Re: [PATCH v2 2/3] btrfs: make btrfs_do_readpage() to do block-by-block read

[David Sterba <dsterba@suse.cz>]

On Wed, Feb 12, 2025 at 01:22:46PM +1030, Qu Wenruo wrote:
> Currently if a btrfs has block size (the older sector size) < page size,
> btrfs_do_readpage() will handle the range extent by extent, this is good
> for performance as it doesn't need to re-lookup the same extent map again
> and again.
> (Although __get_extent_map() already does extra cached em check, thus

Minor thing, __get_extent_map has been renamed to get_extent_map.

> the optimization is not that obvious)
> 
> This is totally fine and is a valid optimization, but it has an
> assumption that, there is no partial uptodate range in the page.
> 
> Meanwhile there is an incoming feature, requiring btrfs to skip the full
> page read if a buffered write range covers a full block but not a full
> page.
> 
> In that case, we can have a page that is partially uptodate, and the
> current per-extent lookup can not handle such case.
> 
> So here we change btrfs_do_readpage() to do block-by-block read, this
> simplifies the following things:
> 
> - Remove the need for @iosize variable
>   Because we just use sectorsize as our increment.
> 
> - Remove @pg_offset, and calculate it inside the loop when needed
>   It's just offset_in_folio().
> 
> - Use a for() loop instead of a while() loop
> 
> This will slightly reduce the read performance for
> block size < page size cases, but for the future where we can skip a
> full page read for a lot of cases, it should still be worthy.
> 
> For block size == page size, this brings no performance change.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>