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When trying to flush qgroups in order to release space we run delayed iputs in order to release space from recently deleted files (their link counted reached zero), and then we start delalloc and wait for any existing ordered extents to complete. However there's a time window here where we end up not doing the final iput on a deleted file which could release necessary space: 1) An unlink operation starts; 2) During the unlink, or right before it completes, delalloc is flushed and an ordered extent is created; 3) When the ordered extent is created, the inode's ref count is incremented (with igrab() at alloc_ordered_extent()); 4) When the unlink finishes it doesn't drop the last reference on the inode and so it doesn't trigger inode eviction to delete all of the inode's items in its root and drop all references on its data extents; 5) Another task enters try_flush_qgroup() to try to release space, it runs all delayed iputs, but there's no delayed iput yet for that deleted file because the ordered extent hasn't completed yet; 6) Then at try_flush_qgroup() we wait for the ordered extent to complete and that results in adding a delayed iput at btrfs_put_ordered_extent() when called from btrfs_finish_one_ordered(); 7) Adding the delayed iput results in waking the cleaner kthread if it's not running already. However it may take some time for it to be scheduled, or it may be running but busy running auto defrag, dropping deleted snapshots or doing other work, so by the time we return from try_flush_qgroup() the space for deleted file isn't released. Improve on this by running delayed iputs only after flushing delalloc and waiting for ordered extent completion. 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.