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stress-ng --mremap triggers severe lruvec lock contention in populate/unmap paths To: Lorenzo Stoakes , Hugh Dickins Cc: Joseph Salisbury , Andrew Morton , David Hildenbrand , Chris Li , Kairui Song , Jason Gunthorpe , Peter Xu , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , linux-mm@kvack.org, LKML References: <4a4f5b48-8a1d-48f8-8760-0f5d43b5d483@nvidia.com> <982e5964-5ea6-eaf7-a11a-0692f14a6943@google.com> Content-Language: en-US From: John Hubbard In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY3PR05CA0004.namprd05.prod.outlook.com (2603:10b6:a03:254::9) To DM3PR12MB9416.namprd12.prod.outlook.com (2603:10b6:0:4b::8) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM3PR12MB9416:EE_|SN7PR12MB6767:EE_ X-MS-Office365-Filtering-Correlation-Id: 26b93637-21d1-4b24-61e5-08de966396b1 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/9/26 11:03 AM, Lorenzo Stoakes wrote: > On Tue, Apr 07, 2026 at 05:35:18PM -0700, Hugh Dickins wrote: >> On Tue, 7 Apr 2026, John Hubbard wrote: >>> On 4/7/26 1:09 PM, Joseph Salisbury wrote: ... >> Thanks for the Cc. I'm not convinced that we should be making such a >> change, just to avoid the stress that an avowed stresstest is showing; >> but can let others debate that - and, need it be said, I have no >> problem with Joseph trying your patch. > > Yeah, the test case (as said by others also) is rather synthetic, and it's a > test designed to saturate, if not I/O throttled by swap then we hammer the > populate path. It feels like a micro-optimisation for something that is not (at > least not yet demonstrated to be) an actual problem. > > stress-ng is not a benchmarking tool per se, it's designed to eek out bugs. > > So really we need to see a real-world case I think. Absolutely. And to be honest, I saw "Oracle" and recalled that they are always doing things will zillions of threads, so I assumed that a real world case was waiting right behind this. But maybe not, after all? ... >>> + * Read VM_LOCKED before __get_user_pages(), which may drop >>> + * mmap_lock when FOLL_UNLOCKABLE is set, after which the vma >>> + * must not be accessed. The read is stable: mmap_lock is held >>> + * for read here, so mlock() (which needs the write lock) >>> + * cannot change VM_LOCKED concurrently. >>> + */ > > BTW, not to nitpick (OK, maybe to nitpick :) this comments feels a bit > redundant. Maybe useful to note that the lock might be dropped (but you don't > indicate why it's OK to still assume state about the VMA), and it's a known > thing that you need a VMA write lock to alter flags, if we had to comment this > each time mm would be mostly comments :) > > So if you want a comment here I'd say something like 'the lock might be dropped > due to FOLL_UNLOCKABLE, but that's ok, we would simply end up doing a redundant > drain in this case'. > > But I'm not sure it's needed? I'm OK with just dropping the whole comment. I've lost my way lately with comment density. :) > >>> + need_drain = vma->vm_flags & VM_LOCKED; > > Please use the new VMA flag interface :) Oops, yes. I'm on the fence about whether to post an updated version of this. Maybe wait until someone pops up with a real need for it? Thoughts? thanks, -- John Hubbard