From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 434C22DC789; Tue, 13 Oct 2026 18:15:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791915349; cv=none; b=Nz3Ncqko10u2udXz3F7PU1S3ExKev6JkC4/ICHZAafEWH9NcsEJ0LQq4VCTaNERswhSZPTZMzCorJOb1r5uRtP9pnyCbf6HOCgmkVJi6A11tE9BP+DwHWoKi1Io0GjpixiKKSxmgdX+rf2sFvjrCqmg9Hnu70173jtOMgjworuI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791915349; c=relaxed/simple; bh=Qv1CvT4tZ96DIlwANVX9XMFIJ4hcjcRRr+xrn/F0hhc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=b1IoQQP+lBoiJSJ+QdgU90EsYLktIin8OIsHDNY59dTVQDg4taH8P1LUskuKiPlSZ2jeVK2Ao599aPjDbXCQnqMhXNkrdI+iul8Xd5uOXJgGffKTSzhUMBVbPbXx5G327JNsxhgrA8uzQfeejWp9VuN5duAKimRNkkH9b50kJT0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=c8L5vrQe; 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="c8L5vrQe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AFDE11F000FF; Tue, 13 Oct 2026 18:15:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791915347; bh=7eFeDKak6G7QTjlikd0utMlJUAH18N9G3C9/xQ9npw4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=c8L5vrQeu1NmmlEVpcK56QAFDFhRdM74zac3J76d3JD5avcFrlJ00RHf0Vl9jjQ38 I4cvg6Q0xuE+R2BxeJ6pZFpQAzgGejak0ZZK9B6dcUP+gbTpuCNUTR1+GPGo3U0f2X jhh+qlHgIZJ5UYZrTdBefc/TL2GWdGQvhnpsO49LmpgtWV7O7lyg9SyNbQ52Z1XRIG amPmILGpJerZa7hqiPyW/5cEtBVUG3xbnZR7+NECMTJv2bswIOdNP7MOn8QkHfEFr2 9wCKs4S0BgKwfXuf50OptGV7Ke3J7zcIovRwUvpmlaEr16tTTXD2I+SKL5I4ycdTyK KqEwGxlDlUjLw== Message-ID: <1f765ae3-434c-4e65-b749-c1e71cce7f33@kernel.org> Date: Tue, 13 Oct 2026 20:15:42 +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 , Andrew Morton Cc: Johannes Weiner , David Hildenbrand , Harry Yoo , Usama Arif , 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> <15a9e03e-c748-4e51-b8cc-583a3610c897@kernel.org> <20261012192834.6934ef85444dee4a60d6d876@linux-foundation.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/13/26 19:37, Kiryl Shutsemau wrote: > On Mon, Oct 12, 2026 at 07:28:34PM -0700, Andrew Morton wrote: >> On Mon, 12 Oct 2026 13:40:36 +0200 "Vlastimil Babka (SUSE)" wrote: >> >> > > 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) ? >> >> I'm concerned about the sheer size of this patch. It fixes significant >> issues, but it's not going to be a simple or safe backport. >> >> So for now please let's optimize for simplicity and backportability and >> defer cleanups for the -rc cycle. >> >> Of course, if the changes you suggest achieve that then go for it. > > They do. The patch with Vlastimil's suggestion is below. It is shorter. > Behaviour is unchanged; the reproducer gives the same results on both > versions. > > Vlastimil, does it look like what you had in mind? Yeah LGTM. Reviewed-by: Vlastimil Babka (SUSE) > >> Perhaps we should defer this change into next -rc? It has cc:stable so >> it'll eventually end up where we want it. > > Do you mean the next -rc cycle? I hoped to get it into 7.4-rc1, as we > agreed earlier. We're fixing a 7.3 commit that went into the stable trees. Too late to rush for the 7.3 final though. The impact is mitigated by defrag_mode not being default, so it's fine to fix it in 7.3.x. Thus 7.4-rc1 would be great, rc2 could also work, but wouldn't delay more. I read the merge window might be 3 weeks this time [1] so perhaps rc1 is comfortable in any case. [1] https://lwn.net/Articles/1099594 > > ----------------------------------->8---------------------------------- > > From: "Kiryl Shutsemau (Meta)" > Subject: [PATCH v4] mm: page_alloc: make defrag_mode retries follow the promoted order > > 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 keeps it in alloc_context for direct reclaim, direct compaction and > the two retry helpers, so no callee has to recompute it. Harry Yoo asked > for the promoted order to be explicit rather than derived in each > callee, and Vlastimil Babka suggested alloc_context over new function > parameters. > > 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) > --- > v4: > - keep the promoted order in alloc_context instead of passing it as a > new parameter (Vlastimil Babka) > > mm/page_alloc.c | 65 +++++++++++++++++++++++++++++-------------------- > mm/page_alloc.h | 2 ++ > 2 files changed, 41 insertions(+), 26 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 12fac9084c48..c593913a14f8 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4127,6 +4127,12 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, > return page; > } > > +static inline bool order_nofrag_promoted(unsigned int order, > + const struct alloc_context *ac) > +{ > + return ac->reclaim_compact_order != order; > +} > + > /* > * Maximum number of compaction retries with a progress before OOM > * killer is consider as the only way to move forward. > @@ -4149,22 +4155,7 @@ __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); > + int compact_order = ac->reclaim_compact_order; > > if (!compact_order) > return NULL; > @@ -4257,7 +4248,7 @@ 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 (!ac->reclaim_compact_order) > return false; > > if (fatal_signal_pending(current)) > @@ -4266,10 +4257,12 @@ 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 fall back instead. > */ > if (compact_result == COMPACT_SKIPPED) { > - ret = compaction_zonelist_suitable(ac, order, alloc_flags, > - gfp_mask); > + if (!order_nofrag_promoted(order, ac)) > + ret = compaction_zonelist_suitable(ac, order, > + alloc_flags, gfp_mask); > goto out; > } > > @@ -4468,11 +4461,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order, > 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); > + int reclaim_order = ac->reclaim_compact_order; > > psi_memstall_enter(&pflags); > *did_some_progress = __perform_reclaim(gfp_mask, reclaim_order, ac); > @@ -4648,9 +4637,12 @@ 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 > + * promoted requests: their reclaim reports progress when it backs off > + * for compaction without freeing anything. > */ > - if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER) > + if (did_some_progress && order <= PAGE_ALLOC_COSTLY_ORDER && > + !order_nofrag_promoted(order, ac)) > *no_progress_loops = 0; > else > (*no_progress_loops)++; > @@ -4934,6 +4926,22 @@ __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); > > + /* > + * 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. > + */ > + ac->reclaim_compact_order = order; > + if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE) > + ac->reclaim_compact_order = max(order, pageblock_order); > + > /* Try direct reclaim and then allocating */ > if (!compact_first) { > page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, > @@ -5015,6 +5023,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > /* Reclaim/compaction failed to prevent the fallback */ > if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT)) { > alloc_flags &= ~ALLOC_NOFRAGMENT; > + /* Retries went into making blocks, start the fallback afresh */ > + if (order_nofrag_promoted(order, ac)) { > + no_progress_loops = 0; > + compaction_retries = 0; > + } > goto retry; > } > > diff --git a/mm/page_alloc.h b/mm/page_alloc.h > index b9259deddb59..2b1017a91928 100644 > --- a/mm/page_alloc.h > +++ b/mm/page_alloc.h > @@ -93,6 +93,8 @@ struct alloc_context { > bool spread_dirty_pages; > /* Only flags that are global to the whole allocation go here. */ > unsigned int alloc_flags; > + /* Set by __alloc_pages_slowpath() before reclaim and compaction */ > + unsigned int reclaim_compact_order; > }; > > /*