linux-kernel - Re: [PATCH RFC 1/2] mm, page_alloc, thp: prevent reclaim for __GFP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aUGITMXjCWdCA22R@tiehlicka>
Date: Tue, 16 Dec 2025 17:26:52 +0100
From: Michal Hocko <mhocko@...e.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Brendan Jackman <jackmanb@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
	David Rientjes <rientjes@...gle.com>,
	David Hildenbrand <david@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Mike Rapoport <rppt@...nel.org>,
	Joshua Hahn <joshua.hahnjy@...il.com>,
	Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] mm, page_alloc, thp: prevent reclaim for
 __GFP_THISNODE THP allocations

On Tue 16-12-25 16:54:21, Vlastimil Babka wrote:
> Since commit cc638f329ef6 ("mm, thp: tweak reclaim/compaction effort of
> local-only and all-node allocations"), THP page fault allocations have
> settled on the following scheme (from the commit log):
> 
> 1. local node only THP allocation with no reclaim, just compaction.
> 2. for madvised VMA's or when synchronous compaction is enabled always - THP
>    allocation from any node with effort determined by global defrag setting
>    and VMA madvise
> 3. fallback to base pages on any node
> 
> Recent customer reports however revealed we have a gap in step 1 above.
> What we have seen is excessive reclaim due to THP page faults on a NUMA
> node that's close to its high watermark, while other nodes have plenty
> of free memory.
> 
> The problem with step 1 is that it promises no reclaim after the
> compaction attempt, however reclaim is only avoided for certain
> compaction outcomes (deferred, or skipped due to insufficient free base
> pages), and not e.g. when compaction is actually performed but fails (we
> did see compact_fail vmstat counter increasing).
> 
> THP page faults can therefore exhibit a zone_reclaim_mode-like behavior,
> which is not the intention.
> 
> Thus add a check for __GFP_THISNODE that corresponds to this exact
> situation and prevents continuing with reclaim/compaction once the
> initial compaction attempt isn't successful in allocating the page.
> 
> Note that commit cc638f329ef6 has not introduced this over-reclaim
> possibility; it appears to exist in some form since commit 2f0799a0ffc0
> ("mm, thp: restore node-local hugepage allocations"). Followup commits
> b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction
> may not succeed") and cc638f329ef6 have moved in the right direction,
> but left the abovementioned gap.
> 
> Fixes: 2f0799a0ffc0 ("mm, thp: restore node-local hugepage allocations")
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Yes, this makes sense as an intermediate state (to make a fix for stable
and other older kernels that might be interested in the fix). I would be
objecting that we should just simplify this whole thing but you have
done that in patch 2

Acked-by: Michal Hocko <mhocko@...e.com>
Thanks

> ---
>  mm/page_alloc.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 822e05f1a964..e6fd1213328b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4788,6 +4788,20 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  			    compact_result == COMPACT_DEFERRED)
>  				goto nopage;
>  
> +			/*
> +			 * THP page faults may attempt local node only first,
> +			 * but are then allowed to only compact, not reclaim,
> +			 * see alloc_pages_mpol()
> +			 *
> +			 * compaction can fail for other reasons than those
> +			 * checked above and we don't want such THP allocations
> +			 * to put reclaim pressure on a single node in a
> +			 * situation where other nodes might have plenty of
> +			 * available memory
> +			 */
> +			if (gfp_mask & __GFP_THISNODE)
> +				goto nopage;
> +
>  			/*
>  			 * Looks like reclaim/compaction is worth trying, but
>  			 * sync compaction could be very expensive, so keep
> 
> -- 
> 2.52.0

-- 
Michal Hocko
SUSE Labs