Re: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF_HIGH allocations

[Vlastimil Babka <vbabka@suse.cz>]

On 8/29/25 10:36, Vlastimil Babka wrote:
> 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@igalia.com>
>> Tested-by: Helen Koike <koike@igalia.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: NeilBrown <neilb@suse.de>
>> Cc: Thierry Reding <thierry.reding@gmail.com>
>> Cc: Vlastimil Babka <vbabka@suse.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.

Oops, forgot to add

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

>> ---
>>  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;
>>  		}
>>  
>