Cache flushing

[Timo Sirainen <tss@iki.fi>]

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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);

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.

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