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[195.135.220.15]) by mx.google.com with ESMTPS id q8si911056edg.87.2019.03.26.01.56.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 Mar 2019 01:56:46 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 90FAAAC17; Tue, 26 Mar 2019 08:56:45 +0000 (UTC) Date: Tue, 26 Mar 2019 09:56:43 +0100 From: Michal Hocko To: Andrea Arcangeli Cc: Andrew Morton , linux-mm@kvack.org, zhong jiang , syzkaller-bugs@googlegroups.com, syzbot+cbb52e396df3e565ab02@syzkaller.appspotmail.com, Mike Rapoport , Mike Kravetz , Peter Xu , Dmitry Vyukov Subject: Re: [PATCH 1/2] userfaultfd: use RCU to free the task struct when fork fails Message-ID: <20190326085643.GG28406@dhcp22.suse.cz> References: <20190325225636.11635-1-aarcange@redhat.com> <20190325225636.11635-2-aarcange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190325225636.11635-2-aarcange@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Mon 25-03-19 18:56:35, Andrea Arcangeli wrote: > MEMCG depends on the task structure not to be freed under > rcu_read_lock() in get_mem_cgroup_from_mm() after it dereferences > mm->owner. Please state the actual problem. Your cover letter mentiones a race condition. Please make it explicit in the changelog. > An alternate possible fix would be to defer the delivery of the > userfaultfd contexts to the monitor until after fork() is guaranteed > to succeed. Such a change would require more changes because it would > create a strict ordering dependency where the uffd methods would need > to be called beyond the last potentially failing branch in order to be > safe. How much more changes are we talking about? Because ... > This solution as opposed only adds the dependency to common code > to set mm->owner to NULL and to free the task struct that was pointed > by mm->owner with RCU, if fork ends up failing. The userfaultfd > methods can still be called anywhere during the fork runtime and the > monitor will keep discarding orphaned "mm" coming from failed forks in > userland. ... this is adding a subtle hack that might break in the future because copy_process error paths are far from trivial and quite error prone IMHO. I am not opposed to the patch in principle but I would really like to see what kind of solutions we are comparing here. > This race condition couldn't trigger if CONFIG_MEMCG was set =n at > build time. All the CONFIG_MEMCG is just ugly as hell. Can we reduce that please? E.g. use if (IS_ENABLED(CONFIG_MEMCG)) where appropriate? [...] > +static __always_inline void mm_clear_owner(struct mm_struct *mm, > + struct task_struct *p) > +{ > +#ifdef CONFIG_MEMCG > + if (mm->owner == p) > + WRITE_ONCE(mm->owner, NULL); > +#endif How can we ever hit this warning and what does that mean? -- Michal Hocko SUSE Labs