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Howlett" , Suren Baghdasaryan , Matthew Wilcox , David Hildenbrand , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 01/10] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON Message-ID: References: <87e668d54927bb4ccdb7d374275e0662de667697.1745307301.git.lorenzo.stoakes@oracle.com> <20250430004703.63rumj4znewlbc2h@master> <8c052822-5365-4178-8e06-ecd4f917cf8a@lucifer.local> <20250430154119.a5ljf5t5tutqzim5@master> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250430154119.a5ljf5t5tutqzim5@master> X-ClientProxiedBy: LO4P265CA0079.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:2bd::12) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|CH3PR10MB7529:EE_ X-MS-Office365-Filtering-Correlation-Id: 60e8fd08-aad2-44d2-a77e-08dd88012370 X-MS-Exchange-SenderADCheck: 1 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 5NySQN/n 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 X-Bogosity: Ham, tests=bogofilter, 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 Wed, Apr 30, 2025 at 03:41:19PM +0000, Wei Yang wrote: > On Wed, Apr 30, 2025 at 02:15:24PM +0100, Lorenzo Stoakes wrote: > >On Wed, Apr 30, 2025 at 12:47:03AM +0000, Wei Yang wrote: > >> On Tue, Apr 22, 2025 at 09:09:20AM +0100, Lorenzo Stoakes wrote: > >> [...] > >> >+bool vma_had_uncowed_children(struct vm_area_struct *vma) > >> >+{ > >> >+ struct anon_vma *anon_vma = vma ? vma->anon_vma : NULL; > >> >+ bool ret; > >> >+ > >> >+ if (!anon_vma) > >> >+ return false; > >> >+ > >> >+ /* > >> >+ * If we're mmap locked then there's no way for this count to change, as > >> >+ * any such change would require this lock not be held. > >> >+ */ > >> >+ if (rwsem_is_locked(&vma->vm_mm->mmap_lock)) > >> >+ return anon_vma->num_children > 1; > >> > >> Hi, Lorenzo > >> > >> May I have a question here? > > > >Just ask the question. > > > > Thanks. > > My question is the function is expected to return true, if we have forked a > vma from this one, right? > > IMO there are cases when it has one forked child and anon_vma->num_children == 1, > which means folios are not exclusively mapped. But the function would return > false. > > Or maybe I misunderstand the logic here. I mean, it'd be helpful if you delineated which cases these were? Presumably you're thiking of something like: 1. Process 1: VMA A is established. num_children == 1 (self-reference is counted). 2. Process 2: Process 1 forks, VMA B references A, a->num_children++ 3. Process 3: Process 2 forks, VMA C is established (maybe you think b->num_children++?) 4. Unmap vma B, oops, a->num_children == 1 but it still has C! But that won't happen, as VMA C will be referencing a->anon_vma, so in reality a->anon_vma->num_children == 3, then after unmap == 2. References to the originally faulted-in anon_vma is propagated through the forks. anon_vma logic is tricky, one of many reasons I want to (significantly) rework it. Though sadly there is a lot of _essential_ complexity, I do think we can do better. > > >However, with respect, the last drive-by review you gave was not helpful, > >so I strongly suggest that this is not a great use of your time. > > > >Again, I _strongly_ suggest you focus on bug fixes or the like. > > Thanks for your suggestion and patience. I would try to focus on bugs and skip > those subtle things. Thanks, you've contributed good bug reports in the past, I'm not just recommending this for no reason! :) David's suggested tests are also a positive way forward. Thanks, Lorenzo > > > > >Thanks. > > > >> > >> >+ > >> >+ /* > >> >+ * Any change that would increase the number of children would be > >> >+ * prevented by a read lock. > >> >+ */ > >> >+ anon_vma_lock_read(anon_vma); > >> >+ ret = anon_vma->num_children > 1; > >> >+ anon_vma_unlock_read(anon_vma); > >> >+ > >> >+ return ret; > >> >+} > >> > >> -- > >> Wei Yang > >> Help you, Help me > > -- > Wei Yang > Help you, Help me