From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7B293253332 for ; Fri, 21 Feb 2025 18:42:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740163373; cv=none; b=WAynxW4Z5elp7htZMBV3RuzqvVjsUDFIbyaWCIRV+3BtDjhF2IDvp2mJXqRiofGSkAnDWE+XfjV53tKujfttpTrknoW/mcFm9FaseGmHviRjbXNfwqXSs+14un4vxbunjiP4dI4gId3lLzrp5NDFg8/2aXnRy7X/LKpder0i9Eg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740163373; c=relaxed/simple; bh=9T+Bu1KAKuB3d1ZhNWvshL6O9sVqY8leEIKv+i0gU7c=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Oop809MWNjQdD+UqvYEpIh6gVsG7To/eF7qTcCfc1EyxbD4lajUB4goxwwJpOJfQ+Tfjk5fXdPd/gYwvLIz4BwxfuOV7mhtaNvM1HB9hqv/vPoBl/NW86iEFg555oDR2VHEbLxAnuAwgdjvE7BEHdnZ7GEWhYoirGuMjqwwmeDc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ri4FxxOL; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ri4FxxOL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1CB42C4CEE4; Fri, 21 Feb 2025 18:42:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1740163373; bh=9T+Bu1KAKuB3d1ZhNWvshL6O9sVqY8leEIKv+i0gU7c=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=ri4FxxOL9kJoLB6pt3B+yDWjrHEhjrHgE00QjLcVSRh8OLSuuF74tstzhznoD3Pqd dCZ04kwgAwqzqLdWkY4BTKD5ovJERr3ZSBmO6sgReR/K7IN7bvlyTxc8ZCZXtNrsSb jYYXmF+3Tv/PUEWsJKRKG28hygqD8JJzEz1dGcnyhaQxIQ0+103HsvqfUFF1zxoEU4 elH+aiLgMBUnimBoLJw0GDoR5rBjII6DyJLOAn9QmEZtoJSW0x94KrM1dK/Kdo7lQM neFCYfZkWMBVgR9JdON8mZ2GzXHJPiHWN/3EUySPP2RhZgMBe3uj1ESM/f5ZJDOgf5 UsouxwpJOIEfA== Message-ID: <42400116f9098ec7f5acc70c2450dd52a2bf8f21.camel@kernel.org> Subject: Re: nfsd: add the ability to enable use of RWF_DONTCACHE for all nfsd IO From: Jeff Layton To: Trond Myklebust , "snitzer@kernel.org" , "chuck.lever@oracle.com" Cc: "okorniev@redhat.com" , "tom@talpey.com" , "linux-nfs@vger.kernel.org" , "Dai.Ngo@oracle.com" , "neilb@suse.de" , "axboe@kernel.dk" Date: Fri, 21 Feb 2025 13:42:50 -0500 In-Reply-To: <7b1574e2499da99986c432f815abccb2e5a6c7f5.camel@hammerspace.com> References: <20250220171205.12092-1-snitzer@kernel.org> <6bd2aa18-e52b-47e6-9151-4ff80d1a39b8@oracle.com> <7b1574e2499da99986c432f815abccb2e5a6c7f5.camel@hammerspace.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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Evolution 3.54.3 (3.54.3-1.fc41) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2025-02-21 at 16:13 +0000, Trond Myklebust wrote: > On Fri, 2025-02-21 at 10:46 -0500, Chuck Lever wrote: > > On 2/21/25 10:36 AM, Mike Snitzer wrote: > > > On Fri, Feb 21, 2025 at 10:25:03AM -0500, Jeff Layton wrote: > > > > On Fri, 2025-02-21 at 10:02 -0500, Mike Snitzer wrote: > > > > > My intent was to make 6.14's DONTCACHE feature able to be > > > > > tested in > > > > > the context of nfsd in a no-frills way.=C2=A0 I realize adding th= e > > > > > nfsd_dontcache knob skews toward too raw, lacks polish.=C2=A0 But > > > > > I'm > > > > > inclined to expose such course-grained opt-in knobs to > > > > > encourage > > > > > others' discovery (and answers to some of the questions you > > > > > pose > > > > > below).=C2=A0 I also hope to enlist all NFSD reviewers' help in > > > > > categorizing/documenting where DONTCACHE helps/hurts. ;) > > > > >=20 > > > > > And I agree that ultimately per-export control is needed.=C2=A0 I= 'll > > > > > take > > > > > the time to implement that, hopeful to have something more > > > > > suitable in > > > > > time for LSF. > > > >=20 > > > > Would it make more sense to hook DONTCACHE up to the IO_ADVISE > > > > operation in RFC7862? IO_ADVISE4_NOREUSE sounds like it has > > > > similar > > > > meaning? That would give the clients a way to do this on a per- > > > > open > > > > basis. > > >=20 > > > Just thinking aloud here but: Using a DONTCACHE scalpel on a per > > > open > > > basis quite likely wouldn't provide the required page reclaim > > > relief > > > if the server is being hammered with normal buffered IO.=C2=A0 Sure t= hat > > > particular DONTCACHE IO wouldn't contribute to the problem but it > > > would still be impacted by those not opting to use DONTCACHE on > > > entry > > > to the server due to needing pages for its DONTCACHE buffered IO. > >=20 > > For this initial work, which is to provide a mechanism for > > experimentation, IMO exposing the setting to clients won't be all > > that helpful. > >=20 > > But there are some applications/workloads on clients where exposure > > could be beneficial -- for instance, a backup job, where NFSD would > > benefit by knowing it doesn't have to maintain the job's written data > > in > > its page cache. I regard that as a later evolutionary improvement, > > though. > >=20 > > Jorge proposed adding the NFSv4.2 IO_ADVISE operation to NFSD, but I > > think we first need to a) work out and document appropriate semantics > > for each hint, because the spec does not provide specifics, and b) > > perform some extensive benchmarking to understand their value and > > impact. > >=20 > >=20 >=20 > That puts the onus on the application running on the client to decide > the caching semantics of the server which: > A. Is a terrible idea=E2=84=A2. The application may know how it wants = to use > the cached data, and be able to somewhat confidently manage its > own pagecache. However in almost all cases, it will have no basis > for understanding how the server should manage its cache. The > latter really is a job for the sysadmin to figure out. > B. Is impractical, because even if you can figure out a policy, it > requires rewriting the application to manage the server cache. > C. Will require additional APIs on the NFSv4.2 client to expose the > IO_ADVISE operation. You cannot just map it to posix_fadvise() > and/or posix_madvise(), because IO_ADVISE is designed to manage a > completely different caching layer. At best, we might be able to > rally one or two more distributed filesystems to implement > similar functionality and share an API, however there is no > chance this API will be useful for ordinary filesystems. >=20 You could map this to RWF_DONTCACHE itself. I know that's really intended as a hint to the local kernel, but it seems reasonable that if the application is giving the kernel a DONTCACHE hint, we could pass that along to the server as well. The server is under no obligation to do anything with it, just like the kernel with RWF_DONTCACHE. We could put an IO_ADVISE in a READ or READ_PLUS compound like so: PUTFH + IO_ADVISE(IO_ADVISE_NOREUSE for ranges being read) + READ_PLUS = or READ ... On the server, we could track those ranges in the compound and enable RWF_DONTCACHE for any subsequent reads or writes. All that said, I don't object to some sort of mechanism to turn this on more globally, particularly since that would allow us to use this with v3 I/O as well. --=20 Jeff Layton