On Thu 11-07-19 17:04:55, Andrew Morton wrote: > On Thu, 11 Jul 2019 14:58:38 +0200 Jan Kara <jack@xxxxxxx> wrote: > > > buffer_migrate_page_norefs() can race with bh users in a following way: > > > > CPU1 CPU2 > > buffer_migrate_page_norefs() > > buffer_migrate_lock_buffers() > > checks bh refs > > spin_unlock(&mapping->private_lock) > > __find_get_block() > > spin_lock(&mapping->private_lock) > > grab bh ref > > spin_unlock(&mapping->private_lock) > > move page do bh work > > > > This can result in various issues like lost updates to buffers (i.e. > > metadata corruption) or use after free issues for the old page. > > > > Closing this race window is relatively difficult. We could hold > > mapping->private_lock in buffer_migrate_page_norefs() until we are > > finished with migrating the page but the lock hold times would be rather > > big. So let's revert to a more careful variant of page migration requiring > > eviction of buffers on migrated page. This is effectively > > fallback_migrate_page() that additionally invalidates bh LRUs in case > > try_to_free_buffers() failed. > > Is this premature optimization? Holding ->private_lock while messing > with the buffers would be the standard way of addressing this. The > longer hold times *might* be an issue, but we don't know this, do we? > If there are indeed such problems then they could be improved by, say, > doing more of the newpage preparation prior to taking ->private_lock. I didn't check how long the private_lock hold times would actually be, it just seems there's a lot of work done before the page is fully migrated a we could release the lock. And since the lock blocks bh lookup, set_page_dirty(), etc. for the whole device, it just seemed as a bad idea. I don't think much of a newpage setup can be moved outside of private_lock - in particular page cache replacement, page copying, page state migration all need to be there so that bh code doesn't get confused. But I guess it's fair to measure at least ballpark numbers of what the lock hold times would be to get idea whether the contention concern is substantiated or not. Finally, I guess I should mention there's one more approach to the problem I was considering: Modify bh code to fully rely on page lock instead of private_lock for bh lookup. That would make sense scalability-wise on its own. The problem with it is that __find_get_block() would become a sleeping function. There aren't that many places calling the function and most of them seem fine with it but still it is non-trivial amount of work to do the conversion and it can have some fallout so it didn't seem like a good solution for a data-corruption issue that needs to go to stable... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
