *workingset) +bool workingset_test_recent(void *shadow, bool file, bool *workingset, + bool flush) { struct mem_cgroup *eviction_memcg; struct lruvec *eviction_lruvec; @@ -467,10 +469,16 @@ bool workingset_test_recent(void *shadow

/* * Flush stats (and potentially sleep) outside the RCU read section. + * + * Note that workingset_test_recent() itself might be called in RCU read + * section (for e.g, in cachestat) - these callers need to skip flushing + * stats (via the flush argument). + * * XXX: With per-memcg flushing and thresholding, is ratelimiting * still needed here? */ - mem_cgroup_flush_stats_ratelimited(eviction_memcg); + if (flush) + mem_cgroup_flush_stats_ratelimited(eviction_memcg);

eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat); refault = atomic_long_read(&eviction_lruvec->nonresident_age); @@ -558,7 +566,7 @@ void workingset_refault(struct folio *fo

mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);

- if (!workingset_test_recent(shadow, file, &workingset)) + if (!workingset_test_recent(shadow, file, &workingset, true)) return;

folio_set_active(folio); _

Patches currently in -mm which might be from nphamcs@gmail.com are

cachestat-do-not-flush-stats-in-recency-check.patch