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[87.59.106.155]) by smtp.gmail.com with ESMTPSA id r5sm243505lfm.2.2021.09.08.10.14.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Sep 2021 10:14:37 -0700 (PDT) From: Jesper Dangaard Brouer X-Google-Original-From: Jesper Dangaard Brouer Cc: brouer@redhat.com Subject: Re: [patch 031/147] mm, slub: protect put_cpu_partial() with disabled irqs instead of cmpxchg To: David Hildenbrand , Vlastimil Babka , Jesper Dangaard Brouer , Andrew Morton , bigeasy@linutronix.de, cl@linux.com, efault@gmx.de, iamjoonsoo.kim@lge.com, jannh@google.com, linux-mm@kvack.org, mgorman@techsingularity.net, mm-commits@vger.kernel.org, penberg@kernel.org, quic_qiancai@quicinc.com, rientjes@google.com, tglx@linutronix.de, torvalds@linux-foundation.org References: <20210908025436.dvsgeCXAh%akpm@linux-foundation.org> <1a8ecf24-dca4-54f2-cdbf-9135b856b773@redhat.com> <6524bba5-f737-3ab4-ee90-d6c70bac04f7@redhat.com> Message-ID: <3a83989f-aa36-3aee-d92f-5ddc912d7fc5@redhat.com> Date: Wed, 8 Sep 2021 19:14:36 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <6524bba5-f737-3ab4-ee90-d6c70bac04f7@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org On 08/09/2021 16.59, David Hildenbrand wrote: > On 08.09.21 16:55, David Hildenbrand wrote: >> On 08.09.21 15:58, Vlastimil Babka wrote: >>> On 9/8/21 15:05, Jesper Dangaard Brouer wrote: >>>> >>>> >>>> On 08/09/2021 04.54, Andrew Morton wrote: >>>>> From: Vlastimil Babka >>>>> Subject: mm, slub: protect put_cpu_partial() with disabled irqs >>>>> instead of cmpxchg >>>>> >>>>> Jann Horn reported [1] the following theoretically possible race: >>>>> >>>>>      task A: put_cpu_partial() calls preempt_disable() >>>>>      task A: oldpage = this_cpu_read(s->cpu_slab->partial) >>>>>      interrupt: kfree() reaches unfreeze_partials() and discards >>>>> the page >>>>>      task B (on another CPU): reallocates page as page cache >>>>>      task A: reads page->pages and page->pobjects, which are actually >>>>>      halves of the pointer page->lru.prev >>>>>      task B (on another CPU): frees page >>>>>      interrupt: allocates page as SLUB page and places it on the >>>>> percpu partial list >>>>>      task A: this_cpu_cmpxchg() succeeds >>>>> >>>>>      which would cause page->pages and page->pobjects to end up >>>>> containing >>>>>      halves of pointers that would then influence when >>>>> put_cpu_partial() >>>>>      happens and show up in root-only sysfs files. Maybe that's >>>>> acceptable, >>>>>      I don't know. But there should probably at least be a comment >>>>> for now >>>>>      to point out that we're reading union fields of a page that >>>>> might be >>>>>      in a completely different state. >>>>> >>>>> Additionally, the this_cpu_cmpxchg() approach in put_cpu_partial() >>>>> is only >>>>> safe against s->cpu_slab->partial manipulation in ___slab_alloc() >>>>> if the >>>>> latter disables irqs, otherwise a __slab_free() in an irq handler >>>>> could >>>>> call put_cpu_partial() in the middle of ___slab_alloc() manipulating >>>>> ->partial and corrupt it.  This becomes an issue on RT after a >>>>> local_lock >>>>> is introduced in later patch.  The fix means taking the local_lock >>>>> also in >>>>> put_cpu_partial() on RT. >>>>> >>>>> After debugging this issue, Mike Galbraith suggested [2] that to avoid >>>>> different locking schemes on RT and !RT, we can just protect >>>>> put_cpu_partial() with disabled irqs (to be converted to >>>>> local_lock_irqsave() later) everywhere.  This should be acceptable >>>>> as it's >>>>> not a fast path, and moving the actual partial unfreezing outside >>>>> of the >>>>> irq disabled section makes it short, and with the retry loop gone >>>>> the code >>>>> can be also simplified.  In addition, the race reported by Jann >>>>> should no >>>>> longer be possible. >>>> >>>> Based on my microbench[0] measurement changing preempt_disable to >>>> local_irq_save will cost us 11 cycles (TSC).  I'm not against the >>>> change, I just want people to keep this in mind. >>> >>> OK, but this is not a fast path for every allocation/free, so it gets >>> amortized. Also it eliminates a this_cpu_cmpxchg loop, and I'd expect >>> cmpxchg to be expensive too? >>> >>>> On my E5-1650 v4 @ 3.60GHz: >>>>     - preempt_disable(+enable)  cost: 11 cycles(tsc) 3.161 ns >>>>     - local_irq_save (+restore) cost: 22 cycles(tsc) 6.331 ns >>>> >>>> Notice the non-save/restore variant is superfast: >>>>     - local_irq_disable(+enable) cost: 6 cycles(tsc) 1.844 ns >>> >>> It actually surprises me that it's that cheap, and would have expected >>> changing the irq state would be the costly part, not the >>> saving/restoring. >>> Incidentally, would you know what's the cost of save+restore when the >>> irqs are already disabled, so it's effectively a no-op? >> >> It surprises me as well. That would imply that protecting short RCU >> sections using >> >> local_irq_disable >> local_irq_enable >> >> instead of via >> >> preempt_disable >> preempt_enable >> >> would actually be very beneficial. Please don't draw this as a general conclusion. As Linus describe in details, the IRQ disable/enable will be very micro-arch specific. The preempt_disable/enable will likely be more stable/consistent across micro-archs. Keep an eye out for kernel config options when juding preempt_disable/enable performance [1] [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c#L363-L367 >> >> Are the numbers trustworthy? :) >> > > .. and especially did the benchmark consider side effects of > enabling/disabling interrupts (pipeline flushes etc ..)? > Of-cause not, this is a microbenchmark... they are per definition not trustworthy :-P -Jesper