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f73853c716
At find_extent_clone() we search twice for the extent item corresponding to the data extent that the current file extent items points to: 1) Once with a call to extent_from_logical(); 2) Once again during backref walking, through iterate_extent_inodes() which eventually leads to find_parent_nodes() where we will search again the extent tree for the same extent item. The extent tree can be huge, so doing this one extra search for every extent we want to send adds up and it's expensive. The first call is there since the send code was introduced and it accomplishes two things: 1) Check that the extent is flagged as a data extent in the extent tree. But it can not be anything else, otherwise we wouldn't have a file extent item in the send root pointing to it. This was probably added to catch bugs in the early days where send was yet too young and the interaction with everything else was far from perfect; 2) Check how many direct references there are on the extent, and if there's too many (more than SEND_MAX_EXTENT_REFS), avoid doing the backred walking as it may take too long and slowdown send. So improve on this by having a callback in the backref walking code that is called when it finds the extent item in the extent tree, and have those checks done in the callback. When the callback returns anything different from 0, it stops the backref walking code. This way we do a single search on the extent tree for the extent item of our data extent. Also, before this change we were only checking the number of references on the data extent against SEND_MAX_EXTENT_REFS, but after starting backref walking we will end up resolving backrefs for extent buffers in the path from a leaf having a file extent item pointing to our data extent, up to roots of trees from which the extent buffer is accessible from, due to shared subtrees resulting from snapshoting. We were therefore allowing for the possibility for send taking too long due to some node in the path from the leaf to a root node being shared too many times. After this change we check for reference counts being greater than SEND_MAX_EXTENT_REFS for both data extents and metadata extents. This change is part of a patchset comprised of the following patches: 01/17 btrfs: fix inode list leak during backref walking at resolve_indirect_refs() 02/17 btrfs: fix inode list leak during backref walking at find_parent_nodes() 03/17 btrfs: fix ulist leaks in error paths of qgroup self tests 04/17 btrfs: remove pointless and double ulist frees in error paths of qgroup tests 05/17 btrfs: send: avoid unnecessary path allocations when finding extent clone 06/17 btrfs: send: update comment at find_extent_clone() 07/17 btrfs: send: drop unnecessary backref context field initializations 08/17 btrfs: send: avoid unnecessary backref lookups when finding clone source 09/17 btrfs: send: optimize clone detection to increase extent sharing 10/17 btrfs: use a single argument for extent offset in backref walking functions 11/17 btrfs: use a structure to pass arguments to backref walking functions 12/17 btrfs: reuse roots ulist on each leaf iteration for iterate_extent_inodes() 13/17 btrfs: constify ulist parameter of ulist_next() 14/17 btrfs: send: cache leaf to roots mapping during backref walking 15/17 btrfs: send: skip unnecessary backref iterations 16/17 btrfs: send: avoid double extent tree search when finding clone source 17/17 btrfs: send: skip resolution of our own backref when finding clone source Performance test results are in the changelog of patch 17/17. 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 Restructured Text 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.