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

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Message-ID: <10cdada2-f28f-4868-b821-4a13ed627e76@suse.cz>
Date: Fri, 29 Aug 2025 10:36:18 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>,
 linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Zi Yan <ziy@...dia.com>, Mel Gorman <mgorman@...hsingularity.net>,
 kernel-dev@...lia.com, Helen Koike <koike@...lia.com>,
 Matthew Wilcox <willy@...radead.org>,
 Thierry Reding <thierry.reding@...il.com>,
 Sergey Senozhatsky <senozhatsky@...omium.org>, neil@...wn.name
Subject: Re: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF_HIGH
 allocations

On 8/14/25 19:22, 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.
> 
> 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.
> 
> From /proc/pagetypeinfo, without the patch:
> 
> Node    0, zone      DMA, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
> Node    0, zone    DMA32, type   HighAtomic      1      8     10      9      7      3      0      0      0      0      0
> Node    0, zone   Normal, type   HighAtomic     64     20     12      5      0      0      0      0      0      0      0
> 
> With the patch:
> 
> Node    0, zone      DMA, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
> Node    0, zone    DMA32, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
> Node    0, zone   Normal, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
> 
> Fixes: 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves")
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
> Tested-by: Helen Koike <koike@...lia.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: NeilBrown <neilb@...e.de>
> Cc: Thierry Reding <thierry.reding@...il.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>

Agreed with others that this change matches the original intention and it
must have been an oversight. Also found nothing to the contrary in the
original threads.

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ef3c07266b3..bf52e3bef626 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4219,7 +4219,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
>  		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
>  			alloc_flags |= ALLOC_NON_BLOCK;
>  
> -			if (order > 0)
> +			if (order > 0 && (alloc_flags & ALLOC_MIN_RESERVE))
>  				alloc_flags |= ALLOC_HIGHATOMIC;
>  		}
>