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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VK8MeTSi; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="VK8MeTSi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 628A11F0089B; Mon, 12 Oct 2026 11:40:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791805241; bh=Oh9Ei+qP6U9FrDZiNh4NKHoeoiIreiapZDZWAguR3tw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=VK8MeTSiVkOV0o/3FjcKoSYGTT+rE03j7mwWNG0G4Ap/R/z3Ly9WStkcEqoB6GG0G xx3+hJPa3GScGXQTl5ji+FaPoedo4wGox6GweyXM7OUPLiZhGsOoCdjD2bsA7jyigV Chcdd4Kh3nRr1CbAYIfh1DVwF441A510AXwpBr6XhoYSASizNIk8kC2tWX8cMyynXL b/apNThdNaCduP8mJUHTExMfzE6Vd/qbzUnezLGDAfltGYJh5OJcLv8nSENHWf1h7f IKIrPTExExZTlm0vevTO0vTDAnpoLfj7MGaN7HMmi1rj0yO2Q97VE/DdyxkgIzQ5Bt 4e9so2yv5ysIA== Message-ID: <15a9e03e-c748-4e51-b8cc-583a3610c897@kernel.org> Date: Mon, 12 Oct 2026 13:40:36 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm: page_alloc: make defrag_mode retries follow the promoted order Content-Language: en-US To: "Kiryl Shutsemau (Meta)" , Andrew Morton , Johannes Weiner , David Hildenbrand , Harry Yoo , Usama Arif Cc: Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Zi Yan , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@meta.com References: <20261009125503.126202-1-kas@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/9/26 14:55, Kiryl Shutsemau (Meta) wrote: > Since commit 7e8756d7ad22 ("mm: page_alloc: fix non-movable reclaim > storm in defrag_mode"), direct reclaim and compaction for non-movable > requests under defrag_mode run at pageblock_order, to produce the whole > blocks that ALLOC_NOFRAGMENT needs. The retry decisions that follow > still use the request order. An order-0 request can therefore retry > indefinitely without ever reaching the ALLOC_NOFRAGMENT fallback: > > - Reclaim at pageblock_order gives up after one pass as soon as a zone > looks compaction_ready(), and do_try_to_free_pages() then returns 1 > even though nothing was reclaimed. It returns before the retry that > would reclaim memory.low-protected cgroups, so when most memory is > protected, the pass that did run finds next to nothing. > > - Compaction at pageblock_order fails or is deferred. > > - should_reclaim_retry() takes the reported progress as progress for > the order-0 request and resets no_progress_loops. The request > retries. > > Order 1-3 requests loop the same way, and should_compact_retry() also > checks their pageblock_order compaction result against the request > order. > > On a production host (64G, defrag_mode, memory.low covering most of the > workload), 95% of direct reclaim runs were order-9 runs that returned 1 > with nothing reclaimed, at up to 60k runs per second. Across ~200M > should_reclaim_retry() calls in a day, no_progress_loops never left 0. > The spinning allocations were SLUB slab refills for inode and dentry > caches. The time spent registers as memory pressure, and pressure-based > OOM killing takes down both workloads and system services. > > For promoted requests: > > - Reclaim progress does not reset no_progress_loops, as for costly > orders. > > - should_compact_retry() checks the compaction result at the promoted > order and does not retry COMPACT_SKIPPED, since the request can fall > back. The compaction priority floor and the COMPACT_SUCCESS retry > limit stay those of the request order, so a non-costly request still > gets its COMPACT_PRIO_SYNC_FULL pass before it falls back. > > When a promoted request takes the fallback, reset the retry counters, > so that the fallback attempt gets a full retry budget before the OOM > killer is considered. A request whose reclaim was not promoted spent > its budget on the request itself and keeps its counters, as Usama Arif > pointed out. > > __alloc_pages_slowpath() computes the promoted order once per iteration > and passes it to direct reclaim, direct compaction and the two retry > helpers next to the request order, so no callee has to recompute it. > Harry Yoo asked for the two orders to be explicit rather than derived > in each callee. > > In a VM reproducer (32G, defrag_mode, inode churn under memory.low): > > before after > should_reclaim_retry() calls 63M 293k > peak memory pressure (PSI some avg10) 99% 12% > > File creation runs 5.7x faster. > > Fixes: 7e8756d7ad22 ("mm: page_alloc: fix non-movable reclaim storm in defrag_mode") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Kiryl Shutsemau (Meta) LGTM, but I think we could simplify and get rid of all the function parameter additions by adding a field to alloc_context instead? We could call it e.g. reclaim_compact_order. The function nofrag_promote_order() probably doesn't need to exist, given it has just one caller, could be just open coded. What could be useful instead is wrapper for comparing order vs ac->reclaim_compact_order. E.g. something like bool order_nofrag_promoted(order, ac) ? Vlastimil > --- > v3: > - reset the retry counters at the fallback only for requests whose > reclaim was promoted; a movable request already spent its budget on > itself (Usama Arif) > > v2: > - keep the