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The linux-next integration testing tree
f13e01b89d
If a write path in COW mode fails, either before submitting a bio for the new extents or an actual IO error happens, we can end up allowing a fast fsync to log file extent items that point to unwritten extents. This is because dropping the extent maps happens when completing ordered extents, at btrfs_finish_one_ordered(), and the completion of an ordered extent is executed in a work queue. This can result in a fast fsync to start logging file extent items based on existing extent maps before the ordered extents complete, therefore resulting in a log that has file extent items that point to unwritten extents, resulting in a corrupt file if a crash happens after and the log tree is replayed the next time the fs is mounted. This can happen for both direct IO writes and buffered writes. For example consider a direct IO write, in COW mode, that fails at btrfs_dio_submit_io() because btrfs_extract_ordered_extent() returned an error: 1) We call btrfs_finish_ordered_extent() with the 'uptodate' parameter set to false, meaning an error happened; 2) That results in marking the ordered extent with the BTRFS_ORDERED_IOERR flag; 3) btrfs_finish_ordered_extent() queues the completion of the ordered extent - so that btrfs_finish_one_ordered() will be executed later in a work queue. That function will drop extent maps in the range when it's executed, since the extent maps point to unwritten locations (signaled by the BTRFS_ORDERED_IOERR flag); 4) After calling btrfs_finish_ordered_extent() we keep going down the write path and unlock the inode; 5) After that a fast fsync starts and locks the inode; 6) Before the work queue executes btrfs_finish_one_ordered(), the fsync task sees the extent maps that point to the unwritten locations and logs file extent items based on them - it does not know they are unwritten, and the fast fsync path does not wait for ordered extents to complete, which is an intentional behaviour in order to reduce latency. For the buffered write case, here's one example: 1) A fast fsync begins, and it starts by flushing delalloc and waiting for the writeback to complete by calling filemap_fdatawait_range(); 2) Flushing the dellaloc created a new extent map X; 3) During the writeback some IO error happened, and at the end io callback (end_bbio_data_write()) we call btrfs_finish_ordered_extent(), which sets the BTRFS_ORDERED_IOERR flag in the ordered extent and queues its completion; 4) After queuing the ordered extent completion, the end io callback clears the writeback flag from all pages (or folios), and from that moment the fast fsync can proceed; 5) The fast fsync proceeds sees extent map X and logs a file extent item based on extent map X, resulting in a log that points to an unwritten data extent - because the ordered extent completion hasn't run yet, it happens only after the logging. To fix this make btrfs_finish_ordered_extent() set the inode flag BTRFS_INODE_NEEDS_FULL_SYNC in case an error happened for a COW write, so that a fast fsync will wait for ordered extent completion. Note that this issues of using extent maps that point to unwritten locations can not happen for reads, because in read paths we start by locking the extent range and wait for any ordered extents in the range to complete before looking for extent maps. Reviewed-by: Qu Wenruo <wqu@suse.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.