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 D6A00239E7D for ; Wed, 10 Sep 2025 11:37:05 +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=1757504225; cv=none; b=Ih3HZGy1hYqcQ+KydVGp7CzUQZXpJsvWnkdW+b0baUK3/0VYD5Iv5eR3BJdC7utA76R1ZlA1SKsvP9pt1arkjxg/lodQtCwzuft8fjRCOPtETPk3hdXw5XCxTyS6iVKobk1XLJZTqLaxRTOtR/01Zo/l+wRvagK9azmKCvT4fmA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757504225; c=relaxed/simple; bh=D2olJUciKC5Du8CAaVaQKhMudJ5nnQH4Wu3BxdUzGc4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=UfoNu+NBRFQh6L4q3+Jdj8a/05MCy5fTmBSIQiC76HjIfugZnb8ap+fRQWM0BP5yGdnNopAARSDizN0HgWAX8KDUq2zlUBDRlFa9CMk4Zp+27ySCJzYD0KsAETecC7BJIo/h44HBsLkUf8hV8AJgHohIy6wYDdZD1SKxUbvct/c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=na1KPN9/; 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="na1KPN9/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C3BE8C4CEF0; Wed, 10 Sep 2025 11:37:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1757504225; bh=D2olJUciKC5Du8CAaVaQKhMudJ5nnQH4Wu3BxdUzGc4=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=na1KPN9/BvkDhu7S6YNNvSYLN6XDO5HDFpxS2sfbxFZ27oIf7zb2Fpr7VzpdXG4x2 zMJg6ONvg5wyNmmB/nylXeuZgMH0sK22njCmOfEOafN6fLJ1bA0OaT4CQd94YVAdoX fr6asNQMVx8MvWhfDsEGQrHdEOiB+H0LZuFsvKjMHa5dWp2nXqyQshdQqodmz3xR3p Isu76IGRhjSs79iD9dr0cXcVCoJKn3rWsEsrQxaZNnARPszpNN6PnJK0S9Ecr5Te0+ OghoML3Cu5T/GZSTcxkcal1sQX1/4Pmyb/JY6IAG9aGQpxheaKMdT4FB3w0Z2ohwNI bK6mnEaTtXNnQ== Message-ID: Subject: Re: [PATCH v1 3/3] NFSD: Implement NFSD_IO_DIRECT for NFS READ From: Jeff Layton To: Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey Cc: linux-nfs@vger.kernel.org, Chuck Lever , Mike Snitzer Date: Wed, 10 Sep 2025 07:37:03 -0400 In-Reply-To: <20250909190525.7214-4-cel@kernel.org> References: <20250909190525.7214-1-cel@kernel.org> <20250909190525.7214-4-cel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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an experimental option that forces NFS READ operations to use > direct I/O instead of reading through the NFS server's page cache. >=20 > There are already other layers of caching: > - The page cache on NFS clients > - The block device underlying the exported file system >=20 > The server's page cache, in many cases, is unlikely to provide > additional benefit. Some benchmarks have demonstrated that the > server's page cache is actively detrimental for workloads whose > working set is larger than the server's available physical memory. >=20 > For instance, on small NFS servers, cached NFS file content can > squeeze out local memory consumers. For large sequential workloads, > an enormous amount of data flows into and out of the page cache > and is consumed by NFS clients exactly once -- caching that data > is expensive to do and totally valueless. >=20 > For now this is a hidden option that can be enabled on test > systems for benchmarking. In the longer term, this option might > be enabled persistently or per-export. When the exported file > system does not support direct I/O, NFSD falls back to using > either DONTCACHE or buffered I/O to fulfill NFS READ requests. > Suggested-by: Mike Snitzer > Signed-off-by: Chuck Lever > --- > fs/nfsd/debugfs.c | 2 ++ > fs/nfsd/nfsd.h | 1 + > fs/nfsd/trace.h | 1 + > fs/nfsd/vfs.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++ > 4 files changed, 82 insertions(+) >=20 > diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c > index ed2b9e066206..00eb1ecef6ac 100644 > --- a/fs/nfsd/debugfs.c > +++ b/fs/nfsd/debugfs.c > @@ -44,6 +44,7 @@ DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, n= fsd_dsr_set, "%llu\n"); > * Contents: > * %0: NFS READ will use buffered IO > * %1: NFS READ will use dontcache (buffered IO w/ dropbehind) > + * %2: NFS READ will use direct IO > * > * This setting takes immediate effect for all NFS versions, > * all exports, and in all NFSD net namespaces. > @@ -64,6 +65,7 @@ static int nfsd_io_cache_read_set(void *data, u64 val) > nfsd_io_cache_read =3D NFSD_IO_BUFFERED; > break; > case NFSD_IO_DONTCACHE: > + case NFSD_IO_DIRECT: > /* > * Must disable splice_read when enabling > * NFSD_IO_DONTCACHE. > diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h > index ea87b42894dd..bdb60ee1f1a4 100644 > --- a/fs/nfsd/nfsd.h > +++ b/fs/nfsd/nfsd.h > @@ -157,6 +157,7 @@ enum { > /* Any new NFSD_IO enum value must be added at the end */ > NFSD_IO_BUFFERED, > NFSD_IO_DONTCACHE, > + NFSD_IO_DIRECT, > }; > =20 > extern u64 nfsd_io_cache_read __read_mostly; > diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h > index e5af0d058fd0..88901df5fbb1 100644 > --- a/fs/nfsd/trace.h > +++ b/fs/nfsd/trace.h > @@ -464,6 +464,7 @@ DEFINE_EVENT(nfsd_io_class, nfsd_##name, \ > DEFINE_NFSD_IO_EVENT(read_start); > DEFINE_NFSD_IO_EVENT(read_splice); > DEFINE_NFSD_IO_EVENT(read_vector); > +DEFINE_NFSD_IO_EVENT(read_direct); > DEFINE_NFSD_IO_EVENT(read_io_done); > DEFINE_NFSD_IO_EVENT(read_done); > DEFINE_NFSD_IO_EVENT(write_start); > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > index 441267d877f9..9665454743eb 100644 > --- a/fs/nfsd/vfs.c > +++ b/fs/nfsd/vfs.c > @@ -1074,6 +1074,79 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, st= ruct svc_fh *fhp, > return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err)= ; > } > =20 > +/* > + * The byte range of the client's READ request is expanded on both > + * ends until it meets the underlying file system's direct I/O > + * alignment requirements. After the internal read is complete, the > + * byte range of the NFS READ payload is reduced to the byte range > + * that was originally requested. > + * > + * Note that a direct read can be done only when the xdr_buf > + * containing the NFS READ reply does not already have contents in > + * its .pages array. This is due to potentially restrictive > + * alignment requirements on the read buffer. When .page_len and > + * @base are zero, the .pages array is guaranteed to be page- > + * aligned. > + */ > +static noinline_for_stack __be32 > +nfsd_direct_read(struct svc_rqst *rqstp, struct svc_fh *fhp, > + struct nfsd_file *nf, loff_t offset, unsigned long *count, > + u32 *eof) > +{ > + loff_t dio_start, dio_end; > + unsigned long v, total; > + struct iov_iter iter; > + struct kiocb kiocb; > + ssize_t host_err; > + size_t len; > + > + init_sync_kiocb(&kiocb, nf->nf_file); > + kiocb.ki_flags |=3D IOCB_DIRECT; > + > + /* Read a properly-aligned region of bytes into rq_bvec */ > + dio_start =3D round_down(offset, nf->nf_dio_read_offset_align); > + dio_end =3D round_up(offset + *count, nf->nf_dio_read_offset_align); > + > + kiocb.ki_pos =3D dio_start; > + > + v =3D 0; > + total =3D dio_end - dio_start; > + while (total) { > + len =3D min_t(size_t, total, PAGE_SIZE); > + bvec_set_page(&rqstp->rq_bvec[v], *(rqstp->rq_next_page++), > + len, 0); > + total -=3D len; > + ++v; > + } > + WARN_ON_ONCE(v > rqstp->rq_maxpages); > + > + trace_nfsd_read_direct(rqstp, fhp, offset, *count); > + iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, dio_end - dio_start)= ; > + > + host_err =3D vfs_iocb_iter_read(nf->nf_file, &kiocb, &iter); > + if (host_err >=3D 0) { > + unsigned int pad =3D offset - dio_start; > + > + /* The returned payload starts after the pad */ > + rqstp->rq_res.page_base =3D pad; > + > + /* Compute the count of bytes to be returned */ > + if (host_err > pad + *count) { > + host_err =3D *count; > + } else if (host_err > pad) { > + host_err -=3D pad; > + } else { > + host_err =3D 0; > + } > + } else if (unlikely(host_err =3D=3D -EINVAL)) { > + pr_info_ratelimited("nfsd: Unexpected direct I/O alignment failure\n")= ; > + host_err =3D -ESERVERFAULT; > + } > + > + return nfsd_finish_read(rqstp, fhp, nf->nf_file, offset, count, > + eof, host_err); > +} > + > /** > * nfsd_iter_read - Perform a VFS read using an iterator > * @rqstp: RPC transaction context > @@ -1106,6 +1179,11 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, stru= ct svc_fh *fhp, > switch (nfsd_io_cache_read) { > case NFSD_IO_BUFFERED: > break; > + case NFSD_IO_DIRECT: > + if (nf->nf_dio_read_offset_align && !base) > + return nfsd_direct_read(rqstp, fhp, nf, offset, > + count, eof); > + fallthrough; > case NFSD_IO_DONTCACHE: > if (file->f_op->fop_flags & FOP_DONTCACHE) > kiocb.ki_flags =3D IOCB_DONTCACHE; This looks fine: Reviewed-by: Jeff Layton Since this is a "hidden" option, it may also be a good idea to add a pr_info that pops when someone enables it. Then if we get a bug report involving this later, that would at least show up in the log.