Re: [NFS] Cache flushing

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Sat, 2007-11-17 at 02:11 +0200, Timo Sirainen wrote:
> Solaris and BSDs support flushing attribute cache safely using
> fchown(fd, (uid_t)-1, (gid_t)-1). Could Linux be changed to support this
> as well? If I'm looking at the sources right, this might work
> (completely untested):
> 
> --- inode.c.old 2007-11-16 22:18:46.000000000 +0200
> +++ inode.c     2007-11-16 22:19:44.000000000 +0200
> @@ -322,6 +322,7 @@
>  nfs_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>         struct inode *inode = dentry->d_inode;
> +       struct nfs_inode *nfsi = NFS_I(inode);
>         struct nfs_fattr fattr;
>         int error;
>  
> @@ -334,8 +335,10 @@
>  
>         /* Optimization: if the end result is no change, don't RPC */
>         attr->ia_valid &= NFS_VALID_ATTRS;
> -       if (attr->ia_valid == 0)
> +       if (attr->ia_valid == 0) {
> +               nfsi->cache_validity |= NFS_INO_INVALID_ATTR;
>                 return 0;
> +       }
>  
>         lock_kernel();
>         nfs_begin_data_update(inode);

Why is this needed?

> Another problem I have is that it's difficult to get a file's data cache
> flushed. The only way I found was to successfully fcntl() lock the file.
> This is pretty bad from performance point of view since often I don't
> want/need to lock the file.
> 
> Solaris and BSDs invalidate also a file's data cache when its attribute
> cache is invalidated. It would be nicer if there was a separate way, but
> I'd settle for fchown(fd, (uid_t)-1, (gid_t)-1) invalidating data cache
> as well.
> 
> Actually I did also look at posix_fadvise(fd, 0, 0,
> POSIX_FADVN_DONTNEED). It appears to work, but I'm a bit worried about
> race conditions that causes pages to randomly not get dropped because
> invalidate_mapping_pages() doesn't drop locked pages.

Again, why do you need this level of data cache invalidation? If you
don't want cached i/o, then use O_DIRECT.

Trond


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