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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36d27209196sm5348024f8f.77.2024.08.09.06.26.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Aug 2024 06:26:18 -0700 (PDT) Message-ID: Date: Fri, 9 Aug 2024 15:26:16 +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 From: David Hildenbrand 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> <79924d5b-b807-440a-a6e8-8f0ae13aa04b@redhat.com> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 09.08.24 15:23, David Hildenbrand wrote: > 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 ... just had another look at lock_folio_maybe_drop_mmap(), and we also usually end up in __folio_lock() / __folio_lock_killable(), folio_trylock() is only used for the fast path. So no, that wouldn't change that much. -- Cheers, David / dhildenb