Re: [PATCH] NFSv3: cached permissions subset enhancement

Myklebust, Trond
2008-08-28T16:05:14Z

On Aug 27, 2008, at 7:28, "Joe Korty" <joe.korty@ccur.com> wrote:

> [NFSv3] cached permissions subset enhancement.
>
> NFSv3 allows file permissions to be cached on the client
> side. The Linux client, when fetching these permissions,
> makes the request for all permissions (rwx) in a single
> operation. However, for some NFSv3 server implementations
> (the only one currently known is PowerMAX OS), an incoming
> request for all rwx permissions in a single request,
> when all are not actually set in the file, returns an
> NFSERR_ACCES failure code to the client, rather than
> the subset of permissions actually available for that file.
>
> This patch modifies the Linux client side code to
> individually fetch the r, w, and x permissions (combining
> these for storage into the cache), if the original
> single-request method fails.
>
> This slower method will not affect performance of those
> client/server pairs for which the original single-request
> method works. In particular there is no performance
> penalty for linux/linux NFSv3 connections.
>
> Author: Linda Dunaphant <ldunaphant@comcast.net>
> Signed-off-by: Joe Korty <joe.korty@ccur.com>
>
> Index: 2.6.27-rc4-git4/fs/nfs/dir.c
> ===================================================================
> --- 2.6.27-rc4-git4.orig/fs/nfs/dir.c 2008-08-26 17:44:34.000000000 -0400
> +++ 2.6.27-rc4-git4/fs/nfs/dir.c 2008-08-26 18:21:27.000000000
> -0400
> @@ -1870,6 +1870,8 @@
> {
> struct nfs_access_entry cache;
> int status;
> + int i;
> + int cmask = 0;
>
> status = nfs_access_get_cached(inode, cred, &cache);
> if (status == 0)
> @@ -1880,8 +1882,27 @@
> cache.cred = cred;
> cache.jiffies = jiffies;
> status = NFS_PROTO(inode)->access(inode, &cache);
> - if (status != 0)
> + if (status == -NFSERR_ACCES) {
> + //
> + // Try again - one mode at a time & combine at the end
> + //
> + for (i = 0; i <= MAY_READ; i++) {
> + cache.mask = 1 << i;
> + status = NFS_PROTO(inode)->access(inode, &cache);
> + if (status == 0)
> + cmask |= cache.mask;
> + else if (status != -NFSERR_ACCES) {
> + return status;
> + }
> + }
> +
> + if (!cmask)
> + return -NFSERR_ACCES;
> +
> + cache.mask = cmask;
> + } else if (status)
> return status;
> +
> nfs_access_add_cache(inode, &cache);
> out:
> if ((mask & ~cache.mask & (MAY_READ | MAY_WRITE | MAY_EXEC)) == 0)

This isn't a good solution. The correct thing to do here is to resend
the request for just those permissions that the VFS requested.

Cheers
Trond