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dkim=pass header.d=gmail.com header.s=20210112 header.b=EqYHk7oQ; spf=pass (imf10.hostedemail.com: domain of shy828301@gmail.com designates 209.85.215.170 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam08 X-Stat-Signature: uc1yeo3bambofqeiejgng1fs5zrusc6u X-Rspamd-Queue-Id: C4756C0072 X-HE-Tag: 1662585842-870501 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 7, 2022 at 2:22 PM Andrew Morton wrote: > > On Wed, 7 Sep 2022 11:01:43 -0700 Yang Shi wrote: > > > Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm: > > introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer > > sufficient to handle concurrent GUP-fast in all cases, it only handles > > traditional IPI-based GUP-fast correctly. On architectures that send > > an IPI broadcast on TLB flush, it works as expected. But on the > > architectures that do not use IPI to broadcast TLB flush, it may have > > the below race: > > > > CPU A CPU B > > THP collapse fast GUP > > gup_pmd_range() <-- see valid pmd > > gup_pte_range() <-- work on pte > > pmdp_collapse_flush() <-- clear pmd and flush > > __collapse_huge_page_isolate() > > check page pinned <-- before GUP bump refcount > > pin the page > > check PTE <-- no change > > __collapse_huge_page_copy() > > copy data to huge page > > ptep_clear() > > install huge pmd for the huge page > > return the stale page > > discard the stale page > > > > The race could be fixed by checking whether PMD is changed or not after > > taking the page pin in fast GUP, just like what it does for PTE. If the > > PMD is changed it means there may be parallel THP collapse, so GUP > > should back off. > > > > Also update the stale comment about serializing against fast GUP in > > khugepaged. > > > > Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()") > > Is this not worth a -stable backport? Yes, I think it is.