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Thu, 20 Aug 2026 22:20:03 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id h0w9IZN9h2qQMAAAD6G6ig (envelope-from ); Thu, 20 Aug 2026 22:20:03 +0000 Date: Thu, 20 Aug 2026 23:20:02 +0100 From: Pedro Falcato To: Lance Yang Cc: kas@kernel.org, usama.arif@linux.dev, hughd@google.com, akpm@linux-foundation.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, david@kernel.org, dev.jain@arm.com, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, nico.pache@linux.dev, ryan.roberts@arm.com, ziy@nvidia.com, nphamcs@gmail.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com, stable@vger.kernel.org, willy@infradead.org Subject: Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap Message-ID: References: <20260820165334.36401-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260820165334.36401-1-lance.yang@linux.dev> X-Rspamd-Action: no action X-Rspam-User: X-Stat-Signature: wwj7cyfhiibff1fs5ypx6rkszewfb4tm X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: F303120003 X-HE-Tag: 1787264414-10264 X-HE-Meta: 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 GNt5oefv k2C2FZAfmFnz7wDp0lNgXIxjmxeTf7N9KIM00MdC+klznHJBQr3M4E7UgvfMFyAvlex+avdqYnobwsm5PRTA2EEFOTb4cwKtD8z6Rgr6tosvjBmiJEfpHpIa2UZQvlMfjzPZCJVJTAqBsWZQ/aqKZeZ5KpefFM2NLXv4L1diGPuOhdgcXXweD2zXP648EddlpeWucwmXA2Iv3mB99I6gtgBcOikkSOS7nlhP4vHPWjFlR1UvY8dscNTU0Gn8KhtrWDg3P/RIkmRUI0I8mF6AlG5wlaazke0zOe6AKUuNLij3Ulflf+7iWPDQwtw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 21, 2026 at 12:53:34AM +0800, Lance Yang wrote: > > >For some context: we were discussing (off-list) the recent report that > >systemd-journald had horrible write amplification, worsed quite a bit > >by large folios. At the moment, there is quite a lot of write amplification, > >but _only_ on mmap writes (if you look at the write(2) paths, you'll see > >write_begin and write_end which tactically only dirty what you actually wrote > >to, block-wise in the BHs or iomap IFS). This doesn't need to be true. > > > >What we conjured up was the following: since WP faults always happen on PTEs, > >you can simply dirty the block(s) corresponding to that particular PTE (which > >naturally points to a page inside the folio, whether it is large or not, doesn't > >matter). That way, you can avoid fully dirtying the folio's blocks. This is > >something that ATM isn't done by any filesystem, but it really should. > > > >Obviously this whole idea is thwarted if we keep writable shared file folios > >PMD-mapped - you can't get meaningful write notifications apart from "someone > >wrote to this folio", which isn't particularly useful once folios get 2M+ large. > >So PMD-mapping on a shared-write fault is antithetical to getting useful, > >granular write notifications to filesystems. > > Just checking I got it right ... for a PMD-order file folio: > > read-only mapping -> PMD-mapped > writable mapping -> PTE-mapped > > Then ->page_mkwrite() could dirty only blocks corresponding to faulting > PTE, avoiding whole-folio write amplification. Yes. To be clear, when I said "mapping" I meant "page table (PTE/PMD) entry". So something like an mmap(PROT_WRITE, MAP_SHARED) could have PMD-mapped folios, until you actually wrote to them (otherwise they're wp'd and huge-PMD-mapped). -- Pedro