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 A87812222CA; Wed, 11 Jun 2025 20:30:00 +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=1749673800; cv=none; b=rMk7UQkoGqFOFJRiUHBsgx0zv/29XU2aNljrmPwyYRI6GRBd64U6GDPYWDD/ddjgGqkeFV4azhzkCQx7wxUODewx0CVIyu8A1j+jSKzPv9PIoCohopgV27LpeI6Ee5T1K42CPN7ZY/K7HYb48t1f9wiGycrB6k6bCfCM9s2w03c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749673800; c=relaxed/simple; bh=DIsIY7/GZbjqILkdCtA1CtbQchPs3eicmrju4/ybhqU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=MSlYLANKy+BI0RmF5kX4RvwwlWr66f69x3xiORT947ky6IicyGH7EmGJXjlo7wfWGDbNe91qfOW66bshTIaX/W3iZMN3zGXu2x0oJA6jW+suB5x6EeLA4CeoR/WbHCRd6e85d1gxuLY9VqYChdjlAIJBBla2TYECztWadJU7lGw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OHHZEqL5; 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="OHHZEqL5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DF423C4CEE3; Wed, 11 Jun 2025 20:29:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749673800; bh=DIsIY7/GZbjqILkdCtA1CtbQchPs3eicmrju4/ybhqU=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=OHHZEqL5RV0OeA4TV/XI67vDerBa6tWsHGR0nu/BqmC8iyLAtlUTcPPtSzKeJs57E c6ImI22F/jB0kn4X1n95CL+TNAtWvh68k8NGw4YVgFW7Jt+XXSHLQemhDG7nnzevFX TIIW8TL5XHJP5MxMCgjnd4NpT7ux5TOAXmUa8LVCnB/+Sp6yWaBKVsLvqZxVRBrfZs c5xEba0DFIx+tRU1fkO/PTMXpMO7R0TYHZsYqtp0iVXtB4F+uV4ArlEfnD2qwx9DV9 h6mQFbwhhggIM+t3mSQcYWa2B+9Qfi63MKTOVawrlIjwnT2XlCt4PVLVQa6nv9DyyH mj0tWgTkzhb4g== Message-ID: <7c48e17c4b575375069a4bd965f346499e66ac3a.camel@kernel.org> Subject: Re: [PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO From: Jeff Layton To: Mike Snitzer , Chuck Lever Cc: linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Jens Axboe Date: Wed, 11 Jun 2025 16:29:58 -0400 In-Reply-To: References: <20250610205737.63343-1-snitzer@kernel.org> <20250610205737.63343-2-snitzer@kernel.org> <4b858fb1-25f6-457f-8908-67339e20318e@oracle.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Wed, 2025-06-11 at 15:18 -0400, Mike Snitzer wrote: > On Wed, Jun 11, 2025 at 10:31:20AM -0400, Chuck Lever wrote: > > On 6/10/25 4:57 PM, Mike Snitzer wrote: > > > Add 'enable-dontcache' to NFSD's debugfs interface so that: Any data > > > read or written by NFSD will either not be cached (thanks to O_DIRECT= ) > > > or will be removed from the page cache upon completion (DONTCACHE). > >=20 > > I thought we were going to do two switches: One for reads and one for > > writes? I could be misremembering. >=20 > We did discuss the possibility of doing that. Still can-do if that's > what you'd prefer. > =20 Having them as separate controls in debugfs is fine for experimentation's sake, but I imagine we'll need to be all-in one way or the other with a real interface. I think if we can crack the problem of receiving WRITE payloads into an already-aligned buffer, then that becomes much more feasible. I think that's a solveable problem. > > After all, you are describing two different facilities here: a form of > > direct I/O for READs, and RWF_DONTCACHE for WRITEs (I think?). >=20 > My thinking was NFSD doesn't need to provide faithful pure > RWF_DONTCACHE if it really doesn't make sense. But the "dontcache" > name can be (ab)used by NFSD to define it how it sees fit (O_DIRECT > doesn't cache so it seems fair). What I arrived at with this patchset > is how I described in my cover letter: >=20 > When 'enable-dontcache' is used: > - all READs will use O_DIRECT (both DIO-aligned and misaligned) > - all DIO-aligned WRITEs will use O_DIRECT (useful for SUNRPC RDMA) > - misaligned WRITEs currently continue to use normal buffered IO >=20 > But we reserve the right to iterate on the implementation details as > we see fit. Still using the umbrella of 'dontcache'. > > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > > > index 7d94fae1dee8..bba3e6f4f56b 100644 > > > --- a/fs/nfsd/vfs.c > > > +++ b/fs/nfsd/vfs.c > > > @@ -49,6 +49,7 @@ > > > #define NFSDDBG_FACILITY NFSDDBG_FILEOP > > > =20 > > > bool nfsd_disable_splice_read __read_mostly; > > > +bool nfsd_enable_dontcache __read_mostly; > > > =20 > > > /** > > > * nfserrno - Map Linux errnos to NFS errnos > > > @@ -1086,6 +1087,7 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, s= truct svc_fh *fhp, > > > unsigned long v, total; > > > struct iov_iter iter; > > > loff_t ppos =3D offset; > > > + rwf_t flags =3D 0; > > > ssize_t host_err; > > > size_t len; > > > =20 > > > @@ -1103,7 +1105,11 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, = struct svc_fh *fhp, > > > =20 > > > trace_nfsd_read_vector(rqstp, fhp, offset, *count); > > > iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count); > > > - host_err =3D vfs_iter_read(file, &iter, &ppos, 0); > > > + > > > + if (nfsd_enable_dontcache) > > > + flags |=3D RWF_DONTCACHE; > >=20 > > Two things: > >=20 > > - Maybe NFSD should record whether the file system is DONTCACHE-enabled > > in @fhp or in the export it is associated with, and then check that > > setting here before asserting RWF_DONTCACHE >=20 > Sure, that'd be safer than allowing RWF_DONTCACHE to be tried only to > get EOPNOTSUPP because the underlying filesystem doesn't enable > support. >=20 > Could follow what I did with nfsd_file only storing the dio_* > alignment data retrieved from statx IFF 'enable-dontcache' was enabled > at the time the nfsd_file was opened. >=20 > By adding check for FOP_DONTCACHE being set in underlying filesystem. > But as-is, we're not actually using RWF_DONTCACHE in the final form of > what I've provided in this patchset. So can easily circle back to > adding this if/when we do decide to use RWF_DONTCACHE. >=20 > > - I thought we were going with O_DIRECT for READs. >=20 > Yes, this is just an intermediate patch that goes away in later > patches. I was more focused on minimal patch to get the > 'enable-dontcache' debugfs interface in place and tweaking it to its > ultimate form in later patch. >=20 > I put in place a more general framework that can evolve... it being > more free-form (e.g. "don't worry your pretty head about the > implementation details, we'll worry for you"). >=20 > Causes some reviewer angst I suppose, so I can just fold patches to do > away with unused intermediate state. >=20 > Mike --=20 Jeff Layton