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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42824af6e8csm126805155e9.1.2024.08.05.03.46.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 05 Aug 2024 03:46:50 -0700 (PDT) Message-ID: <418e818a-f385-459e-a84d-e3880ac08ad5@redhat.com> Date: Mon, 5 Aug 2024 12:46:49 +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 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> 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 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 :) -- Cheers, David / dhildenb