Re: [PATCH] VFS: Add read-only users count to superblock

[Erez Zadok]

In message <20090713192743.GA27582@shell>, Valerie Aurora writes:
> During the last FS summit, Al Viro suggested creating a superblock
> level read-only marker so that union mounts could guarantee that the
> underlying fs would not become writable.  This patch implements the
> VFS support, but doesn't add any users.  The patch making union mounts
> use the support is in our union mounts tree.  I think we also need
> some way to pass this through NFS mounts, since a read-only NFS mount
> for the bottom layer of a union mount is a common use case.
> 
> -VAL
[...]

Val, I've often wondered if a generic readonly superblock solution will
obviate the need for the sb->s_vfs_rename_mutex "kludge" (as commented in
fs.h) and the whole way directory-renames are done wrt locking.  Can the
rename code be the first user of such patch, or the patch isn't quite ready
for this?

I realize that marking a whole superblock readonly during a directory rename
can hurt performance in some cases, as compared to the current way of doing
things, using lock_rename() to lock a subtree of the namespace.  But I
suspect that many concurrent directory renames are an exception, and thus
practical performance won't be hurt much with such a patch -- but result in
major benefits of code simplicity for several file systems (esp. for
unioning layers).

Erez.
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