linux-kernel - Re: [PATCH RFC 2/2] mm, page_alloc: fail costly __GFP

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Message-ID: <9881b540-7e22-404b-aeaa-282dc5eeb5d5@suse.cz>
Date: Wed, 17 Dec 2025 09:46:34 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, 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 2/2] mm, page_alloc: fail costly __GFP_NORETRY
 allocations faster

On 12/16/25 21:32, Johannes Weiner wrote:
> On Tue, Dec 16, 2025 at 04:54:22PM +0100, Vlastimil Babka wrote:
>> It might make therefore more sense to just fail unconditionally after
>> the initial compaction attempt, so do that instead. Costly allocations
>> that do want the reclaim/compaction to happen at least once can omit
>> __GFP_NORETRY, or even specify __GFP_RETRY_MAYFAIL for more than one
>> attempt.
>>
>> There is a slight potential unfairness in that costly __GFP_NORETRY
>> allocations that can't perform direct compaction (i.e. lack __GFP_IO)
>> will still be allowed to direct reclaim, while those that can direct
>> compact will now never attempt direct reclaim. However, in cases of
>> memory pressure causing compaction to be skipped due to insufficient
>> base pages, direct reclaim was already not done before, so there should
>> be no functional regressions from this change.
> 
> Hm, kind of. There could be enough basepages for compaction_suitable()
> but compaction odds are still higher with more free pages. So there
> might be cases it regresses.
> 
> __GFP_NORETRY semantics say it'll try reclaim at least once. We should
> be able to keep that and still simplify, no?
> 
>>  		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
>> -			if (gfp_mask & __GFP_THISNODE)
>> -				goto nopage;
>> +			goto nopage;
> 
> IOW, maybe directly select for the NUMA-THP special case here?
> 
> 		/* Optimistic node-local huge page - only compact once */
> 		if (costly_order &&
>                     ((gfp_mask & (__GFP_NORETRY|__GFP_THISNODE)) ==
>                      (__GFP_NORETRY|__GFP_THISNODE)))
> 			goto nopage;
> 
> and then let other __GFP_NORETRY fall through.

I did consider it as an alternative when realizing the potential unfairness
mentioned above, but then went with the simpler code option.
With your suggestion we keep the THP-specific check but at least remove the
arguably illogical compaction feedback.