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[2003:cb:c70b:e100:38d6:8aa1:11b0:a20a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434d5307400sm17785025e9.38.2024.12.04.01.28.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Dec 2024 01:28:41 -0800 (PST) Message-ID: Date: Wed, 4 Dec 2024 10:28:39 +0100 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RESEND v2 4/6] mm/page_alloc: sort out the alloc_contig_range() gfp flags mess To: Oscar Salvador , Vlastimil Babka Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, Andrew Morton , Zi Yan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Madhavan Srinivasan References: <20241203094732.200195-1-david@redhat.com> <20241203094732.200195-5-david@redhat.com> <96c92857-4850-4f85-9474-ac193c5ea48c@redhat.com> <04c1d28e-bbea-4499-9a6d-770f9ca53ba9@suse.cz> <15096b27-6f27-45fc-8a8b-de781a9c42a5@suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.12.24 10:15, Oscar Salvador wrote: > On Wed, Dec 04, 2024 at 10:03:28AM +0100, Vlastimil Babka wrote: >> 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? > > Yes, that is right, I thought we derive the policy from the old page > somehow when migrating it, but reading the code does not seem to be the > case. > > Looking at prepare_alloc_pages(), if !ac->nodemask, which would be the > case here, we would get the policy from the current task > (alloc_contig_range()) when cpusets are enabled. > > So yes, I am a bit puzzled why __GFP_HARDWALL was chosen in the first > place. I suspect because "GFP_USER" felt like the appropriate thing to do. Before: commit f90b1d2f1aaaa40c6519a32e69615edc25bb97d5 Author: Paul Jackson Date: Tue Sep 6 15:18:10 2005 -0700 [PATCH] cpusets: new __GFP_HARDWALL flag Add another GFP flag: __GFP_HARDWALL. GFP_USER and GFP_KERNEL were the same thing. But memory offlining/alloc_contig were added later. -- Cheers, David / dhildenb