Re: [PATCH] btrfs: add a find_contiguous_extent_bit helper and use it for safe isize

[David Sterba <dsterba@suse.cz>]

On Wed, Feb 12, 2020 at 01:38:31PM -0500, Josef Bacik wrote:
> Filipe noticed a race where we would sometimes get the wrong answer for
> the i_disk_size for !NO_HOLES with my patch.  That is because I expected
> that find_first_extent_bit() would find the contiguous range, since I'm
> only ever setting EXTENT_DIRTY.  However the way set_extent_bit() works
> is it'll temporarily split the range, loop around and set our bits, and
> then merge the state.  When it loops it drops the tree->lock, so there
> is a window where we can have two adjacent states instead of one large
> state.  Fix this by walking forward until we find a non-contiguous
> state, and set our end_ret to the end of our logically contiguous area.
> This fixes the problem without relying on specific behavior from
> set_extent_bit().
> 
> Fixes: 79ceff7f6e5d ("btrfs: introduce per-inode file extent tree")
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> Dave, I assume you'll want to fold this in to the referenced patch, if not let
> me know and I'll rework the series to include this as a different patch.

Folding looks like a good option, the patch adds other helper for
btrfs_inode_clear_file_extent_range so this patch fits in nicely.

> +/**
> + * find_contiguous_extent_bit: find a contiguous area of bits
> + * @tree - io tree to check
> + * @start - offset to start the search from
> + * @start_ret - the first offset we found with the bits set
> + * @end_ret - the final contiguous range of the bits that were set
> + *
> + * set_extent_bit anc clear_extent_bit can temporarily split contiguous ranges
> + * to set bits appropriately, and then merge them again.  During this time it
> + * will drop the tree->lock, so use this helper if you want to find the actual
> + * contiguous area for given bits.  We will search to the first bit we find, and
> + * then walk down the tree until we find a non-contiguous area.  The area
> + * returned will be the full contiguous area with the bits set.
> + */

This summarizes why it's needed so the changelog does not need to be
updated AFAICS. Patch updated and misc-next pushed. Thanks.