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Mon, 24 Mar 2025 11:55:55 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHKT0AIGOD74uSCO7EpMRdLffUkeO90SPxsewBmpglboi/Ese+zvtNAM2TxfbNzdmxW7TizeQ== X-Received: by 2002:a05:6214:2027:b0:6e6:9b86:85d0 with SMTP id 6a1803df08f44-6eb3493113bmr263136076d6.8.1742842555398; Mon, 24 Mar 2025 11:55:55 -0700 (PDT) Received: from [172.22.33.10] ([99.209.85.25]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6eb3efda6f2sm47138366d6.116.2025.03.24.11.55.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Mar 2025 11:55:55 -0700 (PDT) Message-ID: Date: Mon, 24 Mar 2025 19:55:54 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Migrating the un-migratable From: David Hildenbrand To: "lsf-pc@lists.linux-foundation.org" Cc: linux-fsdevel@vger.kernel.org, "linux-mm@kvack.org" , Jeff Layton , Zi Yan , Shakeel Butt , Joanne Koong , Miklos Szeredi References: <882b566c-34d6-4e68-9447-6c74a0693f18@redhat.com> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 29.01.25 17:10, David Hildenbrand wrote: > Hi, > > ___GFP_MOVABLE allocations are supposed to be movable -> migratable: the > page allocator can place them on > MIGRATE_CMA/ZONE_MOVABLE/MIGRATE_MOVABLE areas: areas where the > expectation is that allocations can be migrated (somewhat reliably) to > different memory areas on demand. > > Mechanisms that turn such allocations unmigratable, such as long-term > page pinning (FOLL_LONGTERM), migrate these allocations at least out of > MIGRATE_CMA/ZONE_MOVABLE areas first. > > Ideally, we'd only perform this migration if really required (e.g., > long-term pinning), and rather "fix" other cases to not turn allocations > unmigratable. > > However, we have some rather obscure cases that can turn migratable > allocations effectively unmigratable for a long/indeterminate time, > possibly controlled by unprivileged user space. > > Possible effects include: > * CMA allocations failing > * Memory hotunplug not making progress > * Memory compaction not working as expected > > Some cases I can fix myself [1], others are harder to tackle. > > As one example, in context of FUSE we recently discovered that folios > that are under writeback cannot be migrated, and user space in control > of when writeback will end. Something similar can happen ->readahead() > where user space is in charge of supplying page content. Networking > filesystems in general seem to be prone to this as well. > > As another example, failing to split large folios can prevent migration > if memory is fragmented. XFS (IOMAP in general) refuses to split folios > that are dirty [3]. Splitting of folios and page migration have a lot in > common. > > This session is to collect cases that are known to be problematic, and > to start discussing possible approaches to make some of these > un-migratable allocations migratable, or alternative strategies to deal > with this. > > > [1] https://lkml.kernel.org/r/20250129115411.2077152-1-david@redhat.com > [2] > https://lkml.kernel.org/r/CAJnrk1ZCgff6ZWmqKzBXFq5uAEbms46OexA1axWS5v-PCZFqJg@mail.gmail.com > [3] > https://lkml.kernel.org/r/4febc035-a4ff-4afe-a9a0-d127826852a9@redhat.com > Slides from today: https://drive.google.com/file/d/1uX80M1x86Oz3DFoHif-JLx1rlC_Nh93R/view?usp=sharing -- Cheers, David / dhildenb