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[2003:cb:c708:1a00:df86:93fe:6505:d096]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36bbd01efe5sm15754160f8f.46.2024.08.07.04.39.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 07 Aug 2024 04:39:40 -0700 (PDT) Message-ID: Date: Wed, 7 Aug 2024 13:39:38 +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> 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 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? > (Adding Mel if at all he has any comments for a compaction use-case) > > What I was trying to say is this (forgive me for the hard-coded value): The hard-coded 2 is wrong indeed :) > > diff --git a/mm/migrate.c b/mm/migrate.c > index a8c6f466e33a..404af46dd661 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1262,6 +1262,8 @@ static int migrate_folio_unmap(new_folio_t > get_new_folio, > } > if (!folio_mapped(src)) { > + if (folio_ref_count(src) != 2) > + goto out; > __migrate_folio_record(dst, old_page_state, anon_vma); > return MIGRATEPAGE_UNMAP; > } > This will give us more chances to win the race. On an average, now > the test fails on 100 iterations of move_pages(). If you multiply > the NR_MAX_PAGES_(A)SYNC_RETRY macros by 3, the average goes above > to 2000. > But if the consensus is that this is just pleasing the test without > any real use-case (compaction?) then I guess I am alright with making > the change in the test. I'd be curious if other activity (besides fault, like concurrent read()/write()/...) can similarly make migration fail. I suspect it can. -- Cheers, David / dhildenb