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Mon, 14 Aug 2023 11:06:17 -0400 X-MC-Unique: I0CXlIs1MWeF5tyF_66jfg-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E7338299E749; Mon, 14 Aug 2023 15:06:16 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.45.225.27]) by smtp.corp.redhat.com (Postfix) with SMTP id 731A8492C13; Mon, 14 Aug 2023 15:06:14 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 14 Aug 2023 17:05:34 +0200 (CEST) Date: Mon, 14 Aug 2023 17:05:31 +0200 From: Oleg Nesterov To: Mateusz Guzik Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, brauner@kernel.org, ebiederm@xmission.com, david@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org, koct9i@gmail.com, dave@stgolabs.net Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement Message-ID: <20230814150530.GB17738@redhat.com> References: <20230813123333.1705833-1-mjguzik@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230813123333.1705833-1-mjguzik@gmail.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 X-Rspamd-Queue-Id: 77FA1C012A X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: gsxdq3sfpc4q4w4kb3epem39o5z41sdx X-HE-Tag: 1692025581-929079 X-HE-Meta: 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 FZRAKlhQ rBqdb71vFJZz/Y51htfMY3n8mt20lY6YMrZpylD9ocl3JzR8976ftXgvjM5xJM2puv9oFPSR1Gi6XBWQIaqEXdF69mwygex/4OjGticEF1kdPLZaVVGMd13MVbjzBX51CYPqYLUZ6JAJTmAMmvnKIciGu2x0OXhzveEQhl5rvBztI/2+hU8MU+JEdZx4KXuK5ZLG3MKc8kbAINvA+th0XNFlgD16me+w8TaMVThBG0B1XHZhDnU5qDFXXX4HCcjzSC54Yds4VwpO6lL8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.001233, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 08/13, Mateusz Guzik wrote: > > fe69d560b5bd ("kernel/fork: always deny write access to current MM > exe_file") added another lock trip to synchronize the state of exe_file > against fork, further defeating the point of xchg. > > As such I think the atomic here only adds complexity for no benefit. > > Just write-lock around the replacement. Well, I tend to agree but can't really comment because I forgot everything about these code paths. But I have to admit that I don't understand the code in replace_mm_exe_file() without this patch... old_exe_file = xchg(&mm->exe_file, new_exe_file); if (old_exe_file) { /* * Don't race with dup_mmap() getting the file and disallowing * write access while someone might open the file writable. */ mmap_read_lock(mm); allow_write_access(old_exe_file); fput(old_exe_file); mmap_read_unlock(mm); } Can someone please explain me which exactly race this mmap_read_lock() tries to avoid and how ? Oleg.