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Received: from 77-249-17-89.cable.dynamic.v4.ziggo.nl ([77.249.17.89] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1tNUmT-00000006Jip-1v5t; Tue, 17 Dec 2024 10:26:22 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 50D1730015F; Tue, 17 Dec 2024 11:26:20 +0100 (CET) Date: Tue, 17 Dec 2024 11:26:20 +0100 From: Peter Zijlstra To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v6 13/16] mm: introduce vma_ensure_detached() Message-ID: <20241217102620.GC11133@noisy.programming.kicks-ass.net> References: <20241216192419.2970941-1-surenb@google.com> <20241216192419.2970941-14-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241216192419.2970941-14-surenb@google.com> X-Rspamd-Queue-Id: D0B4AA0008 X-Stat-Signature: wpadcbx6r56jfss47bfs6ujpchh38nrf X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1734431152-189387 X-HE-Meta: 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 VjBmLlEm 0lKBfRJEElzTSnUfS5ozsoQkP2r9gD/R/kwu911XOyRFhG20PZUsAmkzMdLWApfXjbxzOiNYQeXgE2Z2kdfq/B7rwJLwQU24KzonmCEw6kGK0UoskFc24xHTULHQxASk68WEmbT7ywqw4viNEZGoz3wTPVM2cNFdcxYsrHOjMs3tZEKDjYAk1yU8oVqfYKmFO0H8LFZsobHvf+hZijxXqdADbUUTy5wLxudMlGzpIK2RwjAw= 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 Mon, Dec 16, 2024 at 11:24:16AM -0800, Suren Baghdasaryan wrote: > vma_start_read() can temporarily raise vm_refcnt of a write-locked and > detached vma: > > // vm_refcnt==1 (attached) > vma_start_write() > vma->vm_lock_seq = mm->mm_lock_seq > > vma_start_read() > vm_refcnt++; // vm_refcnt==2 > > vma_mark_detached() > vm_refcnt--; // vm_refcnt==1 > > // vma is detached but vm_refcnt!=0 temporarily > > if (vma->vm_lock_seq == mm->mm_lock_seq) > vma_refcount_put() > vm_refcnt--; // vm_refcnt==0 > > This is currently not a problem when freeing the vma because RCU grace > period should pass before kmem_cache_free(vma) gets called and by that > time vma_start_read() should be done and vm_refcnt is 0. However once > we introduce possibility of vma reuse before RCU grace period is over, > this will become a problem (reused vma might be in non-detached state). > Introduce vma_ensure_detached() for the writer to wait for readers until > they exit vma_start_read(). So aside from the lockdep problem (which I think is fixable), the normal way to fix the above is to make dec_and_test() do the kmem_cache_free(). Then the last user does the free and everything just works.