Re: [PATCH v2 0/5] btrfs: scrub: make scrub uses less memory for metadata scrub

[Qu Wenruo <wqu@suse.com>]

Ping?

This series would begin a new wave of changes of moving various members 
to scrub_block, thus being able to further reduce memory usage.
(Per-sector to per-block)

Thus it would be better to get this series merged before newer changes 
to arrive.

Thanks,
Qu

On 2022/7/19 15:24, Qu Wenruo wrote:
> [Changelog]
> v2:
> - Rebased to latest misc-next
>    The conflicts are mostly from the member renaming.
>    Has re-ran the tests on both aarch64 64K page size, and x86_64.
>    For both scrub and replace groups.
> 
> 
> Although btrfs scrub works for subpage from day one, it has a small
> pitfall:
> 
>    Scrub will always allocate a full page for each sector.
> 
> This causes increased memory usage, although not a big deal, it's still
> not ideal.
> 
> The patchset will change the behavior by integrating all pages into
> scrub_block::pages[], instead of using scrub_sector::page.
> 
> Now scrub_sector will no longer hold a page pointer, but uses its
> logical bytenr to caculate which page and page range it should use.
> 
> This behavior unfortunately still only affects memory usage on metadata
> scrub, which uses nodesize for scrub.
> 
> For the best case, 64K node size with 64K page size, we waste no memory
> to scrub one tree block.
> 
> For the worst case, 4K node size with 64K page size, we are no worse
> than the existing behavior (still one 64K page for the tree block)
> 
> For the default case (16K nodesize), we use one 64K page, compared to
> 4x64K pages previously.
> 
> For data scrubing, we uses sector size, thus it causes no difference.
> In the future, we may want to enlarge the data scrub size so that
> subpage can waste less memory.
> 
> [PATCHSET STRUCTURE]
> The first 3 patches are just cleanups, mostly to make scrub_sector
> allocation much easier.
> 
> The 4th patch is to introduce the new page array for sblock, and
> the last one to completely remove the usage of scrub_sector::page.
> 
> Qu Wenruo (5):
>    btrfs: scrub: use pointer array to replace @sblocks_for_recheck
>    btrfs: extract the initialization of scrub_block into a helper
>      function
>    btrfs: extract the allocation and initialization of scrub_sector into
>      a helper
>    btrfs: scrub: introduce scrub_block::pages for more efficient memory
>      usage for subpage
>    btrfs: scrub: remove scrub_sector::page and use scrub_block::pages
>      instead
> 
>   fs/btrfs/scrub.c | 398 +++++++++++++++++++++++++++++++----------------
>   1 file changed, 266 insertions(+), 132 deletions(-)
>