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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id d19-20020a05620a167300b0075b23e55640sm2844871qko.123.2023.06.12.06.36.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Jun 2023 06:36:35 -0700 (PDT) Date: Mon, 12 Jun 2023 09:36:33 -0400 From: Peter Xu To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz, minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com, lstoakes@gmail.com, hdanton@sina.com, apopple@nvidia.com, ying.huang@intel.com, david@redhat.com, yuzhao@google.com, dhowells@redhat.com, hughd@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, pasha.tatashin@soleen.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration Message-ID: References: <20230609005158.2421285-1-surenb@google.com> <20230609005158.2421285-5-surenb@google.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 5F1CC180006 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: xhbtrikzy3kfx94gsy4iu6ic6nf1y4ko X-HE-Tag: 1686577000-158929 X-HE-Meta: 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 nc21aT2o U6Lw4iOJHfrwLBMLYXkLuFvmJsRM7CwsQdzmn3mGepBaFoubILa/5EoDgo0YnDYYjJwf7CgU9zcD8FDkk5VQ8TYdmuw24nGNyNM4cc2TD3mu5wsyyOyCQ4ru8ZHqYDQxcxxB+i5hv2lPIPPnxmbxrSvPjzTsAOo1mp5p3bYffZbCF/KI1eyzH1TelIAFHrX9jcLwNr7TAP7k/4CZdUS96+kx7QvnQepDq5znLcgsttvrUGI9eoIvXMxXFCDDoldQcKQhkRsNL3wReXDY8Gn6zoJS2lrnq+6jdeCPxXqK35Ld9rdp4B4AuKgsujt1bMEt7Lymaa7izLp+rSE1qY9YcOEH8zux3pgt4pR1ulUzeHzGMddFeBe9ZAIJTZtK6AQD0qc4Mu2MRlwxNwUp/b0SjdxQYb1yLX75XZulh 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: On Fri, Jun 09, 2023 at 06:29:43PM -0700, Suren Baghdasaryan wrote: > On Fri, Jun 9, 2023 at 3:30 PM Suren Baghdasaryan wrote: > > > > On Fri, Jun 9, 2023 at 1:42 PM Peter Xu wrote: > > > > > > On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote: > > > > migration_entry_wait does not need VMA lock, therefore it can be dropped > > > > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA > > > > lock was dropped while in handle_mm_fault(). > > > > Note that once VMA lock is dropped, the VMA reference can't be used as > > > > there are no guarantees it was not freed. > > > > > > Then vma lock behaves differently from mmap read lock, am I right? Can we > > > still make them match on behaviors, or there's reason not to do so? > > > > I think we could match their behavior by also dropping mmap_lock here > > when fault is handled under mmap_lock (!(fault->flags & > > FAULT_FLAG_VMA_LOCK)). > > I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping > > mmap_lock in do_page_fault(), so indeed, I might be able to use > > VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well > > instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea > > of reusing existing flags? > Sorry, I meant VM_FAULT_VMA_UNLOCKED, not FAULT_FLAG_VMA_LOCK in the > above reply. > > I took a closer look into using VM_FAULT_COMPLETED instead of > VM_FAULT_VMA_UNLOCKED but when we fall back from per-vma lock to > mmap_lock we need to retry with an indication that the per-vma lock > was dropped. Returning (VM_FAULT_RETRY | VM_FAULT_COMPLETE) to > indicate such state seems strange to me ("retry" and "complete" seem Not relevant to this migration patch, but for the whole idea I was thinking whether it should just work if we simply: fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs); - vma_end_read(vma); + if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED))) + vma_end_read(vma); ? GUP may need more caution on NOWAIT, but vma lock is only in fault paths so IIUC it's fine? -- Peter Xu