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[115.124.30.44]) by mx.google.com with ESMTPS id s7si4121114pgr.287.2019.05.09.11.22.44 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 09 May 2019 11:22:44 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.44 as permitted sender) client-ip=115.124.30.44; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.44 as permitted sender) smtp.mailfrom=yang.shi@linux.alibaba.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07486;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=12;SR=0;TI=SMTPD_---0TRHI4lD_1557426158; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TRHI4lD_1557426158) by smtp.aliyun-inc.com(127.0.0.1); Fri, 10 May 2019 02:22:41 +0800 Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush To: Peter Zijlstra , Will Deacon Cc: jstancek@redhat.com, akpm@linux-foundation.org, stable@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, npiggin@gmail.com, namit@vmware.com, minchan@kernel.org, Mel Gorman References: <1557264889-109594-1-git-send-email-yang.shi@linux.alibaba.com> <20190509083726.GA2209@brain-police> <20190509103813.GP2589@hirez.programming.kicks-ass.net> From: Yang Shi Message-ID: Date: Thu, 9 May 2019 11:22:37 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20190509103813.GP2589@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US 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 5/9/19 3:38 AM, Peter Zijlstra wrote: > On Thu, May 09, 2019 at 09:37:26AM +0100, Will Deacon wrote: >> Hi all, [+Peter] > Right, mm/mmu_gather.c has a MAINTAINERS entry; use it. Sorry for that, I didn't realize we have mmu_gather maintainers. I should run maintainer.pl. > > Also added Nadav and Minchan who've poked at this issue before. And Mel, > because he loves these things :-) > >> Apologies for the delay; I'm attending a conference this week so it's tricky >> to keep up with email. >> >> On Wed, May 08, 2019 at 05:34:49AM +0800, Yang Shi wrote: >>> A few new fields were added to mmu_gather to make TLB flush smarter for >>> huge page by telling what level of page table is changed. >>> >>> __tlb_reset_range() is used to reset all these page table state to >>> unchanged, which is called by TLB flush for parallel mapping changes for >>> the same range under non-exclusive lock (i.e. read mmap_sem). Before >>> commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in >>> munmap"), MADV_DONTNEED is the only one who may do page zapping in >>> parallel and it doesn't remove page tables. But, the forementioned commit >>> may do munmap() under read mmap_sem and free page tables. This causes a >>> bug [1] reported by Jan Stancek since __tlb_reset_range() may pass the > Please don't _EVER_ refer to external sources to describe the actual bug > a patch is fixing. That is the primary purpose of the Changelog. > > Worse, the email you reference does _NOT_ describe the actual problem. > Nor do you. Sure, will articulate the real bug in the commit log. > >>> wrong page table state to architecture specific TLB flush operations. >> Yikes. Is it actually safe to run free_pgtables() concurrently for a given >> mm? > Yeah.. sorta.. it's been a source of 'interesting' things. This really > isn't the first issue here. > > Also, change_protection_range() is 'fun' too. > >>> So, removing __tlb_reset_range() sounds sane. This may cause more TLB >>> flush for MADV_DONTNEED, but it should be not called very often, hence >>> the impact should be negligible. >>> >>> The original proposed fix came from Jan Stancek who mainly debugged this >>> issue, I just wrapped up everything together. >> I'm still paging the nested flush logic back in, but I have some comments on >> the patch below. >> >>> [1] https://lore.kernel.org/linux-mm/342bf1fd-f1bf-ed62-1127-e911b5032274@linux.alibaba.com/T/#m7a2ab6c878d5a256560650e56189cfae4e73217f >>> >>> Reported-by: Jan Stancek >>> Tested-by: Jan Stancek >>> Cc: Will Deacon >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Yang Shi >>> Signed-off-by: Jan Stancek >>> --- >>> mm/mmu_gather.c | 7 ++++--- >>> 1 file changed, 4 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c >>> index 99740e1..9fd5272 100644 >>> --- a/mm/mmu_gather.c >>> +++ b/mm/mmu_gather.c >>> @@ -249,11 +249,12 @@ void tlb_finish_mmu(struct mmu_gather *tlb, >>> * flush by batching, a thread has stable TLB entry can fail to flush >> Urgh, we should rewrite this comment while we're here so that it makes sense... > Yeah, that's atrocious. We should put the actual race in there. > >>> * the TLB by observing pte_none|!pte_dirty, for example so flush TLB >>> * forcefully if we detect parallel PTE batching threads. >>> + * >>> + * munmap() may change mapping under non-excluse lock and also free >>> + * page tables. Do not call __tlb_reset_range() for it. >>> */ >>> - if (mm_tlb_flush_nested(tlb->mm)) { >>> - __tlb_reset_range(tlb); >>> + if (mm_tlb_flush_nested(tlb->mm)) >>> __tlb_adjust_range(tlb, start, end - start); >>> - } >> I don't think we can elide the call __tlb_reset_range() entirely, since I >> think we do want to clear the freed_pXX bits to ensure that we walk the >> range with the smallest mapping granule that we have. Otherwise couldn't we >> have a problem if we hit a PMD that had been cleared, but the TLB >> invalidation for the PTEs that used to be linked below it was still pending? > That's tlb->cleared_p*, and yes agreed. That is, right until some > architecture has level dependent TLBI instructions, at which point we'll > need to have them all set instead of cleared. > >> Perhaps we should just set fullmm if we see that here's a concurrent >> unmapper rather than do a worst-case range invalidation. Do you have a feeling >> for often the mm_tlb_flush_nested() triggers in practice? > Quite a bit for certain workloads I imagine, that was the whole point of > doing it. > > Anyway; am I correct in understanding that the actual problem is that > we've cleared freed_tables and the ARM64 tlb_flush() will then not > invalidate the cache and badness happens? Yes. > > Because so far nobody has actually provided a coherent description of > the actual problem we're trying to solve. But I'm thinking something > like the below ought to do. Thanks for the suggestion, will do in v2. > > > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index 99740e1dd273..fe768f8d612e 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -244,15 +244,20 @@ void tlb_finish_mmu(struct mmu_gather *tlb, > unsigned long start, unsigned long end) > { > /* > - * If there are parallel threads are doing PTE changes on same range > - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB > - * flush by batching, a thread has stable TLB entry can fail to flush > - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB > - * forcefully if we detect parallel PTE batching threads. > + * Sensible comment goes here.. > */ > - if (mm_tlb_flush_nested(tlb->mm)) { > - __tlb_reset_range(tlb); > - __tlb_adjust_range(tlb, start, end - start); > + if (mm_tlb_flush_nested(tlb->mm) && !tlb->full_mm) { > + /* > + * Since we're can't tell what we actually should have > + * flushed flush everything in the given range. > + */ > + tlb->start = start; > + tlb->end = end; > + tlb->freed_tables = 1; > + tlb->cleared_ptes = 1; > + tlb->cleared_pmds = 1; > + tlb->cleared_puds = 1; > + tlb->cleared_p4ds = 1; > } > > tlb_flush_mmu(tlb);