Re: [PATCH v2 07/18] btrfs: extent_io: make grab_extent_buffer_from_page() to handle subpage case

[Josef Bacik <josef@toxicpanda.com>]

On 12/10/20 1:38 AM, Qu Wenruo wrote:
> For subpage case, grab_extent_buffer_from_page() can't really get an
> extent buffer just from btrfs_subpage.
> 
> Although we have btrfs_subpage::tree_block_bitmap, which can be used to
> grab the bytenr of an existing extent buffer, and can then go radix tree
> search to grab that existing eb.
> 
> However we are still doing radix tree insert check in
> alloc_extent_buffer(), thus we don't really need to do the extra hassle,
> just let alloc_extent_buffer() to handle existing eb in radix tree.
> 
> So for grab_extent_buffer_from_page(), just always return NULL for
> subpage case.

This is fundamentally flawed.  The extent buffer radix tree look up is done 
_after_ the pages are init'ed.  This is why there's that complicated dance of 
checking for existing extent buffers attached to to a page, because we can race 
at the initialization stage and attach an EB to a page before it's in the radix 
tree.  What you'll end up doing here is overwriting your existing subpage stuff 
anytime there's a race, and it'll end very badly.  Thanks,

Josef