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Received: from willy by casper.infradead.org with local (Exim 4.98 #2 (Red Hat Linux)) id 1t9VGe-00000009GQk-02b6; Fri, 08 Nov 2024 20:07:40 +0000 Date: Fri, 8 Nov 2024 20:07:39 +0000 From: Matthew Wilcox To: Jens Axboe Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 11/13] iomap: make buffered writes work with RWF_UNCACHED Message-ID: References: <20241108174505.1214230-1-axboe@kernel.dk> <20241108174505.1214230-12-axboe@kernel.dk> <45ac1a3c-7198-4f5b-b6e3-c980c425f944@kernel.dk> <30f5066a-0d3a-425f-a336-16a2af330473@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30f5066a-0d3a-425f-a336-16a2af330473@kernel.dk> X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: D377810000C X-Stat-Signature: x1afbo7uarsaeot7warjkju5bwjt9z3t X-Rspam-User: X-HE-Tag: 1731096393-770068 X-HE-Meta: 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 eUdvbzOF pmiHQr0KrwmdOEaTgvNEq8hnL+9xk9Fu24aBJkNUUWdx5OBPIxmbegPtw/jmBvxEPsa6ZyzHNC77LHX17oUWlsTUUG8JY9sgVnVRkYlYoI7nxJ6H3VqFvMh+VBbroMN3ivJY63AFIFtaX2zXumc2LCvptemerHerhPppjHKK6I/7RQ0nYfrfEytlF1EX08kflUIKgjbu+OMV4nNy/gy0dssen1ldNQOflilnimFVFreWA9PYfjrmJY8BH1D7zUjWAn5qR2SuDlIBgh3vuB8/qQjk8yA== 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: List-Subscribe: List-Unsubscribe: On Fri, Nov 08, 2024 at 12:49:58PM -0700, Jens Axboe wrote: > On 11/8/24 12:26 PM, Jens Axboe wrote: > > On 11/8/24 11:46 AM, Matthew Wilcox wrote: > >> On Fri, Nov 08, 2024 at 10:43:34AM -0700, Jens Axboe wrote: > >>> +++ b/fs/iomap/buffered-io.c > >>> @@ -959,6 +959,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i) > >>> } > >>> if (iter->iomap.flags & IOMAP_F_STALE) > >>> break; > >>> + if (iter->flags & IOMAP_UNCACHED) > >>> + folio_set_uncached(folio); > >> > >> This seems like it'd convert an existing page cache folio into being > >> uncached? Is this just leftover from a previous version or is that a > >> design decision you made? > > > > I'll see if we can improve that. Currently both the read and write side > > do drop whatever it touches. We could feasibly just have it drop > > newly instantiated pages - iow, uncached just won't create new persistent > > folios, but it'll happily use the ones that are there already. > > Well that was nonsense on the read side, it deliberately only prunes > entries that has uncached set. For the write side, this is a bit > trickier. We'd essentially need to know if the folio populated by > write_begin was found in the page cache, or create from new. Any way we > can do that? One way is to change ->write_begin() so it takes a kiocb > rather than a file, but that's an amount of churn I'd rather avoid! > Maybe there's a way I'm just not seeing? Umm. We can solve it for iomap with a new FGP_UNCACHED flag and checking IOMAP_UNCACHED in iomap_get_folio(). Not sure how we solve it for other filesystems though. Any filesystem which uses FGP_NOWAIT has _a_ solution, but eg btrfs will need to plumb through a third boolean flag (or, more efficiently, just start passing FGP flags to prepare_one_folio()).