On Thu, Sep 03, 2015 at 01:33:14PM -0400, Jeff Layton wrote:
> There are cases with NFSv3 where the client doesn't actually care about
> WCC attributes in replies. If the server is mainly acting as a DS for
> flexfiles, then the client just throws out those attributes anyway.
> Also, in the case where the client is primarily doing direct I/O, post
> op attributes aren't terribly useful
>
> Another reason to allow turning these off is that NFS will flush all
> buffered writes prior to issuing a GETATTR, and it also takes the
> i_mutex in its ->getattr operation.
>
> If we're doing a vfs_getattr after most RPCs, then we can end up
> deadlocking or (at best) prematurely flushing buffered writes, which
> kills performance.
So you're talking about the NFS re-export case? Do we know of any other
case when a ->getattr is so expensive?
--b.
> This patch adds a new export option -- "nowcc" that disables the return
> of WCC attributes in NFSv3 replies. I also have a userland patch that
> adds support for the same option to nfs-utils that I'll send along as
> well.
>
> Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> ---
> fs/nfsd/export.c | 1 +
> fs/nfsd/nfs3xdr.c | 5 ++++-
> fs/nfsd/nfsfh.c | 4 ++++
> fs/nfsd/nfsfh.h | 5 ++++-
> include/uapi/linux/nfsd/export.h | 3 ++-
> 5 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index b4d84b579f20..97258009ce1e 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -1092,6 +1092,7 @@ static struct flags {
> { NFSEXP_NOAUTHNLM, {"insecure_locks", ""}},
> { NFSEXP_V4ROOT, {"v4root", ""}},
> { NFSEXP_PNFS, {"pnfs", ""}},
> + { NFSEXP_NOWCC, {"nowcc", ""}},
> { 0, {"", ""}}
> };
>
> 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..32093b7dce55 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -267,6 +267,9 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>
> fhp->fh_dentry = dentry;
> fhp->fh_export = exp;
> + if (exp->ex_flags & NFSEXP_NOWCC && rqstp->rq_vers == 3)
> + fhp->fh_no_wcc = true;
> +
> return 0;
> out:
> exp_put(exp);
> @@ -641,6 +644,7 @@ 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/uapi/linux/nfsd/export.h b/include/uapi/linux/nfsd/export.h
> index 0df7bd5d2fb1..4c132290f414 100644
> --- a/include/uapi/linux/nfsd/export.h
> +++ b/include/uapi/linux/nfsd/export.h
> @@ -51,9 +51,10 @@
> */
> #define NFSEXP_V4ROOT 0x10000
> #define NFSEXP_PNFS 0x20000
> +#define NFSEXP_NOWCC 0x40000
>
> /* All flags that we claim to support. (Note we don't support NOACL.) */
> -#define NFSEXP_ALLFLAGS 0x3FE7F
> +#define NFSEXP_ALLFLAGS 0x7FE7F
>
> /* The flags that may vary depending on security flavor: */
> #define NFSEXP_SECINFO_FLAGS (NFSEXP_READONLY | NFSEXP_ROOTSQUASH \
> --
> 2.4.3
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