compaction priority floor and the COMPACT_SUCCESS retry > limit on the request order, so non-costly requests still escalate to > SYNC_FULL before falling back (Harry Yoo, Johannes Weiner) > - compute the promoted order once in __alloc_pages_slowpath() and pass > it to direct reclaim, direct compaction and the retry helpers, instead > of each of them deriving it (Harry Yoo) > > mm/page_alloc.c | 126 +++++++++++++++++++++++++++++++----------------- > 1 file changed, 81 insertions(+), 45 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 12fac9084c48..c0be88d2c3ad 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4127,6 +4127,32 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, > return page; > } > > +/* > + * If fallbacks are not permitted (defrag_mode), we either need to > + * reclaim space in a block of matching type, or clear out an entire > + * block to allow __rmqueue_claim() to convert. > + * > + * Reclaim by itself is primarily freeing space in movable blocks, > + * since that's where the LRU pages live. So this works for movable > + * requests, but not for others. > + * > + * For those, promote the order of reclaim and compaction to help make > + * blocks, instead of spinning in reclaim alone unproductively. Retry > + * decisions based on the outcome of that work - reclaim progress and > + * compaction results - must account for the promotion as well, so > + * __alloc_pages_slowpath() computes the promoted order once and passes > + * it alongside the request order. > + */ > +static inline unsigned int nofrag_promote_order(unsigned int order, > + unsigned int alloc_flags, > + const struct alloc_context *ac) > +{ > + if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE) > + return max(order, pageblock_order); > + > + return order; > +} > + > /* > * Maximum number of compaction retries with a progress before OOM > * killer is consider as the only way to move forward. > @@ -4137,8 +4163,9 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, > /* Try memory compaction for high-order allocations before reclaim */ > static struct page * > __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > - unsigned int alloc_flags, const struct alloc_context *ac, > - enum compact_priority prio, enum compact_result *compact_result) > + unsigned int compact_order, unsigned int alloc_flags, > + const struct alloc_context *ac, enum compact_priority prio, > + enum compact_result *compact_result) > { > struct page *page = NULL; > unsigned long pflags; > @@ -4149,22 +4176,6 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > .order = order, > .page = NULL, > }; > - int compact_order = order; > - > - /* > - * If fallbacks are not permitted (defrag_mode), we either > - * need to reclaim space in a block of matching type, or clear > - * out an entire block to allow __rmqueue_claim() to convert. > - * > - * Reclaim by itself is primarily freeing space in movable > - * blocks, since that's where the LRU pages live. So this > - * works for movable requests, but not for others. > - * > - * For those, promote the order to help make blocks, instead > - * of spinning in reclaim alone unproductively. > - */ > - if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE) > - compact_order = max(order, pageblock_order); > > if (!compact_order) > return NULL; > @@ -4246,7 +4257,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > > static inline bool > should_compact_retry(gfp_t gfp_mask, struct alloc_context *ac, int order, > - int alloc_flags, > + int compact_order, int alloc_flags, > enum compact_result compact_result, > enum compact_priority *compact_priority, > int *compaction_retries) > @@ -4257,7 +4268,7 @@ should_compact_retry(gfp_t gfp_mask, struct alloc_context *ac, int order, > int retries = *compaction_retries; > enum compact_priority priority = *compact_priority; > > - if (!order) > + if (!compact_order) > return false; > > if (fatal_signal_pending(current)) > @@ -4266,10 +4277,14 @@ should_compact_retry(gfp_t gfp_mask, struct alloc_context *ac, int order, > /* > * Compaction was skipped due to a lack of free order-0 > * migration targets. Continue if reclaim can help. > + * > + * Promoted requests have exhausted their reclaim retries at > + * this point, and they can fall back instead. > */ > if (compact_result == COMPACT_SKIPPED) { > - ret = compaction_zonelist_suitable(ac, order, alloc_flags, > - gfp_mask); > + if (compact_order == order) > + ret = compaction_zonelist_suitable(ac, order, alloc_flags, > + gfp_mask); > goto out; > } > > @@ -4314,8 +4329,9 @@ should_compact_retry(gfp_t gfp_mask, struct alloc_context *ac, int order, > #else > static inline struct page * > __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > - unsigned int alloc_flags, const struct alloc_context *ac, > - enum compact_priority prio, enum compact_result *compact_result) > + unsigned int compact_order, unsigned int alloc_flags, > + const struct alloc_context *ac, enum compact_priority prio, > + enum compact_result *compact_result) > { > *compact_result = COMPACT_SKIPPED; > return NULL; > @@ -4323,7 +4339,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > > static