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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b36861b05sm19469420f8f.94.2024.08.01.06.48.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 06:48:24 -0700 (PDT) Message-ID: <9359caf7-81a8-45d9-9787-9009b3b2eed3@redhat.com> Date: Thu, 1 Aug 2024 15:48:22 +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: Will Deacon Cc: Dev Jain , 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 -- Cheers, David / dhildenb