Btrfs: rework delayed ref total_bytes_pinned accounting
The total_bytes_pinned counter is completely broken when accounting delayed refs: - If two drops for the same extent are merged, we will decrement total_bytes_pinned twice but only increment it once. - If an add is merged into a drop or vice versa, we will decrement the total_bytes_pinned counter but never increment it. - If multiple references to an extent are dropped, we will account it multiple times, potentially vastly over-estimating the number of bytes that will be freed by a commit and doing unnecessary work when we're close to ENOSPC. The last issue is relatively minor, but the first two make the total_bytes_pinned counter leak or underflow very often. These accounting issues were introduced in b150a4f1 ("Btrfs: use a percpu to keep track of possibly pinned bytes"), but they were papered over by zeroing out the counter on every commit until d288db5d ("Btrfs: fix race of using total_bytes_pinned"). We need to make sure that an extent is accounted as pinned exactly once if and only if we will drop references to it when when the transaction is committed. Ideally we would only add to total_bytes_pinned when the *last* reference is dropped, but this information isn't readily available for data extents. Again, this over-estimation can lead to extra commits when we're close to ENOSPC, but it's not as bad as before. The fix implemented here is to increment total_bytes_pinned when the total refmod count for an extent goes negative and decrement it if the refmod count goes back to non-negative or after we've run all of the delayed refs for that extent. Signed-off-by: NOmar Sandoval <osandov@fb.com> Tested-by: NHolger Hoffstätte <holger@applied-asynchrony.com> Reviewed-by: NLiu Bo <bo.li.liu@oracle.com> Signed-off-by: NDavid Sterba <dsterba@suse.com>
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