On Thu, Jul 11, 2019 at 05:04:55PM -0700, 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. > To some extent, we do not know how much of a problem this patch will be either or what impact avoiding dirty block pages during migration is either. So both approaches have their downsides. However, failing a high-order allocation is typically benign and it is an inevitable problem that depends on the workload. I don't think we could ever hit a case whereby there was enough spinning to cause a soft lockup but on the other hand, I don't think there is much scope for doing more of the preparation steps before acquiring private_lock either. -- Mel Gorman SUSE Labs
