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Thu, 20 Aug 2026 16:53:52 +0000 X-Mizu-Trace-ID: ca54f914b327986e X-Migadu-Flow: FLOW_OUT From: Lance Yang To: pfalcato@suse.de 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, Lance Yang Subject: Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap Date: Fri, 21 Aug 2026 00:53:34 +0800 Message-Id: <20260820165334.36401-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 6E9DE180003 X-Stat-Signature: sz31gxogormxabpumxth56tho5wtp98b X-Rspam-User: X-HE-Tag: 1787244834-564062 X-HE-Meta: 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 bmlC3BoW r32w2tHWlSYcxeoai0XZ7yqo39EFecIyrD6FpXG1Ykm8Z03S/XSqP2WPtaW90Sj6ivPrSi0DV85SoECGw+MOp3NTQfgPkrufn2L+3/Ux31wTZnZDIuI71IdUe0XGpK+/2daSJBT8LBzZD0hQMgLazpaCot3u6XCgM5ZlFNV/86jqP4qH/BIHrOXvhloF99meR8fimd+wLu3+3MCxoQnrCE6AG27IshO5zV5JdGX3ReXpIu3E+xTNCsuEiXvTt0LKfYCKTQzISIov8g/osNp9GkNNOT8yUTKmnxQ9+kPEY4rOzoXCEXO86qf0roMpOFRybya5FWTRHbcG/yO0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >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. Is that what you have in mind? Cheers, Lance