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Fri, 4 Aug 2023 16:25:48 -0700 (PDT) In-Reply-To: References: <20230708191212.4147700-1-surenb@google.com> <20230708191212.4147700-3-surenb@google.com> <20230804214620.btgwhsszsd7rh6nf@f> From: Mateusz Guzik Date: Sat, 5 Aug 2023 01:25:48 +0200 Message-ID: Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking To: Linus Torvalds Cc: Suren Baghdasaryan , akpm@linux-foundation.org, regressions@leemhuis.info, bagasdotme@gmail.com, jacobly.alt@gmail.com, willy@infradead.org, liam.howlett@oracle.com, david@redhat.com, peterx@redhat.com, ldufour@linux.ibm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, gregkh@linuxfoundation.org, regressions@lists.linux.dev, Jiri Slaby , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: E525A80015 X-Rspam-User: X-Stat-Signature: w5748et8z8uwjzx9a9p8b5xyh8abrspy X-Rspamd-Server: rspam01 X-HE-Tag: 1691191549-239716 X-HE-Meta: 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 0zqm4ubY 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000017, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/5/23, Linus Torvalds wrote: > On Fri, 4 Aug 2023 at 14:46, Mateusz Guzik wrote: >> >> I don't see it mentioned in the discussion, so at a risk of ruffling >> feathers or looking really bad I'm going to ask: is the locking of any >> use if the forking process is single-threaded? T > > Sadly, we've always been able to access the mm from other processes, > so the locking is - I think - unavoidable. > > And some of those "access from other processes" aren't even uncommon > or special. It's things like "ps" etc, that do it just to see the > process name and arguments. > I know of these guys, I think they are excluded as is -- they go through access_remote_vm, starting with: if (mmap_read_lock_killable(mm)) return 0; while dup_mmap already write locks the parent's mm. I don't see any surprise relocks of the semaphore. Granted, should someone *bypass* this mechanism the above would be moot. -- Mateusz Guzik