[PATCH] vfs: Fix potential circular locking through setxattr() and removexattr()

[Matthew Wilcox]

On Wed, Jul 31, 2024 at 08:16:57PM +0200, Jan Kara wrote:
> To fix this, either we'd have to keep the lower cache filesystem private to
> cachefiles (but I don't think that works with the usecases) or we have to
> somehow untangle this mmap_lock knot. This "page fault does quite some fs
> locking under mmap_lock" problem is not causing filesystems headaches for
> the first time. I would *love* to be able to always drop mmap_lock in the
> page fault handler, fill the data into the page cache and then retry the
> fault (so that filemap_map_pages() would then handle the fault without
> filesystem involvement). It would make many things in filesystem locking
> simpler. As far as I'm checking there are now not that many places that
> could not handle dropping of mmap_lock during fault (traditionally the
> problem is with get_user_pages() / pin_user_pages() users). So maybe this
> dream would be feasible after all.

The traditional problem was the array of VMAs which was removed in
commit b2cac248191b -- if we dropped the mmap_lock, any previous
entries in that array would become invalid.  Now that array is gone,
do we have any remaining dependencies on the VMAs remaining valid?