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Our subpage testing started hanging on generic/560 and I bisected it down to1cab1375ba("btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations"). This is subtle because we use eb->start to figure out where in the folio we're copying to when we're subpage, as our ->start may refer to an area inside of the folio. For example, assume a 16K page size machine with a 4K node size, and assume that we already have a cloned extent buffer when we cloned the previous search. copy_extent_buffer_full() will do the following when copying the extent buffer path->nodes[0] (src) into cloned (dest): src->start = 8k; // this is the new leaf we're cloning cloned->start = 4k; // this is left over from the previous clone src_addr = folio_address(src->folios[0]); dest_addr = folio_address(dest->folios[0]); memcpy(dest_addr + get_eb_offset_in_folio(dst, 0), src_addr + get_eb_offset_in_folio(src, 0), src->len); Now get_eb_offset_in_folio() is where the problems occur, because for sub-pagesize blocksize we can have multiple eb's per folio, the code for this is as follows size_t get_eb_offset_in_folio(eb, offset) { return (eb->start + offset & (folio_size(eb->folio[0]) - 1)); } So in the above example we are copying into offset 4K inside the folio. However once we update cloned->start to 8K to match the src the math for get_eb_offset_in_folio() changes, and any subsequent reads (i.e. btrfs_item_key_to_cpu()) will start reading from the offset 8K instead of 4K where we copied to, giving us garbage. Fix this by setting start before we co copy_extent_buffer_full() to make sure that we're copying into the same offset inside of the folio that we will read from later. All other sites of copy_extent_buffer_full() are correct because we either set ->start beforehand or we simply don't change it in the case of the tree-log usage. With this fix we now pass generic/560 on our subpage tests. Fixes:1cab1375ba("btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations") Reviewed-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: Qu Wenruo <wqu@suse.com> Signed-off-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: David Sterba <dsterba@suse.com>
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.