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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b367c028bsm19386843f8f.24.2024.08.01.06.26.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 06:26:59 -0700 (PDT) Message-ID: <6486a2b1-45ef-44b6-bd84-d402fc121373@redhat.com> Date: Thu, 1 Aug 2024 15:26:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Race condition observed between page migration and page fault handling on arm64 machines From: David Hildenbrand To: Dev Jain , akpm@linux-foundation.org, willy@infradead.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, osalvador@suse.de, baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, ioworker0@gmail.com, gshan@redhat.com, mark.rutland@arm.com, kirill.shutemov@linux.intel.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, broonie@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20240801081657.1386743-1-dev.jain@arm.com> <3b82e195-5871-4880-9ce5-d01bb751f471@redhat.com> <92df0ee1-d3c9-41e2-834c-284127ae2c4c@arm.com> <19902a48-c59b-4e3b-afc5-e792506c2fd6@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 64C7B40027 X-Stat-Signature: ti9ctsc7euhi1irdzxrcby7sxrcmxfrm X-HE-Tag: 1722518824-797223 X-HE-Meta: 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 aYAu6q8t sQCgpZM6KA2Ju/YA2c5+XmERLaE3WErJEQz+tYYc8qb7b6fCW6cFvXMkm8KBbHpUK8Ax/ngGCO3x9nNfoC+0O4xcQRpLbAi2Nnr8e5qfoSi6SoQ+TBIlVYMAAkV58+c2jf/PfeaDa2M+nUqrFuZZoXadZh0mKfHw20prAr10HwbY9SMHocehoP9ionLr6qLSkaj3MH5nWMFVZUGkESL9y3654zbqkKCsC6dOL5MmTCSN4YW6R3DkjNo89DAbrJAv96e2j00pTfrrrT7xxvmxb3REE4hJBQSPv+9XRWCmKIeXLYeP/p9heI1JE6rJSnNWlCKGP3VZos2jV+ig= 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: List-Subscribe: List-Unsubscribe: On 01.08.24 15:13, David Hildenbrand wrote: >>>> To dampen the tradeoff, we could do this in shmem_fault() instead? But >>>> then, this would mean that we do this in all >>>> >>>> kinds of vma->vm_ops->fault, only when we discover another reference >>>> count race condition :) Doing this in do_fault() >>>> >>>> should solve this once and for all. In fact, do_pte_missing() may call >>>> do_anonymous_page() or do_fault(), and I just >>>> >>>> noticed that the former already checks this using vmf_pte_changed(). >>> >>> What I am still missing is why this is (a) arm64 only; and (b) if this >>> is something we should really worry about. There are other reasons >>> (e.g., speculative references) why migration could temporarily fail, >>> does it happen that often that it is really something we have to worry >>> about? >> >> >> (a) See discussion at [1]; I guess it passes on x86, which is quite >> strange since the race is clearly arch-independent. > > Yes, I think this is what we have to understand. Is the race simply less > likely to trigger on x86? > > I would assume that it would trigger on any arch. > > I just ran it on a x86 VM with 2 NUMA nodes and it also seems to work here. > > Is this maybe related to deferred flushing? Such that the other CPU will > by accident just observe the !pte_none a little less likely? > > But arm64 also usually defers flushes, right? At least unless > ARM64_WORKAROUND_REPEAT_TLBI is around. With that we never do deferred > flushes. Bingo! diff --git a/mm/rmap.c b/mm/rmap.c index e51ed44f8b53..ce94b810586b 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -718,10 +718,7 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm, pte_t pteval, */ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags) { - if (!(flags & TTU_BATCH_FLUSH)) - return false; - - return arch_tlbbatch_should_defer(mm); + return false; } On x86: # ./migration TAP version 13 1..1 # Starting 1 tests from 1 test cases. # RUN migration.shared_anon ... Didn't migrate 1 pages # migration.c:170:shared_anon:Expected migrate(ptr, self->n1, self->n2) (-2) == 0 (0) # shared_anon: Test terminated by assertion # FAIL migration.shared_anon not ok 1 migration.shared_anon It fails all of the time! -- Cheers, David / dhildenb