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Message-ID: <20150911212957.GH11677@fieldses.org>
Date: Fri, 11 Sep 2015 17:29:57 -0400
From: "J. Bruce Fields" <bfields@...ldses.org>
To: Jeff Layton <jlayton@...chiereds.net>
Cc: linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
linux-fsdevel@...r.kernel.org, HPDD-discuss@...ts.01.org,
ceph-devel@...r.kernel.org, cluster-devel@...hat.com,
fuse-devel@...ts.sourceforge.net, ocfs2-devel@....oracle.com
Subject: Re: [PATCH] nfsd: add a new EXPORT_OP_NOWCC flag to struct
export_operations
On Fri, Sep 11, 2015 at 06:20:30AM -0400, Jeff Layton wrote:
> With NFSv3 nfsd will always attempt to send along WCC data to the
> client. This generally involves saving off the in-core inode information
> prior to doing the operation on the given filehandle, and then issuing a
> vfs_getattr to it after the op.
>
> Some filesystems (particularly clustered or networked ones) have an
> expensive ->getattr inode operation. Atomicitiy is also often difficult
> or impossible to guarantee on such filesystems. For those, we're best
> off not trying to provide WCC information to the client at all, and to
> simply allow it to poll for that information as needed with a GETATTR
> RPC.
>
> This patch adds a new flags field to struct export_operations, and
> defines a new EXPORT_OP_NOWCC flag that filesystems can use to indicate
> that nfsd should not attempt to provide WCC info in NFSv3 replies. It
> also adds a blurb about the new flags field and flag to the exporting
> documentation.
>
> The server will also now skip collecting this information for NFSv2 as
> well, since that info is never used there anyway.
>
> Note that this patch does not add this flag to any filesystem
> export_operations structures. This was originally developed to allow
> reexporting nfs via nfsd. That code is not (and may never be) suitable
> for merging into mainline.
>
> Other filesystems may want to consider enabling this flag too. It's hard
> to tell however which ones have export operations to enable export via
> knfsd and which ones mostly rely on them for open-by-filehandle support,
Are there any in the latter class? I'm not sure how or why you'd
support open-by-filehandle without supporting nfs exports.
> so I'm leaving that up to the individual maintainers to decide. I am
> cc'ing the relevant lists for those filesystems that I think may want to
> consider adding this though.
I'd definitely like to see evidence from maintainers of those
filesystems that this would be useful to them.
--b.
>
> Cc: HPDD-discuss@...ts.01.org
> Cc: ceph-devel@...r.kernel.org
> Cc: cluster-devel@...hat.com
> Cc: fuse-devel@...ts.sourceforge.net
> Cc: ocfs2-devel@....oracle.com
> Signed-off-by: Jeff Layton <jeff.layton@...marydata.com>
> ---
> Documentation/filesystems/nfs/Exporting | 27 +++++++++++++++++++++++++++
> fs/nfsd/nfs3xdr.c | 5 ++++-
> fs/nfsd/nfsfh.c | 14 ++++++++++++++
> fs/nfsd/nfsfh.h | 5 ++++-
> include/linux/exportfs.h | 2 ++
> 5 files changed, 51 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/nfs/Exporting b/Documentation/filesystems/nfs/Exporting
> index 520a4becb75c..fa636cde3907 100644
> --- a/Documentation/filesystems/nfs/Exporting
> +++ b/Documentation/filesystems/nfs/Exporting
> @@ -138,6 +138,11 @@ struct which has the following members:
> to find potential names, and matches inode numbers to find the correct
> match.
>
> + flags
> + Some filesystems may need to be handled differently than others. The
> + export_operations struct also includes a flags field that allows the
> + filesystem to communicate such information to nfsd. See the Export
> + Operations Flags section below for more explanation.
>
> A filehandle fragment consists of an array of 1 or more 4byte words,
> together with a one byte "type".
> @@ -147,3 +152,25 @@ generated by encode_fh, in which case it will have been padded with
> nuls. Rather, the encode_fh routine should choose a "type" which
> indicates the decode_fh how much of the filehandle is valid, and how
> it should be interpreted.
