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?
