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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qlmxn-001Zto-Gx; Thu, 28 Sep 2023 09:05:39 +0000 Date: Thu, 28 Sep 2023 10:05:39 +0100 From: Matthew Wilcox To: Yang Shi Cc: "Vishal Moola (Oracle)" , linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/2] mm/khugepaged: Remove compound_pagelist Message-ID: References: <20230922193639.10158-1-vishal.moola@gmail.com> <20230922193639.10158-3-vishal.moola@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: A61EB40017 X-Stat-Signature: mbr4d97fxobgbis74iqn1nff3kesn78y X-HE-Tag: 1695891944-26361 X-HE-Meta: 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 itUQpnVX +bafUtpqShYtiNNTkXNlJ2zzGXJN/qRV362BqYMS69LT05PHBedZNebLwI3Y/Fk6nkh1c24GUKxINJyfbGIWZVbrAl2b5JJC5DwlSTo2VYOOjou6Acpo2WAWnnDoqpd8t3CcmXHTaolg6+gJG30fgDVci9N1+ySt/2g+8S2QFpLzcHc/xl+keZeTrJxTlOggsWdIUXykNhuI/GDk423VrGgclmScOTULahbUx5Pco3AEoNrKIdPaNRjpeI+ZTbXNu/EBAI4ZkpFwmJvyZQ/NzIv2CTg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000008, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Sep 26, 2023 at 03:07:18PM -0700, Yang Shi wrote: > On Fri, Sep 22, 2023 at 9:33 PM Vishal Moola (Oracle) > wrote: > > > > Currently, khugepaged builds a compound_pagelist while scanning, which > > is used to properly account for compound pages. We can now account > > for a compound page as a singular folio instead, so remove this list. > > > > Large folios are guaranteed to have consecutive ptes and addresses, so > > once the first pte of a large folio is found skip over the rest. > > The address space may just map a partial folio, for example, in the > extreme case the HUGE_PMD size range may have HUGE_PMD_NR folios with > mapping one subpage from each folio per PTE. So assuming the PTE > mapped folio is mapped consecutively may be wrong. How? You can do that with two VMAs, but this is limited to scanning within a single VMA. If we've COWed a large folio, we currently do so as a single page folio, and I'm not seeing any demand to change that. If we did COW as a large folio, we'd COW every page in that folio. How do we interleave two large folios in the same VMA? > Please refer to collapse_compound_extreme() in > tools/testing/selftests/mm/khugepaged.c. I agree that running that part of the test-suite would be useful, but could you point to which test specifically would create a problem here?