> +
> +Export Operations Flags
> +-----------------------
> +In addition to the operation vector pointers, struct export_operations also
> +contains a "flags" field that allows the filesystem to communicate to nfsd
> +that it may want to do things differently when dealing with it. The
> +following flags are defined:
> +
> + EXPORT_OP_NOWCC
> + RFC 1813 recommends that servers always send weak cache consistency
> + (WCC) data to the client after each operation. The server should
> + atomically collect attributes about the inode, do an operation on it,
> + and then collect the attributes afterward. This allows the client to
> + skip issuing GETATTRs in some situations but means that the server
> + is calling vfs_getattr for almost all RPCs. On some filesystems
> + (particularly those that are clustered or networked) this is expensive
> + and atomicity is difficult to guarantee. This flag indicates to nfsd
> + that it should skip providing WCC attributes to the client in NFSv3
> + replies when doing operations on this filesystem. Consider enabling
> + this on filesystems that have an expensive ->getattr inode operation,
> + or when atomicity between pre and post operation attribute collection
> + is impossible to guarantee.
> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> index 01dcd494f781..c30c8c604e2a 100644
> --- a/fs/nfsd/nfs3xdr.c
> +++ b/fs/nfsd/nfs3xdr.c
> @@ -203,7 +203,7 @@ static __be32 *
> encode_post_op_attr(struct svc_rqst *rqstp, __be32 *p, struct svc_fh *fhp)
> {
> struct dentry *dentry = fhp->fh_dentry;
> - if (dentry && d_really_is_positive(dentry)) {
> + if (!fhp->fh_no_wcc && dentry && d_really_is_positive(dentry)) {
> __be32 err;
> struct kstat stat;
>
> @@ -256,6 +256,9 @@ void fill_post_wcc(struct svc_fh *fhp)
> {
> __be32 err;
>
> + if (fhp->fh_no_wcc)
> + return;
> +
> if (fhp->fh_post_saved)
> printk("nfsd: inode locked twice during operation.\n");
>
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 350041a40fe5..29ae37f62b9b 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -267,6 +267,16 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>
> fhp->fh_dentry = dentry;
> fhp->fh_export = exp;
> +
> + switch (rqstp->rq_vers) {
> + case 3:
> + if (!(dentry->d_sb->s_export_op->flags & EXPORT_OP_NOWCC))
> + break;
> + /* Fallthrough */
> + case 2:
> + fhp->fh_no_wcc = true;
> + }
> +
> return 0;
> out:
> exp_put(exp);
> @@ -535,6 +545,9 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
> */
> set_version_and_fsid_type(fhp, exp, ref_fh);
>
> + /* If we have a ref_fh, then copy the fh_no_wcc setting from it. */
> + fhp->fh_no_wcc = ref_fh ? ref_fh->fh_no_wcc : false;
> +
> if (ref_fh == fhp)
> fh_put(ref_fh);
>
> @@ -641,6 +654,7 @@ fh_put(struct svc_fh *fhp)
> exp_put(exp);
> fhp->fh_export = NULL;
> }
> + fhp->fh_no_wcc = false;
> return;
> }
>
> diff --git a/fs/nfsd/nfsfh.h b/fs/nfsd/nfsfh.h
> index 1e90dad4926b..9ddead4d98f8 100644
> --- a/fs/nfsd/nfsfh.h
> +++ b/fs/nfsd/nfsfh.h
> @@ -32,6 +32,7 @@ typedef struct svc_fh {
>
> unsigned char fh_locked; /* inode locked by us */
> unsigned char fh_want_write; /* remount protection taken */
> + bool fh_no_wcc; /* no wcc data needed */
>
> #ifdef CONFIG_NFSD_V3
> unsigned char fh_post_saved; /* post-op attrs saved */
> @@ -51,7 +52,6 @@ typedef struct svc_fh {
> struct kstat fh_post_attr; /* full attrs after operation */
> u64 fh_post_change; /* nfsv4 change; see above */
> #endif /* CONFIG_NFSD_V3 */
> -
> } svc_fh;
>
> enum nfsd_fsid {
> @@ -225,6 +225,9 @@ fill_pre_wcc(struct svc_fh *fhp)
> {
> struct inode *inode;
>
> + if (fhp->fh_no_wcc)
> + return;
> +
> inode = d_inode(fhp->fh_dentry);
> if (!fhp->fh_pre_saved) {
> fhp->fh_pre_mtime = inode->i_mtime;
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index fa05e04c5531..600c3fccc999 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -214,6 +214,8 @@ struct export_operations {
> bool write, u32 *device_generation);
> int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
> int nr_iomaps, struct iattr *iattr);
> +#define EXPORT_OP_NOWCC (0x1) /* Don't collect wcc data for NFSv3 replies */
> + unsigned long flags;
> };
>
> extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
> --
> 2.4.3
>
> --
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