Re: [PATCH 0/6] shmem: high order folios support in write path

[David Hildenbrand]

On 15.09.23 11:51, Daniel Gomez wrote:
> This series add support for high order folios in shmem write
> path.
>
> This is a continuation of the shmem work from Luis here [1]
> following Matthew Wilcox's suggestion [2] regarding the path to take
> for the folio allocation order calculation.
>
> [1] RFC v2 add support for blocksize > PAGE_SIZE
> https://lore.kernel.org/all/ZHBowMEDfyrAAOWH@xxxxxxxxxxxxxxxxxxxxxx/T/#md3e93ab46ce2ad9254e1eb54ffe71211988b5632
> [2] https://lore.kernel.org/all/ZHD9zmIeNXICDaRJ@xxxxxxxxxxxxxxxxxxxx/
>
> Patches have been tested and sent from next-230911. They do apply
> cleanly to the latest next-230914.
>
> fsx and fstests has been performed on tmpfs with noswap with the
> following results:
> - fsx: 2d test, 21,5B
> - fstests: Same result as baseline for next-230911 [3][4][5]
>
> [3] Baseline next-230911 failures are: generic/080 generic/126
> generic/193 generic/633 generic/689
> [4] fstests logs baseline: https://gitlab.com/-/snippets/3598621
> [5] fstests logs patches: https://gitlab.com/-/snippets/3598628
>
> There are at least 2 cases/topics to handle that I'd appreciate
> feedback.
> 1. With the new strategy, you might end up with a folio order matching
> HPAGE_PMD_ORDER. However, we won't respect the 'huge' flag anymore if
> THP is enabled.
> 2. When the above (1.) occurs, the code skips the huge path, so
> xa_find with hindex is skipped.

Similar to large anon folios (but different to large non-shmem folios in 
the pagecache), this can result in memory waste.

We discussed that topic in the last bi-weekly mm meeting, and also how 
to eventually configure that for shmem.

Refer to of a summary. [1]

[1] https://lkml.kernel.org/r/4966f496-9f71-460c-b2ab-8661384ce626@xxxxxxx

--
Cheers,

David / dhildenb