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U2FsdGVkX19qXmqX3IjJpIyiWUGXw9Q+6TmPQtbe0CHb3SY+phc+oz2tzlmL56KVW/2OdsbXke+Is6XL/N7e8a6w3K2k07wNI6gPrqPIR++VVYN9lW1m5gWGIdfoyadEfmMu6e5TLO/gPDmFmPpJ5VHgY6z/CGa+/vU1urQOsE/Lghp8BOrcJV8SyJq6pIcrDd3ZDwrgEJm856271/p/sbkCISoifn+pjq9DgDuvxhFsKILmJPgyOmk15xNmtF+La6HDOZdVN839HAKninCcYfjN2XHq8FNDMflk57BsKHtqF6hypXI14AYQ5JIbc+poGnH0gZZVs19pBGDNvV0NLMUWghO62LszAog0G+iPONReXdRmyjVKI+xXAAxfVm65brFbvI0B2W+aCjfuvl1wKHtrkY/oYknw/CO4pOp1mdkDb7lT1bl8HTN9JL/8T39+0Y692H4+3xfqYVYu4kwwmXoeWiBAr56NFGHqLUm4Ip2BrMytbhS494EdX0yrHDwXGz9xMJCF0jjadYUr0yQC5xiG3wyjhQic94+9DVWdNaFpm4jPawK3AyQXDUJHs2gXEe7FuoQIwsBMm1QSy0BPe+u45s123esJuQTTWvIfs+cTsci3zyigF9qMBWe2c+v+lN3CiEtY1NhX/siwNruv07sn2kkbrdGie6/FtHWsgG3Q/fAnaQlDc0mMgIrh3vyX/MtA7DRqTdVMTnWI9+2o5Q4MXel4FMCeFOXcB2FdX6XFRpUbTZZLhmKfOXJ+/f2FIwqWUkNP1cuNHHa94TBki8LJzyYlMfzWwITJZ7zvN6+kcxGPZv7Sou41+p5t4BN29UTtspcW8QyBipS4k/nT8TFTGUNIPezjRKgc5G9SlXUtOz1jX7D0gyOIRFfwq/5ARkV26LFDOvuqQWJ3WwWw/rmjMVvSL5OEElqYpUa2YqaQ4dcVPYXvAmo0Dv8gqKjhXfEHZfGD3jf0TgAinjv mQxUjLK6 k0e4MhNmyU0t/smjJcgvT1hxmxPL2nKmbK3FP6cJbR0lqZXDzZf05WUR5kJeMEBcGRIyqz2/AbNF5E9nWpRpNlbIFnnReL7qQylxiGmSJPOJImB1hIIEzqUNUt7L9o3G6hnnwlroX0U5ZwMAbpTZM+2RMWNmk5zUaAb2q82mXjZWb2H/IZ2kqfewx7c0rT93ricjzqwRRSA75qRg64VPzYDprHqwz5QFjjMW4gN/ADEFTeUCX/JU2pA/vcOZOtlAoWWT/9SmTRDzrpBBIP/s8ZVYOQzP+3rdgmL7bvvF43k/82Id8hMOe0zZkcL8n/212li26f+xVepawFJvqNONd3LWuIiezzyA54EYI6upFxCeixhA9wvTvQHVcq3x/g2d1LLAYu4NpidU3hSP2rcjV9PuBW4XjfZLswysC6G37daJik9PCjBI/B3aMR9T4P92+gmuubGoYXpdOGnLbnUdTlVcfbZ6RtugC1S0SucDXfD4urFw1k+N9H+3YW6zJuWeoK/0TzcgGB0aVH2SOE5vqGXZumo2EVbNGgYBQPQSaOwm2iE1atE/MslUHEZrY86i28VW/To+7rtvQ/8DIWIIr0NV5tvXvvXmukjcyNCjjLl0qAlUut/FVpN1QfSJzPOfry0y2Dqgnd2na+IDPwA1pfr8FHHtGS0PrOS8GwReYVGIwSrLyHSAXw/iBjUQ63SyV6TnyVVeBlv1PU10= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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; } > 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