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

[Daniel Gomez]

On Fri, Sep 15, 2023 at 05:29:51PM +0200, David Hildenbrand wrote:
> 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

Thanks for the summary David (I was missing linux-MM from kvack in lei).

I think the PMD_ORDER-1 as max would suffice here to honor/respect the
huge flag. Although, we would end up having a different max value
than pagecache/readahead.
>
> --
> Cheers,
>
> David / dhildenb
>