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b=qoZ457FSJ41XR2WrW1mZL2ObusGMYGFNfDToNiar5cUHA7/N1zhs8/UGzIPl+qBhh l8DmXkoSdzLGRJfVjdMOImOuBrEZ3B4XqFyITWWKjXK+K5ULs++oAKjyzLlmPQiYeM CYd3QSDaB2YaqzkkFZGN2S4wE58IrNGcyAbvu4DUw3CHPU0SOnjeff97l97f08xPgG hRj84AfWUv/T7Ik25XpVxdcQf62HkSdaU/Nz5Af+MAVR0VFXRkJNXPzxfKamF4PdvL z92q9kAgps7eJPPoPDRIuqSDpSu8Lnzup7kMob826aT/wCdJi3Iaa4wOrO2kEfHa9c +4jUkPU71Rr6Q== Message-ID: <917a2a5a-f499-4759-8160-c8e7d9c0ed65@kernel.org> Date: Mon, 2 Mar 2026 20:43:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: fix folio isn't locked in softleaf_to_folio() To: Jinjiang Tu , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, fengwei.yin@intel.com, baohua@kernel.org, ryan.roberts@arm.com, linux-mm@kvack.org Cc: wangkefeng.wang@huawei.com, sunnanyong@huawei.com References: <20260225081240.253057-1-tujinjiang@huawei.com> <2a46b99d-7f03-4e47-b0ec-da2475d49af8@huawei.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: A639A4000D X-Stat-Signature: 9px1bw6i97czo9s687sk6qmyhujny4jc X-HE-Tag: 1772480635-773512 X-HE-Meta: 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 uTtTJPAN yhy0bwLb8+u6+79Awo1bi6MsYIH05eYUFEgOTbit8XvgCrmq4XSJdrWcutqM0RDQ/GJZ5pmEjRf5nI0Jr6IQmWfsS91VqKCcID4i+IewyRZlnN/UKuWLob2gZVh4/in+DLADl/TuRVzR7qff3OgLAhOUvjt1Zx0z1bgzlpG50ZOu/HiAw39h1gSegFH+AC8nn/K2xzBl7oZJIbQOPlDf16KvDeOiB3Ip/oC60i2OcNsinrn/GrpnC6SZ/T4SllG0OfoRLifP3Gcp53bSauuxNl2EgA6P9cjg45nEDqd74b9IFOXfs1shaFyiuuu5pDtNNR+kJ2VgOgg8tGbz8cMPmqUG4Dat9NoBFktNr Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/26/26 03:01, Jinjiang Tu wrote: > > 在 2026/2/25 17:15, David Hildenbrand (Arm) 写道: >> On 2/25/26 09:12, Jinjiang Tu wrote: >>> On arm64 server, we found folio that get from migration entry isn't >>> locked >>> in softleaf_to_folio(). This issue triggers when mTHP splitting and >>> zap_nonpresent_ptes() races, and the root cause is lack of memory >>> barrier >>> in softleaf_to_folio(). The race is as follows: >>> >>>     CPU0                                             CPU1 >>> >>> deferred_split_scan()                              zap_nonpresent_ptes() >>>    lock folio >>>    split_folio() >>>      unmap_folio() >>>        change ptes to migration entries >>>      __split_folio_to_order()                         >>> softleaf_to_folio() >>>        set flags(including PG_locked) for tail pages    folio = >>> pfn_folio(softleaf_to_pfn(entry)) >>>        smp_wmb()                                        >>> VM_WARN_ON_ONCE(!folio_test_locked(folio)) >>>        prep_compound_page() for tail pages >>> >> In general, relying on a "struct page" for a migration entry is shaky, >> because it can change any time from being a large folio to being a small >> folio. >> >> So we generally only check properties that would be true for either the >> old (large) or the new (smaller) folio, like folio_test_ksm() or >> folio_test_anon(). >> >> It's important that these properties were written for the new folio >> before a migration entry user might look at the page indeed. >> >> So it's not just about the locked state. >> >>> In __split_folio_to_order(), smp_wmb() guarantees page flags of tail >>> pages >>> are visible before the tail page becomes non-compound. smp_wmb() should >>> be paired with smp_rmb() in softleaf_to_folio(), which is missed. As a >>> result, if zap_nonpresent_ptes() accesses migration entry that stores >>> tail pfn, softleaf_to_folio() may see the updated compound_head of tail >>> page before page->flags. >>> >>> Although the code exists for long time, this issue should only exist >>> after >>> mTHP