On Fri, Jul 12, 2019 at 11:17:46AM +0200, Jan Kara wrote: > 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. > I think it would be tricky to measure and quantify how much the contention is an issue. While it would be possible to construct a microbenchmark that should illustrate the problem, it would tell us relatively little about how much of a problem it is generally. It would be relatively difficult to detect the contention and stalls in block lookups due to migration would be tricky to spot. Careful use of lock_stat might help but enabling that has consequences of its own. However, a rise in allocation failures due to dirty pages not being migrated is relatively easy to detect and the consequences are relatively benign -- failed high-order allocation that is usually ok versus a stall on block lookups that could have a wider general impact. On that basis, I think the patch you proposed is the more appropriate as a fix for the race which has the potential for data corruption. So; Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > 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... > Maybe *if* it's shown there is a major issue with increased high-order allocation failures, it would be worth looking into but right now, I think it's overkill with relatively high risk and closing the potential race is more important. -- Mel Gorman SUSE Labs
