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AJvYcCWccDaSxlhfdl0XE1vlqy8l37zz4eMfAGXI1gt/D7G9fDN/swCFws1px6NVa6/eJEnIgGDwkCRb2iix16w=@vger.kernel.org X-Gm-Message-State: AOJu0Yydp6RPCUpD8NT6+TE3mN4LxasZ6duEYq4iEs6kR7s+c/evuSka CYMwxo4FXmE/lIbhFo5kXxz3WscASRdWlv657BI0siTRbbF17mtiBFr4/Sjdu0E= X-Google-Smtp-Source: AGHT+IFLmWi9D/02ZHOGsiKNUZIejllZA3TE1dM/x2+sf2uL7j+2l7Scl+cOm+alMwnqVVLvVvizEQ== X-Received: by 2002:a05:6a21:32a1:b0:1db:f087:5b1d with SMTP id adf61e73a8af0-1dc22b94a7cmr5051588637.37.1731097121568; Fri, 08 Nov 2024 12:18:41 -0800 (PST) Received: from [192.168.1.150] ([198.8.77.157]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2e9a5f52d4csm4023444a91.7.2024.11.08.12.18.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 08 Nov 2024 12:18:41 -0800 (PST) Message-ID: <00b97317-610a-4ae3-8d97-f5871972a401@kernel.dk> Date: Fri, 8 Nov 2024 13:18:40 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 11/13] iomap: make buffered writes work with RWF_UNCACHED To: Matthew Wilcox Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org 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> Content-Language: en-US From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/8/24 1:07 PM, Matthew Wilcox wrote: > 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()). Yeah that's true, forgot we already have the IOMAP_UNCACHED flag there and it's available in creation. Thanks, I'll start with that. -- Jens Axboe