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Fri, 12 Aug 2022 03:50:31 -0700 (PDT) Message-ID: <8a862c86-bc59-a76b-7ca0-96f304c612db@bytedance.com> Date: Fri, 12 Aug 2022 18:50:26 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v2] mm/mempolicy: fix lock contention on mems_allowed Content-Language: en-US To: Michal Hocko Cc: Andrew Morton , Vlastimil Babka , Mel Gorman , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.orga, Wei Yang References: <20220811124157.74888-1-wuyun.abel@bytedance.com> From: Abel Wu In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660301433; a=rsa-sha256; cv=none; b=Gh2NiSohGF9fA9Bfk4hIlaTn8iQZDjpus7pTHq0WfpLSSXbUr/QMrwZccmby0GoVJ59rCe AUKN18wBVevnJoelHaOypqec8dSHURGY7KmN0BUPDF5F1QT2IMGiE/rYzP5l7S8wBk92f4 CLZ6RGViWY2P6674roxsSRlwxexaMyo= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=jVoVB2g7; 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spf=pass (imf24.hostedemail.com: domain of wuyun.abel@bytedance.com designates 209.85.216.42 as permitted sender) smtp.mailfrom=wuyun.abel@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Rspam-User: X-HE-Tag: 1660301432-939053 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 8/11/22 10:06 PM, Michal Hocko Wrote: > [Cc Wei Yang who is author of 78b132e9bae9] > > On Thu 11-08-22 20:41:57, Abel Wu wrote: >> The mems_allowed field can be modified by other tasks, so it isn't >> safe to access it with alloc_lock unlocked even in the current >> process context. >> >> Say there are two tasks: A from cpusetA is performing set_mempolicy(2), >> and B is changing cpusetA's cpuset.mems: >> >> A (set_mempolicy) B (echo xx > cpuset.mems) >> ------------------------------------------------------- >> pol = mpol_new(); >> update_tasks_nodemask(cpusetA) { >> foreach t in cpusetA { >> cpuset_change_task_nodemask(t) { >> mpol_set_nodemask(pol) { >> task_lock(t); // t could be A >> new = f(A->mems_allowed); >> update t->mems_allowed; >> pol.create(pol, new); >> task_unlock(t); >> } >> } >> } >> } >> task_lock(A); >> A->mempolicy = pol; >> task_unlock(A); >> >> In this case A's pol->nodes is computed by old mems_allowed, and could >> be inconsistent with A's new mems_allowed. > > Just to clarify. With an unfortunate timing and those two nodemasks > overlap the end user effect could be a premature OOM because some nodes > wouldn't be considered, right? Yes, indeed! > >> While it is different when replacing vmas' policy: the pol->nodes is >> gone wild only when current_cpuset_is_being_rebound(): >> >> A (mbind) B (echo xx > cpuset.mems) >> ------------------------------------------------------- >> pol = mpol_new(); >> mmap_write_lock(A->mm); >> cpuset_being_rebound = cpusetA; >> update_tasks_nodemask(cpusetA) { >> foreach t in cpusetA { >> cpuset_change_task_nodemask(t) { >> mpol_set_nodemask(pol) { >> task_lock(t); // t could be A >> mask = f(A->mems_allowed); >> update t->mems_allowed; >> pol.create(pol, mask); >> task_unlock(t); >> } >> } >> foreach v in A->mm { >> if (cpuset_being_rebound == cpusetA) >> pol.rebind(pol, cpuset.mems); >> v->vma_policy = pol; >> } >> mmap_write_unlock(A->mm); >> mmap_write_lock(t->mm); >> mpol_rebind_mm(t->mm); >> mmap_write_unlock(t->mm); >> } >> } >> cpuset_being_rebound = NULL; >> >> In this case, the cpuset.mems, which has already done updating, is >> finally used for calculating pol->nodes, rather than A->mems_allowed. >> So it is OK to call mpol_set_nodemask() with alloc_lock unlocked when >> doing mbind(2). >> >> Fixes: 78b132e9bae9 ("mm/mempolicy: remove or narrow the lock on current") >> Signed-off-by: Abel Wu > > The fix looks correct. > >> --- >> mm/mempolicy.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/mm/mempolicy.c b/mm/mempolicy.c >> index d39b01fd52fe..61e4e6f5cfe8 100644 >> --- a/mm/mempolicy.c >> +++ b/mm/mempolicy.c >> @@ -855,12 +855,14 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags, >> goto out; >> } >> >> + task_lock(current); >> ret = mpol_set_nodemask(new, nodes, scratch); >> if (ret) { >> + task_unlock(current); >> mpol_put(new); >> goto out; >> } >> - task_lock(current); >> + >> old = current->mempolicy; >> current->mempolicy = new; >> if (new && new->mode == MPOL_INTERLEAVE) >> -- >> 2.31.1 >