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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jan Kara , Rik van Riel , Harry Yoo , Lance Yang , Jann Horn , Alexander Viro , Christian Brauner , "Darrick J. Wong" , Carlos Maiolino , Usama Arif , Pedro Falcato , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [RFC PATCH 0/5] mm: sub-folio dirty tracking for PTE-mapped mmap writes Message-ID: References: <20260903182943.662461-1-kirill@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: 13td6cjtfs6nrqnptemn3cts1kjrzrdb X-Rspamd-Queue-Id: 218951C0008 X-HE-Tag: 1789579190-566005 X-HE-Meta: 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 cgkWvwRe Gkc66ddufYUvp/i9LDvj6zxAnigvwGuxVNjfPcHuZ5iXUAxatHAj28JET8Nvhxlxkvgu5L52NW8iWgGFVHO3GdmJdzZjyTAiMEeXwPepRF9IvZ6gmRbsTNjUtKO1UqmgIM56SmtYos4AkfkyHmhmXouUO3doIPfqnWN/iTfY1NORLZru3HgUvbD8embHYZqzZhGQ1qWID2NjtkvLYxpxAiMNvWcq7kOsG870pKu8cjuCSDeu4OjblLSE0tEFrQXiPya9ZzXURAT+br3xCfG9yrrhA46AZu79Pcr75 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 16, 2026 at 06:09:17PM +0100, Kiryl Shutsemau wrote: > On Wed, Sep 16, 2026 at 05:27:42PM +0100, Matthew Wilcox wrote: > > On Mon, Sep 07, 2026 at 11:15:15AM +0100, Kiryl Shutsemau wrote: > > > My proposal makes the distinction between making the folio writable and > > > making it dirty meaningful. ->page_mkwrite() allocates whatever is needed > > > on the filesystem side to track dirty state and drive writeback for the > > > *folio*, while ->dirty_folio_range() marks part of the folio dirty. > > > > Why do you think that's a meaningful distinction? We create a writable > > PTE because we've taken a page fault for write. There's probably a few > > naoseconds where the PTE is writable+clean before it becomes > > writable+dirty, but even then sometimes we do both pte_mkwrite() and > > pte_mkdirty() as an optimisation in the write fault path. > > This is true for the PTE that the fault was for. But we don't necessarily > want to dirty the other 511 pages at the same time. > > The basic idea is to make the whole folio writable at fault and shift > dirtying to be per-PTE on write to it. Hmm. Is that something that filesystem people want? I suspect not. I think they'd prefer to be told when we write-fault on each page. But let's see if we can gather feedback from FS people. > > It's very odd to be optimising for shared-writable-mmap. This is a > > horrid model for I/O. https://cs.brown.edu/people/acrotty/pubs/p13-crotty.pdf > > Sure. > > But not everybody got the memo[1] :P > > I think it worth considering if we want to make large folio adoption > smoother. > > [1] https://www.reddit.com/r/bcachefs/comments/1vepk4a/comment/p1sn4v1/ Since I'm an idiot, I'm not inclined to be helpful. But I think my proposal does fix this; we call page_mkwrite() for each page fault and then the filesystem knows which pages are actually dirty without having to be told under the PTL when they can't reserve space.