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 Zp9wJ4cD 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 04, 2026 at 10:58:58AM +0100, Vlastimil Babka wrote: > On 3/4/26 4:05 AM, Harry Yoo wrote: > > On Mon, Mar 02, 2026 at 10:55:37AM +0100, Vlastimil Babka (SUSE) wrote: > >> Ming Lei reported [1] a regression in the ublk null target benchmark due > >> to sheaves. The profile shows that the alloc_from_pcs() fastpath fails > >> and allocations fall back to ___slab_alloc(). It also shows the > >> allocations happen through mempool_alloc(). > >> > >> The strategy of mempool_alloc() is to call the underlying allocator > >> (here slab) without __GFP_DIRECT_RECLAIM first. This does not play well > >> with __pcs_replace_empty_main() checking for gfpflags_allow_blocking() > >> to decide if it should refill an empty sheaf or fallback to the > >> slowpath, so we end up falling back. > >> > >> We could change the mempool strategy but there might be other paths > >> doing the same ting. So instead allow sheaf refill when blocking is not > >> allowed, changing the condition to gfpflags_allow_spinning(). The > >> original condition was unnecessarily restrictive. > >> > >> Note this doesn't fully resolve the regression [1] as another component > >> of that are memoryless nodes, which is to be addressed separately. > >> > >> Reported-by: Ming Lei > >> Fixes: e47c897a2949 ("slab: add sheaves to most caches") > >> Link: https://lore.kernel.org/all/aZ0SbIqaIkwoW2mB@fedora/ > >> Signed-off-by: Vlastimil Babka (SUSE) > >> --- > >> mm/slub.c | 21 +++++++++------------ > >> 1 file changed, 9 insertions(+), 12 deletions(-) > >> > >> diff --git a/mm/slub.c b/mm/slub.c > >> index b1e9f16ba435..17b200695e9b 100644 > >> --- a/mm/slub.c > >> +++ b/mm/slub.c > >> @@ -4632,11 +4631,8 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs, > >> if (!full) > >> return NULL; > >> > >> - /* > >> - * we can reach here only when gfpflags_allow_blocking > >> - * so this must not be an irq > >> - */ > >> - local_lock(&s->cpu_sheaves->lock); > >> + if (!local_trylock(&s->cpu_sheaves->lock)) > >> + goto barn_put; > > > > My AI buddy says (don't worry, I filtered it): > > | When local_trylock() fails above, the function jumps to barn_put and returns > > | pcs without holding the lock. This appears to violate the function's contract > > | documented in the comment at the beginning of __pcs_replace_empty_main(): > > | > > | "If not successful, returns NULL and the local lock unlocked." > > | > > | The caller in alloc_from_pcs() checks for NULL to detect failure: > > | > > | if (unlikely(pcs->main->size == 0)) { > > | pcs = __pcs_replace_empty_main(s, pcs, gfp); > > | if (unlikely(!pcs)) > > | return NULL; > > | } > > | > > | If the trylock fails and pcs (non-NULL) is returned, the caller proceeds > > | without realizing the lock was never re-acquired. This leads to accessing > > | pcs->main without the lock and later trying to unlock a lock that isn't held. > > > > And the analysis sounds correct to me. > > > > perhaps it should be: > > > > if (!local_trylock(&s->cpu_sheaves->lock)) { > > pcs = NULL; > > goto barn_put; > > } > > Thanks a lot Harry. In fact I realized this mistake after initially > sending the patch to Ming in a reply, and fixed it locally (same as you > suggest). > Or so I thought, because the fix got apparently lost. That happens sometimes, yeah :) > So I'll do that now in slab/for-next-fixes Thanks. > Or actually I think a more robust way is to set pcs = NULL after the > unlock, unconditionally, so I'll do that. Oh, that sounds better! > >> pcs = this_cpu_ptr(s->cpu_sheaves); > >> > >> /* > >> @@ -4667,6 +4663,7 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs, > >> return pcs; > >> } > >> > >> +barn_put: > >> barn_put_full_sheaf(barn, full); > >> stat(s, BARN_PUT); -- Cheers, Harry / Hyeonggon