Re: [PATCH v4] Btrfs: compress_file_range() change page dirty status once

[David Sterba <dsterba@suse.cz>]

On Tue, Oct 24, 2017 at 01:29:48AM +0300, Timofey Titovets wrote:
> We need to call extent_range_clear_dirty_for_io()
> on compression range to prevent application from changing
> page content, while pages compressing.
> 
> extent_range_clear_dirty_for_io() run on each loop iteration,
> "(end - start)" can be much (up to 1024 times) bigger
> then compression range (BTRFS_MAX_UNCOMPRESSED).
> 
> That produce extra calls to page managment code.
> 
> Fix that behaviour by call extent_range_clear_dirty_for_io()
> only once.
> 
> v1 -> v2:
>  - Make that more obviously and more safeprone
> 
> v2 -> v3:
>  - Rebased on:
>    Btrfs: compress_file_range() remove dead variable num_bytes
>  - Update change log
>  - Add comments
> 
> v3 -> v4:
>  - Rebased on: kdave for-next
>  - To avoid dirty bit clear/set behaviour change
>    call clear_bit once, istead of per compression range

This version looks safe to me. There's no functional difference if we
redirty the pages repeatedly, but there's probably a noticeable runtime
difference if we do it just once.

Reviewed-by: David Sterba <dsterba@suse.com>