btrfs: fix RWF_NOWAIT writes blocking on extent locks and waiting for IO
A RWF_NOWAIT write is not supposed to wait on filesystem locks that can be held for a long time or for ongoing IO to complete. However when calling check_can_nocow(), if the inode has prealloc extents or has the NOCOW flag set, we can block on extent (file range) locks through the call to btrfs_lock_and_flush_ordered_range(). Such lock can take a significant amount of time to be available. For example, a fiemap task may be running, and iterating through the entire file range checking all extents and doing backref walking to determine if they are shared, or a readpage operation may be in progress. Also at btrfs_lock_and_flush_ordered_range(), called by check_can_nocow(), after locking the file range we wait for any existing ordered extent that is in progress to complete. Another operation that can take a significant amount of time and defeat the purpose of RWF_NOWAIT. So fix this by trying to lock the file range and if it's currently locked return -EAGAIN to user space. If we are able to lock the file range without waiting and there is an ordered extent in the range, return -EAGAIN as well, instead of waiting for it to complete. Finally, don't bother trying to lock the snapshot lock of the root when attempting a RWF_NOWAIT write, as that is only important for buffered writes. Fixes: edf064e7 ("btrfs: nowait aio support") Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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