On Thu, Oct 19, 2023 at 11:52:11AM -0400, Kent Overstreet wrote: > On Thu, Oct 19, 2023 at 09:37:24AM -0400, Brian Foster wrote: > > Hi Kent, All, > > > > I recently observed a data corruption problem that is related to the > > recently discovered issue of mounted fs' running with the early bucket > > allocator instead of the freelist allocator. The immediate failure is > > generic/113 producing various splats, the most common of which is a > > duplicate backpointer issue. generic/113 is primarily an aio/dio stress > > test. > > > > I eventually tracked this down to an actual duplicate bucket allocation > > in the early bucket allocator code. The race generally looks as follows: > > > > - Task 1 lands in bch2_bucket_alloc_early(), selects key A from the > > alloc btree, and then schedules (perhaps due to freelist_lock). > > > > - Task 2 runs through the same alloc path and selects the same key K, > > but proceeds to open the associated bucket, alloc/write to it, > > complete the I/O and release the bucket (removing it from the hash). > > > > - Task 1 continues with alloc key K. bch2_bucket_is_open() returns false > > because the previously opened bucket has been removed from the hash > > list. Therefore task 1 opens a new bucket for what is now no longer free > > space and uses it for the its associated write operation. > > This shouldn't be possible because task 1 is holding the alloc key > locked, and task 2 has to update that same alloc key before releasing > the open bucket. > > Except perhaps not - perhaps this is a key cache coherency issue? > > We're not using a BTREE_ITER_CACHED iterator, because we're scanning and > we can't scan with key cache iterators. It's still supposed to be > coherent with the key cache; bch2_btree_iter_peek_slot() -> > btree_trans_peek_key_cache() checks if a key exists in the key cache and > returns that key instead of the key in the btree if it exists. > > But it doesn't return with that slot locked in the key cache locked if > the key didn't exist in the key cache. Oops. >
Ah, interesting. I wasn't aware of the lower level locking involved here. This sounds like a plausible theory wrt key cache, but I'll have to dig more into it to grok the locking. Thanks for the additional context. Brian > So we're going to need to keep an eye out for this issue occuring > elsewhere, and maybe come up with a real fix in the btree iterator code: > looking up a key in a cached btree without BTREE_ITER_CACHED _does_ > return the correct key at that particular point in time, but it does > _not_ necessarily keep it locked for the duration of the transaction. > > For now, we can fix this locally in bch2_bucket_alloc_early() with a > second BTREE_ITER_CACHED iterator - run some tests with freespace > initialization disabled, confirm that that's the issue, then go from > there. >
