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[2003:cb:c708:e100:4f41:ff29:a59f:8c7a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e4c1f2esm5266467f8f.98.2025.01.10.11.17.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Jan 2025 11:17:04 -0800 (PST) Message-ID: <25e02685-4f1d-47fa-be5b-01ff85bb0ce2@redhat.com> Date: Fri, 10 Jan 2025 20:17:02 +0100 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: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: fix memory loads ordering From: David Hildenbrand To: Matthew Wilcox Cc: Yu Zhao , Andrew Morton , Mateusz Guzik , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Will Deacon References: <20250107043505.351925-1-yuzhao@google.com> <85ea0bc9-0892-489d-b42b-430ff8a1f368@redhat.com> <559ea5ca-fea2-4a0a-a099-23e2d972f6e6@redhat.com> <57447786-9184-427b-b5de-321784e946f5@redhat.com> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.01.25 20:04, David Hildenbrand wrote: > On 07.01.25 18:02, David Hildenbrand wrote: >> On 07.01.25 17:35, Matthew Wilcox wrote: >>> On Tue, Jan 07, 2025 at 09:49:18AM +0100, David Hildenbrand wrote: >>>>> +++ b/include/linux/page-flags.h >>>>> @@ -212,7 +212,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page >>>>> * cold cacheline in some cases. >>>>> */ >>>>> if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) && >>>>> - test_bit(PG_head, &page->flags)) { >>>>> + test_bit_acquire(PG_head, &page->flags)) { >>>> >>>> This change will affect all page_fixed_fake_head() users, like ordinary >>>> PageTail even on !hugetlb. >>> >>> I've been looking at the callers of PageTail() because it's going to >>> be a bit of a weird thing to be checking in the separate-page-and-folio >>> world. Obviously we can implement it, but there's a bit of a "But why >>> would you want to ask that question" question. >>> >>> Most current occurrences of PageTail() are in assertions of one form or >>> another. Fair enough, not performance critical. >>> >>> make_device_exclusive_range() is a little weird; looks like it's trying >>> to make sure that each folio is only made exclusive once, and ignore any >>> partial folios which overlap the start of the area. >> >> I could have sworn we only support small folios here, but looks like >> we do support large folios. >> >> IIUC, there is no way to identify reliably "this folio is device exclusive", >> the only hint is "no mappings". The following might do: >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index c6c4d4ea29a7e..1424d0a351a86 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2543,7 +2543,13 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start, >> >> for (i = 0; i < npages; i++, start += PAGE_SIZE) { >> struct folio *folio = page_folio(pages[i]); >> - if (PageTail(pages[i]) || !folio_trylock(folio)) { >> + >> + /* >> + * If there are no mappings, either the folio is actually >> + * unmapped or only device-exclusive swap entries point at >> + * this folio. >> + */ >> + if (!folio_mapped(folio) || !folio_trylock(folio)) { >> folio_put(folio); >> pages[i] = NULL; >> continue; > > I stared longer at this, and not sure if that will work. > > The PageTail() is in place because we return with the folio locked on > success, so we won't trylock again on tail pages. > > But staring at page_make_device_exclusive_one(), I am not sure if it > does what we want in all cases ... > > ... and the hmm selftests just keeps failing upstream as well?! huh. :) > > I'll try spending some time on this to see if I can grasp what needs to > be done and how it could be handled ... better. > As expected ... # echo never > /sys/kernel/mm/transparent_hugepage/enabled # ./hmm-tests ... # RUN hmm.hmm_device_private.exclusive ... # OK hmm.hmm_device_private.exclusive ok 21 hmm.hmm_device_private.exclusive # RUN hmm.hmm_device_private.exclusive_mprotect ... # OK hmm.hmm_device_private.exclusive_mprotect ok 22 hmm.hmm_device_private.exclusive_mprotect # RUN hmm.hmm_device_private.exclusive_cow ... # OK hmm.hmm_device_private.exclusive_cow ok 23 hmm.hmm_device_private.exclusive_cow # RUN hmm.hmm_device_private.hmm_gup_test ... # OK hmm.hmm_device_private.hmm_gup_test ... # echo always > /sys/kernel/mm/transparent_hugepage/enabled ... # RUN hmm.hmm_device_private.exclusive ... # hmm-tests.c:1751:exclusive:Expected ret (-16) == 0 (0) # exclusive: Test terminated by assertion # FAIL hmm.hmm_device_private.exclusive not ok 21 hmm.hmm_device_private.exclusive # RUN hmm.hmm_device_private.exclusive_mprotect ... # hmm-tests.c:1805:exclusive_mprotect:Expected ret (-16) == 0 (0) # exclusive_mprotect: Test terminated by assertion # FAIL hmm.hmm_device_private.exclusive_mprotect not ok 22 hmm.hmm_device_private.exclusive_mprotect # RUN hmm.hmm_device_private.exclusive_cow ... # hmm-tests.c:1858:exclusive_cow:Expected ret (-16) == 0 (0) # exclusive_cow: Test terminated by assertion # FAIL hmm.hmm_device_private.exclusive_cow not ok 23 hmm.hmm_device_private.exclusive_cow So rejecting folio_test_large() would likely achieve the same thing right now. -- Cheers, David / dhildenb