linux-kernel - Re: [PATCH RESEND v2 4/6] mm/page_alloc: sort out the alloc_contig

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Message-ID: <15096b27-6f27-45fc-8a8b-de781a9c42a5@suse.cz>
Date: Wed, 4 Dec 2024 10:03:28 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Oscar Salvador <osalvador@...e.de>, David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linuxppc-dev@...ts.ozlabs.org, Andrew Morton <akpm@...ux-foundation.org>,
 Zi Yan <ziy@...dia.com>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Naveen N Rao <naveen@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>
Subject: Re: [PATCH RESEND v2 4/6] mm/page_alloc: sort out the
 alloc_contig_range() gfp flags mess

On 12/4/24 09:59, Oscar Salvador wrote:
> On Tue, Dec 03, 2024 at 08:19:02PM +0100, David Hildenbrand wrote:
>> It was always set using "GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL",
>> and I removed the same flag combination in #2 from memory offline code, and
>> we do have the exact same thing in do_migrate_range() in
>> mm/memory_hotplug.c.
>> 
>> We should investigate if__GFP_HARDWALL is the right thing to use here, and
>> if we can get rid of that by switching to GFP_KERNEL in all these places.
> 
> Why would not we want __GFP_HARDWALL set?
> Without it, we could potentially migrate the page to a node which is not
> part of the cpuset of the task that originally allocated it, thus violating the
> policy? Is not that a problem?

The task doing the alloc_contig_range() will likely not be the same task as
the one that originally allocated the page, so its policy would be
different, no? So even with __GFP_HARDWALL we might be already migrating
outside the original tasks's constraint? Am I missing something?