From: Johannes Weiner <hannes@cmpxchg.org>
To: Salvatore Dipietro <dipiets@amazon.it>
Cc: akpm@linux-foundation.org, abuehaze@amazon.com,
alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org,
brendan.jackman@linux.dev, david@redhat.com, dgc@kernel.org,
dipietro.salvatore@gmail.com, djwong@kernel.org,
hch@infradead.org, hch@lst.de, jackmanb@google.com,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, linux-xfs@vger.kernel.org, mhocko@suse.com,
ritesh.list@gmail.com, rvvandan@amazon.com,
stable@vger.kernel.org, surenb@google.com, vbabka@kernel.org,
vbabka@suse.cz, willy@infradead.org, ziy@nvidia.com
Subject: Re: [PATCH v5] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations
Date: Tue, 15 Sep 2026 12:17:34 -0400 [thread overview]
Message-ID: <aqlvnm8AKp5f_UhG@cmpxchg.org> (raw)
In-Reply-To: <20260915160042.1265958-1-dipiets@amazon.it>
On Tue, Sep 15, 2026 at 04:00:42PM +0000, Salvatore Dipietro wrote:
>
> On Fri, 11 Sep 2026 09:32:00 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > does any of this look real?
>
> The mechanism seems real, but it only shows up with vm.defrag_mode=1,
> which is off by default.
>
> With defrag_mode=1, a costly __GFP_NORETRY allocation that used to fail
> cleanly can now succeed by fragmenting another migratetype's pageblock,
> which seems like the wrong trade for a caller whose premise is a cheap
> lower-order fallback. Johannes, do we want to keep it strict?
Yes, avoiding the fallback is a good idea. The user already
communicated they can handle failure and have fallback options.
> If so, we could add a condition like below. It is equally
> applicable to v4 and v5. Happy to send it separately or in v6.
>
> /*
> * Reclaim/compaction cannot run, so defrag_mode's strategy
> * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow
> * fallbacks rather than failing the allocation outright.
> * Not for costly __GFP_NORETRY: those have a cheap lower
> * order fallback, so failing beats fragmenting.
> */
> if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) &&
> !(costly_order && (gfp_mask & __GFP_NORETRY)) &&
It would be possible to remove the costly_order gate here and just
bail on __GFP_NORETRY. But no strong feelings.
next prev parent reply other threads:[~2026-09-15 16:17 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-11 14:21 [PATCH v5] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations Salvatore Dipietro
2026-09-11 15:37 ` Zi Yan
2026-09-11 16:32 ` Andrew Morton
2026-09-15 16:00 ` Salvatore Dipietro
2026-09-15 16:17 ` Johannes Weiner [this message]
2026-09-18 6:32 ` Salvatore Dipietro
2026-09-18 6:40 ` Vlastimil Babka (SUSE)
2026-09-18 21:35 ` Andrew Morton
2026-10-01 8:31 ` Salvatore Dipietro
2026-09-11 20:44 ` Johannes Weiner
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