On Wed 31-07-24 19:27:43, Matthew Wilcox wrote: > 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? So as far as I've checked the callers of get_user_pages() / pin_user_pages() I didn't find any that fundamentally could not handle dropping of mmap_lock. So at least for callers I've seen it was mostly about teaching them to handle dropped mmap_lock, reacquire it and possibly reestablish some state which could get invalidated after the mmap_lock got dropped. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
