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[91.12.97.37]) by smtp.gmail.com with ESMTPSA id o2sm2529573wrh.13.2021.09.08.07.59.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Sep 2021 07:59:11 -0700 (PDT) Subject: Re: [patch 031/147] mm, slub: protect put_cpu_partial() with disabled irqs instead of cmpxchg From: David Hildenbrand To: 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 Cc: brouer@redhat.com References: <20210908025436.dvsgeCXAh%akpm@linux-foundation.org> <1a8ecf24-dca4-54f2-cdbf-9135b856b773@redhat.com> Organization: Red Hat Message-ID: <6524bba5-f737-3ab4-ee90-d6c70bac04f7@redhat.com> Date: Wed, 8 Sep 2021 16:59:10 +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: <1a8ecf24-dca4-54f2-cdbf-9135b856b773@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org 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. > > Are the numbers trustworthy? :) > .. and especially did the benchmark consider side effects of enabling/disabling interrupts (pipeline flushes etc ..)? -- Thanks, David / dhildenb