splitting is supported. For THP splitting, there is only a pmd >>> migration entry >> When splitting, we first install a PTE table, no? >> >> unmap_folio() passes TTU_SPLIT_HUGE_PMD. >> >> and it's impossible to access migration entry that stores > > Indeed, I misunderstood the code. So the fix tag is incorrect. > >>> tail page pfn. >>> >>> To fix it, add missing smp_rmb() if the softleaf entry is migration >>> entry >>> in softleaf_to_folio() and softleaf_to_page(). >>> >>> Fixes: 7dc7c5ef6463 ("mm: allow deferred splitting of arbitrary anon >>> large folios") >>> Signed-off-by: Jinjiang Tu >>> --- >>>   include/linux/leafops.h | 39 ++++++++++++++++++++++++++------------- >>>   1 file changed, 26 insertions(+), 13 deletions(-) >>> >>> diff --git a/include/linux/leafops.h b/include/linux/leafops.h >>> index a9ff94b744f2..f823f390ba6b 100644 >>> --- a/include/linux/leafops.h >>> +++ b/include/linux/leafops.h >>> @@ -371,14 +371,21 @@ static inline unsigned long >>> softleaf_to_pfn(softleaf_t entry) >>>    */ >>>   static inline struct page *softleaf_to_page(softleaf_t entry) >>>   { >>> -    struct page *page = pfn_to_page(softleaf_to_pfn(entry)); >>> +    struct page *page; >>>         VM_WARN_ON_ONCE(!softleaf_has_pfn(entry)); >>> -    /* >>> -     * Any use of migration entries may only occur while the >>> -     * corresponding page is locked >>> -     */ >>> -    VM_WARN_ON_ONCE(softleaf_is_migration(entry) && !PageLocked(page)); >>> + >>> +    page = pfn_to_page(softleaf_to_pfn(entry)); >>> +    if (softleaf_is_migration(entry)) { >>> +        /* See __split_folio_to_order() comment */ >>> +        smp_rmb(); >>> + >>> +        /* >>> +         * Any use of migration entries may only occur while the >>> +         * corresponding page is locked >>> +         */ >>> +        VM_WARN_ON_ONCE(!PageLocked(page)); >>> +    } >> Conceptually, wouldn't the smb_rmb() have to happen *after* the >> page_folio(), like we have in softleaf_to_folio()? > > the comments of page_folio() says: >  * Context: No reference, nor lock is required on @page.  If the caller >  * does not hold a reference, this call may race with a folio split, so >  * it should re-check the folio still contains this page after gaining >  * a reference on the folio. >  * Return: The folio which contains this page. > > The old large folio is locked and freezed during splitting, page_folio() > couldn't be called with reference held, so the result is unstable, the > caller should recheck after gaining a reference, at that time splitting > finishes, and folio_get() contains memory barrier already, ensuring the > folio flags is seen after compound_head. Let me elaborate what I mean: __split_folio_to_order() does: /* setup flags and mappings etc. for the new folio */ new_folio->flags.f = ... new_folio->mapping = ... smp_wmb(); /* Now re-route page_folio(). */ clear_compound_head(new_head); if (new_order) { prep_compound_page(new_head, new_order); folio_set_large_rmappable(new_folio); } So I would expect the opposite direction to do: /* Lookup either the old or the new folio. */ page = pfn_to_page(softleaf_to_pfn(entry)); folio = page_folio(page); /* looks up compound head etc. */ /* Make sure we'll see proper flags, mappings of new folio. */ smp_rmb(); /* Continue using new folio */ folio_test_locked() ... folio_test_anon() ... Instead of page = pfn_to_page(softleaf_to_pfn(entry)); smp_rmb(); PageLocked(page) /* internally does page_folio() */ folio = page_folio(page); Maybe that is fine, but it sure is harder to argue about correctness? -- Cheers, David