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AJvYcCWBodYNd21Vezmr3ChZMfXdNVSgXxTU654mXId7iCrqKAAAotINbq13y5GNhsGtAIPQfGtL8F70KQ==@kvack.org X-Gm-Message-State: AOJu0YzvHHxMLNmgiUQFlvK0AI5R8Dqrue6LkVWnE77mOuKFtepus7SP pOax22lOx/6pqBNgYWfQ+RKtYZbdbhPWt0JAvNMgKAkIJXpiCjBlXMHf+bc20lmk0cBEvg69CBJ qObZVVeTyumOJhhglvuX3OMWrzSSFVn9wDp8u X-Gm-Gg: ASbGnctJGr536brKlKui9Ht8N8YFIE5pDIIwf1uKpOTbxetR9hF341UYZu9AfnkPrgc t2QSveyXJo+UCHtuUtEfK1buNTxT+05xrnbwk3QMcH7Thw+fiqH9su1B185qP7GaFwoFJ X-Google-Smtp-Source: AGHT+IFoNxbLSivCB8O+mxImiir970GdW7OaKshYzGwkWOn3yHVrguyW7CMWfUXbyO03XcT7Rr+UjXFED1Gcx3L2cUs= X-Received: by 2002:a05:622a:1115:b0:468:f7e0:ad40 with SMTP id d75a77b69052e-468f971ac80mr4409181cf.14.1734451106340; Tue, 17 Dec 2024 07:58:26 -0800 (PST) MIME-Version: 1.0 References: <20241216192419.2970941-1-surenb@google.com> <20241216192419.2970941-14-surenb@google.com> <20241217102620.GC11133@noisy.programming.kicks-ass.net> In-Reply-To: <20241217102620.GC11133@noisy.programming.kicks-ass.net> From: Suren Baghdasaryan Date: Tue, 17 Dec 2024 07:58:15 -0800 Message-ID: Subject: Re: [PATCH v6 13/16] mm: introduce vma_ensure_detached() To: Peter Zijlstra 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: m3qbfp1q3zp6jiqeqygd8js9147qxo39 X-Rspamd-Queue-Id: 2BB6740002 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1734451083-561088 X-HE-Meta: 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 yDd2S6tw ROFlUwNq+C0AQ76ZqzoUMsqGgE48pd2bmDFBJAOB6qsYS6qpZM5GEzWDHzqmk9DNz+4z88OVkCrp3qDrbscHZYAtCMiQEvwNvaUaKM/EWfrU6v3e9/zkcWX9/Za6YqI9BBx24F6rYNEmL3ST/i27oBQA3mmfeciRDmI1ZlGgoyBZXzgS9YNXguFIsAnUMyO4aB3POKXB+i67ilCGo5KX4B9EqBvFk2LjL8w++6G0HbQWbGlWztHNy8/BtRG+7u0tuNzpeOWQR5Ti4xiJgGlIPShASkxHhDnKXLzOUUG4wQ4h5AKk= X-Bogosity: Ham, tests=bogofilter, spamicity=0.136358, 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 Tue, Dec 17, 2024 at 2:26=E2=80=AFAM Peter Zijlstra wrote: > > 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=3D=3D1 (attached) > > vma_start_write() > > vma->vm_lock_seq =3D mm->mm_lock_seq > > > > vma_start_read() > > vm_refcnt++; // vm_refcnt=3D=3D2 > > > > vma_mark_detached() > > vm_refcnt--; // vm_refcnt=3D=3D1 > > > > // vma is detached but vm_refcnt!=3D0 temporarily > > > > if (vma->vm_lock_seq =3D=3D mm->mm_lock_seq) > > vma_refcount_put() > > vm_refcnt--; // vm_refcnt=3D=3D0 > > > > 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 unti= l > > 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. I see your point. Let me reply in the other patch where you have more comments about this.