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[PATCH] mm/gup: Drain batched mlock folio processing before attempting migration To: David Hildenbrand , Will Deacon , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Keir Fraser , Jason Gunthorpe , Frederick Mayle , Andrew Morton , Peter Xu , Rik van Riel , Vlastimil Babka , Ge Yang References: <20250815101858.24352-1-will@kernel.org> <9e7d31b9-1eaf-4599-ce42-b80c0c4bb25d@google.com> <8376d8a3-cc36-ae70-0fa8-427e9ca17b9b@google.com> <7ce169c2-09b7-39e3-d00b-ba1db6dd258c@google.com> <655d2d29-8fe6-4684-aba4-4803bed0d4d0@redhat.com> Content-Language: en-US From: John Hubbard In-Reply-To: <655d2d29-8fe6-4684-aba4-4803bed0d4d0@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY3PR10CA0010.namprd10.prod.outlook.com (2603:10b6:a03:255::15) To BY5PR12MB4116.namprd12.prod.outlook.com (2603:10b6:a03:210::13) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY5PR12MB4116:EE_|LV8PR12MB9417:EE_ 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spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/9/25 4:50 AM, David Hildenbrand wrote: > On 09.09.25 13:39, Will Deacon wrote: >> On Fri, Aug 29, 2025 at 08:46:52AM -0700, Hugh Dickins wrote: >>> On Fri, 29 Aug 2025, Will Deacon wrote: >>>> On Thu, Aug 28, 2025 at 01:47:14AM -0700, Hugh Dickins wrote: ... > Migration code itself will retry multiple times, which usually takes > care of most races. > > Not all of course. > > Now, I recall John was working on that at some point (I recall an RFC > patch, but I might be daydreaming), and I recall discussions at LSF/MM Yes. I was trying to improve the gup scenario of "try to long-term pin pages that are currently in ZONE_MOVABLE, and must therefore be migrated before pinning them". The problem is that the current code just retries in a loop, when what we really want is for it to go to sleep and wait for the pages to become migratable. Anyway, I had trouble finding something to wait *on*. David here at the last LSF/MM suggested waiting on a page group, but I only partially understand the approach--and what really happened is that I am "temporarily" (for a few years) consumed by this entertaining new Nova (Rust for Linux) project. And so I've failed to make any progress there this year. > around improving handling when we are mixing a flood of short-therm gup > with a single long-term gup that wants to migrate these (short-term > pinned) pages. > > Essentially, we would have to temporarily prevent new short-term GUP > pins in order to make the long-term GUP-pin succeed in migrating the folio. > thanks, -- John Hubbard