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[70.27.3.10]) by smtp.gmail.com with ESMTPSA id d5-20020ac86685000000b00342f7beccd0sm8823057qtp.45.2022.08.31.11.52.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 Aug 2022 11:52:51 -0700 (PDT) Date: Wed, 31 Aug 2022 14:52:49 -0400 From: Peter Xu To: Yang Shi Cc: David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Andrea Arcangeli , Hugh Dickins , "Kirill A. Shutemov" Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page() Message-ID: References: <20220831083024.37138-1-david@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline ARC-Authentication-Results: i=1; imf29.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=WzejkKiE; spf=pass (imf29.hostedemail.com: domain of peterx@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661971974; a=rsa-sha256; cv=none; b=nSX9W7DMzJWNc8XS3xRscquw1Zu2SuDb+NwI/yHlor5pDLwhV2Q7sU0Sh5Zl57msZwc/l2 0dpwOy9dwybZRl+gEDn6TxPgAAufKE404O03489bc9XbV/N/Q3LAqgmOfsDfsnfrc9fnxk tyu3xOuvKBd/1GssC0zJcUMNE8yTTyI= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661971974; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=rMGNs2YUPmBpJtRzjX0jo2J/6RpATG/lLCVwNu3BD9E=; b=yZ/hmwp+Cw4rBMO7nKSvAH2SbCjiVNeZIfBdn8C8PE7fskWfVjyfW481ElskkSdd88OUTi O2Z0kbsz0Rvr/9H62OpfFmcHeAE4mHGqW0KKtHzzNdz/pvTMh2FODlI9kD1uVR5/8e+hRa E2KOfPV0MpvHvA2IV4J2PZM7Zz9gbNE= Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=WzejkKiE; spf=pass (imf29.hostedemail.com: domain of peterx@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspam-User: X-Stat-Signature: o1mafqtrh6gxwb4igy68u5d88gt73mhc X-Rspamd-Queue-Id: 12F0112007F X-Rspamd-Server: rspam09 X-HE-Tag: 1661971973-557277 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, Aug 31, 2022 at 10:55:43AM -0700, 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 Maybe the pmd level tlb flush is still needed, but on pte level it's optional (where we can rely on fast-gup rechecking on the pte change)? -- Peter Xu