linux-kernel - Re: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF

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Message-ID: <20250814201211.GI115258@cmpxchg.org>
Date: Thu, 14 Aug 2025 16:12:11 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Brendan Jackman <jackmanb@...gle.com>, Zi Yan <ziy@...dia.com>,
	Mel Gorman <mgorman@...hsingularity.net>, kernel-dev@...lia.com,
	Helen Koike <koike@...lia.com>,
	Matthew Wilcox <willy@...radead.org>, NeilBrown <neilb@...e.de>,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF_HIGH
 allocations

Hello Thadeu,

On Thu, Aug 14, 2025 at 02:22:45PM -0300, Thadeu Lima de Souza Cascardo wrote:
> Commit 524c48072e56 ("mm/page_alloc: rename ALLOC_HIGH to
> ALLOC_MIN_RESERVE") is the start of a series that explains how __GFP_HIGH,
> which implies ALLOC_MIN_RESERVE, is going to be used instead of
> __GFP_ATOMIC for high atomic reserves.
> 
> Commit eb2e2b425c69 ("mm/page_alloc: explicitly record high-order atomic
> allocations in alloc_flags") introduced ALLOC_HIGHATOMIC for such
> allocations of order higher than 0. It still used __GFP_ATOMIC, though.
> 
> Then, commit 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH
> non-blocking allocations accesses reserves") just turned that check for
> !__GFP_DIRECT_RECLAIM, ignoring that high atomic reserves were expected to
> test for __GFP_HIGH.

It indeed looks accidental. From the cover letter,

    High-order atomic allocations are explicitly handled with the caveat that
    no __GFP_ATOMIC flag means that any high-order allocation that specifies
    GFP_HIGH and cannot enter direct reclaim will be treated as if it was
    GFP_ATOMIC.

it sounds like the intent was what your patch does, and not to extend
those privileges to anybody who is !gfp_direct_reclaim.

> This leads to high atomic reserves being added for high-order GFP_NOWAIT
> allocations and others that clear __GFP_DIRECT_RECLAIM, which is
> unexpected. Later, those reserves lead to 0-order allocations going to the
> slow path and starting reclaim.

Can you please provide more background on the workload and the
environment in which you observed this?

Which GFP_NOWAIT requests you saw participating in the reserves etc.

I would feel better with Mel or Vlastimil chiming in as well, but your
fix looks correct to me.