inline bool > should_compact_retry(gfp_t gfp_mask, struct alloc_context *ac, int order, > - int alloc_flags, > + int compact_order, int alloc_flags, > enum compact_result compact_result, > enum compact_priority *compact_priority, > int *compaction_retries) > @@ -4462,17 +4478,12 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order, > /* The really slow allocator path where we enter direct reclaim */ > static inline struct page * > __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order, > - unsigned int alloc_flags, const struct alloc_context *ac, > - unsigned long *did_some_progress) > + unsigned int reclaim_order, unsigned int alloc_flags, > + const struct alloc_context *ac, unsigned long *did_some_progress) > { > struct page *page = NULL; > unsigned long pflags; > bool drained = false; > - int reclaim_order = order; > - > - /* Match the slowpath compaction promotion in __alloc_pages_direct_compact */ > - if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE) > - reclaim_order = max(order, pageblock_order); > > psi_memstall_enter(&pflags); > *did_some_progress = __perform_reclaim(gfp_mask, reclaim_order, ac); > @@ -4638,8 +4649,9 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask) > */ > static inline bool > should_reclaim_retry(gfp_t gfp_mask, unsigned order, > - struct alloc_context *ac, int alloc_flags, > - bool did_some_progress, int *no_progress_loops) > + unsigned int reclaim_order, struct alloc_context *ac, > + int alloc_flags, bool did_some_progress, > + int *no_progress_loops) > { > struct zone *zone; > struct zoneref *z; > @@ -4648,9 +4660,17 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order, > /* > * Costly allocations might have made a progress but this doesn't mean > * their order will become available due to high fragmentation so > - * always increment the no progress counter for them > + * always increment the no progress counter for them. > + * > + * The same goes for requests whose reclaim is promoted to make whole > + * blocks. At that order, reclaim also reports progress when it backs > + * off for compaction without freeing anything. > + * > + * The watermark check below stays at the request order: it asks > + * whether the request itself could succeed after reclaim. > */ > - if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) > + if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER && > + reclaim_order == order) > *no_progress_loops = 0; > else > (*no_progress_loops)++; > @@ -4790,6 +4810,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER; > struct page *page = NULL; > unsigned int alloc_flags; > + unsigned int reclaim_order; > unsigned long did_some_progress; > enum compact_priority compact_priority; > enum compact_result compact_result; > @@ -4934,17 +4955,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > /* If allocation has taken excessively long, warn about it */ > check_alloc_stall_warn(gfp_mask, ac->nodemask, order, alloc_start_time); > > + /* The order reclaim and compaction work at, see nofrag_promote_order() */ > + reclaim_order = nofrag_promote_order(order, alloc_flags, ac); > + > /* Try direct reclaim and then allocating */ > if (!compact_first) { > - page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, > - ac, &did_some_progress); > + page = __alloc_pages_direct_reclaim(gfp_mask, order, reclaim_order, > + alloc_flags, ac, > + &did_some_progress); > if (page) > goto got_pg; > } > > /* Try direct compaction and then allocating */ > - page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags, ac, > - compact_priority, &compact_result); > + page = __alloc_pages_direct_compact(gfp_mask, order, reclaim_order, > + alloc_flags, ac, compact_priority, > + &compact_result); > if (page) > goto got_pg; > > @@ -4996,8 +5022,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > check_retry_zonelist(zonelist_iter_cookie)) > goto restart; > > - if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags, > - did_some_progress > 0, &no_progress_loops)) > + if (should_reclaim_retry(gfp_mask, order, reclaim_order, ac, > + alloc_flags, did_some_progress > 0, > + &no_progress_loops)) > goto retry; > > /* > @@ -5007,14 +5034,23 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > * of free memory (see __compaction_suitable) > */ > if (did_some_progress > 0 && can_compact && > - should_compact_retry(gfp_mask, ac, order, alloc_flags, > - compact_result, &compact_priority, > + should_compact_retry(gfp_mask, ac, order, reclaim_order, > + alloc_flags, compact_result, &compact_priority, > &compaction_retries)) > goto retry; > > - /* Reclaim/compaction failed to prevent the fallback */ > + /* > + * Reclaim/compaction failed to prevent the fallback. A promoted > + * request spent its retry budget on making blocks, not on the > + * request itself; give its fallback a fresh one before > + * considering OOM. > + */ > if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT)) { > alloc_flags &= ~ALLOC_NOFRAGMENT; > + if (reclaim_order != order) { > + no_progress_loops = 0; > + compaction_retries = 0; > + } > goto retry; > } >