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[2003:cb:c70b:e100:38d6:8aa1:11b0:a20a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434d04defb7sm41582415e9.0.2024.12.04.03.05.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Dec 2024 03:05:23 -0800 (PST) Message-ID: <09e544d9-30b2-4422-bf19-7e0b2e436648@redhat.com> Date: Wed, 4 Dec 2024 12:05:22 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: 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 Cc: Vlastimil Babka , 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 Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.12.24 11:04, Oscar Salvador wrote: > On Wed, Dec 04, 2024 at 10:28:39AM +0100, David Hildenbrand wrote: >> 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. > > Looking back at when the whole contiguous allocator patchset was posted, > it seems that it kinda copied what memory-offline code was doing, which > was migrating pages with GFP_HIGHUSER_MOVABLE (hotremove_migrate_alloc()). > > Then, the HIGHMEM modifier was dropped due to HIGHMEM restrictions on > some systems, ending up with GFP_USER. Looking at some other migration_target_control users, some of them also shouldn't be setting GFP_USER->HARDWALL either I think. Essentially, whenever we are migrating a page that is not guaranteed to be "ours" in the context of the caller. mm/damon/paddr.c:__damon_pa_migrate_folio_list() for example, which obtained the addresses by scanning a chunk of physical address space. For others it's less clear: soft_offline_in_use_page() may be called either using madvise() from process context, but also using sysfs using a PFN. -- Cheers, David / dhildenb