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(p200300cbc706290016134308aca32786.dip0.t-ipconnect.de. [2003:cb:c706:2900:1613:4308:aca3:2786]) by smtp.gmail.com with ESMTPSA id m5-20020a5d4a05000000b00225213fd4a9sm13151586wrq.33.2022.08.31.11.29.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Aug 2022 11:29:10 -0700 (PDT) Message-ID: <4845ae71-b7dd-1707-ebc3-2eb3521e7fa0@redhat.com> Date: Wed, 31 Aug 2022 20:29:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Content-Language: en-US To: Yang Shi Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Andrea Arcangeli , Hugh Dickins , Peter Xu , "Kirill A. Shutemov" References: <20220831083024.37138-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page() In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 31.08.22 19:55, Yang Shi wrote: > On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand wrote: >> >> The comment is stale, because a TLB flush is no longer sufficient and >> required to synchronize against concurrent GUP-fast. This used to be true >> in the past, whereby a TLB flush would have implied an IPI on architectures >> that support GUP-fast, resulting in GUP-fast that disables local interrupts >> from completing before completing the flush. > > Hmm... it seems there might be problem for THP collapse IIUC. THP > collapse clears and flushes pmd before doing anything on pte and > relies on interrupt disable of fast GUP to serialize against fast GUP. > But if TLB flush is no longer sufficient, then we may run into the > below race IIUC: > > CPU A CPU B > THP collapse fast GUP > > gup_pmd_range() <-- see valid pmd > > gup_pte_range() <-- work on pte > clear pmd and flush TLB > __collapse_huge_page_isolate() > isolate page <-- before GUP bump refcount > > pin the page > __collapse_huge_page_copy() > copy data to huge page > clear pte (don't flush TLB) > Install huge pmd for huge page > > return the obsolete page Hm, the is_refcount_suitable() check runs while the PTE hasn't been cleared yet. And we don't check if the PMD changed once we're in gup_pte_range(). The comment most certainly should be stale as well -- unless there is some kind of an implicit IPI broadcast being done. 2667f50e8b81 mentions: "The RCU page table free logic coupled with an IPI broadcast on THP split (which is a rare event), allows one to protect a page table walker by merely disabling the interrupts during the walk." I'm not able to quickly locate that IPI broadcast -- maybe there is one being done here (in collapse) as well? -- Thanks, David / dhildenb