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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429059cbd33sm127195435e9.41.2024.08.09.06.23.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Aug 2024 06:23:47 -0700 (PDT) Message-ID: <79924d5b-b807-440a-a6e8-8f0ae13aa04b@redhat.com> Date: Fri, 9 Aug 2024 15:23:45 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Race condition observed between page migration and page fault handling on arm64 machines To: Dev Jain , Will Deacon Cc: akpm@linux-foundation.org, willy@infradead.org, 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, 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, mgorman@techsingularity.net 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> <6486a2b1-45ef-44b6-bd84-d402fc121373@redhat.com> <20240801134358.GB4794@willie-the-truck> <9359caf7-81a8-45d9-9787-9009b3b2eed3@redhat.com> <418e818a-f385-459e-a84d-e3880ac08ad5@redhat.com> <8158c9d6-cbfe-4767-be8e-dc227b29200c@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 07.08.24 14:58, Dev Jain wrote: > > On 8/7/24 17:09, David Hildenbrand wrote: >> On 05.08.24 16:14, Dev Jain wrote: >>> >>> On 8/5/24 16:16, David Hildenbrand wrote: >>>> On 05.08.24 11:51, Dev Jain wrote: >>>>> >>>>> On 8/1/24 19:18, David Hildenbrand wrote: >>>>>> On 01.08.24 15:43, Will Deacon wrote: >>>>>>> On Thu, Aug 01, 2024 at 03:26:57PM +0200, David Hildenbrand wrote: >>>>>>>> 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! >>>>>>> >>>>>>> Nice work! I suppose that makes sense as, with the eager TLB >>>>>>> invalidation, the window between the other CPU faulting and the >>>>>>> migration entry being written is fairly wide. >>>>>>> >>>>>>> Not sure about a fix though :/ It feels a bit overkill to add a new >>>>>>> invalid pte encoding just for this. >>>>>> >>>>>> Something like that might make the test happy in most cases: >>>>>> >>>>>> diff --git a/tools/testing/selftests/mm/migration.c >>>>>> b/tools/testing/selftests/mm/migration.c >>>>>> index 6908569ef406..4c18bfc13b94 100644 >>>>>> --- a/tools/testing/selftests/mm/migration.c >>>>>> +++ b/tools/testing/selftests/mm/migration.c >>>>>> @@ -65,6 +65,7 @@ int migrate(uint64_t *ptr, int n1, int n2) >>>>>>           int ret, tmp; >>>>>>           int status = 0; >>>>>>           struct timespec ts1, ts2; >>>>>> +       int errors = 0; >>>>>> >>>>>>           if (clock_gettime(CLOCK_MONOTONIC, &ts1)) >>>>>>                   return -1; >>>>>> @@ -79,12 +80,17 @@ int migrate(uint64_t *ptr, int n1, int n2) >>>>>>                   ret = move_pages(0, 1, (void **) &ptr, &n2, >>>>>> &status, >>>>>>                                   MPOL_MF_MOVE_ALL); >>>>>>                   if (ret) { >>>>>> -                       if (ret > 0) >>>>>> +                       if (ret > 0) { >>>>>> +                               if (++errors < 100) >>>>>> +                                       continue; >>>>>>                                   printf("Didn't migrate %d pages\n", >>>>>> ret); >>>>>> -                       else >>>>>> +                       } else { >>>>>>                                   perror("Couldn't migrate pages"); >>>>>> +                       } >>>>>>                           return -2; >>>>>>                   } >>>>>> +               /* Progress! */ >>>>>> +               errors = 0; >>>>>> >>>>>>                   tmp = n2; >>>>>>                   n2 = n1; >>>>>> >>>>>> >>>>>> [root@localhost mm]# ./migration >>>>>> TAP version 13 >>>>>> 1..1 >>>>>> # Starting 1 tests from 1 test cases. >>>>>> #  RUN           migration.shared_anon ... >>>>>> #            OK  migration.shared_anon >>>>>> ok 1 migration.shared_anon >>>>>> # PASSED: 1 / 1 tests passed. >>>>>> # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0 >>>>> >>>>> >>>>> This does make the test pass, to my surprise, since what you are doing >>>>> from userspace >>>>> >>>>> should have been done by the kernel, because it retries folio >>>>> unmapping >>>>> and moving >>>>> >>>>> NR_MAX_MIGRATE_(A)SYNC_RETRY times; I had already tested pumping up >>>>> these >>>>> >>>>> macros and the original test was still failing. Now, I digged in more, >>>>> and, if the >>>>> >>>>> following assertion is correct: >>>>> >>>>> >>>>> Any thread having a reference on a folio will end up calling >>>>> folio_lock() >>>>> >>>> >>>> Good point. I suspect concurrent things like read/write would also be >>>> able to trigger this (did not check, though). >>>> >>>>> >>>>> then it seems to me that the retry for loop wrapped around >>>>> migrate_folio_move(), inside >>>>> >>>>> migrate_pages_batch(), is useless; if migrate_folio_move() fails on >>>>> the >>>>> first iteration, it is >>>>> >>>>> going to fail for all iterations since, if I am reading the code path >>>>> correctly, the only way it >>>>> >>>>> fails is when the actual refcount is not equal to expected refcount >>>>> (in >>>>> folio_migrate_mapping()), >>>>> >>>>> and there is no way that the extra refcount is going to get released >>>>> since the migration path >>>>> >>>>> has the folio lock. >>>>> >>>>> And therefore, this begs the question: isn't it logical to assert the >>>>> actual refcount against the >>>>> >>>>> expected refcount, just after we have changed the PTEs, so that if >>>>> this >>>>> assertion fails, we can >>>>> >>>>> go to the next iteration of the for loop for migrate_folio_unmap() >>>>> inside migrate_pages_batch() >>>>> >>>>> by calling migrate_folio_undo_src()/dst() to restore the old state? >>>>> I am >>>>> trying to implement >>>>> >>>>> this but is not as straightforward as it seemed to me this morning. >>>> >>>> I agree with your assessment that migration code currently doesn't >>>> handle the case well when some other thread does an unconditional >>>> folio_lock(). folio_trylock() users would be handled, but that's not >>>> what we want with FGP_LOCK I assume. >>>> >>>> So IIUC, your idea would be to unlock the folio in migration code and >>>> try again their. Sounds reasonable, without looking into the details :) >>> >>> >> >> BTW, I was trying to find the spot that would do the folio_lock(), but >> filemap_fault() does the lock_folio_maybe_drop_mmap() where we do a >> folio_trylock(). >> >> Where exactly is the folio_lock() on the fault path that would >> prohibit us from making progress? > > Not filemap_fault(); it enters shmem_fault() which eventually calls > shmem_get_folio_gfp(), retrieving the folio from the pagecache, and > calling folio_lock(). Ah, thanks! ... which raises the question if we should handle it similar to filemap_fault(), essentially drop the reference and retry using VM_FAULT_RETRY. Hmmmmm -- Cheers, David / dhildenb