Re: [PATCH v2 0/3] don't use mapcount() to check large folio sharing

[Yu Zhao]

On Mon, Aug 7, 2023 at 8:10 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
> folio_mapcount() is used to check whether the folio is shared. But it's
> not correct as folio_mapcount() returns total mapcount of large folio.
>
> Use folio_estimated_sharers() here as the estimated number is enough.
>
>
> This patchset will fix the cases:
> User space application call madvise() with MADV_FREE, MADV_COLD and
> MADV_PAGEOUT for specific address range. There are THP mapped to the
> range. Without the patchset, the THP is skipped. With the patch, the
> THP will be split and handled accordingly.
>
> David reported the cow self test skip some cases because of
> MADV_PAGEOUT skip THP:
> https://lore.kernel.org/linux-mm/9e92e42d-488f-47db-ac9d-75b24cd0d037@xxxxxxxxx/T/#mbf0f2ec7fbe45da47526de1d7036183981691e81
> and I confirmed this patchset make it work again.
>
>
> Changelog from v1:
>   - Avoid two Fixes tags make backport harder. Thank Andrew for pointing
>     this out.
>
>   - Add note section to mention this is a temporary fix which is fine
>     to reduce user-visble effects. For long term fix, we should wait for
>     David's solution. Thank Ryan and David for pointing this out.
>
>   - Spell user-visible effects out. Then people could decide whether
>     these patches are necessary for stable branch. Thank Andrew for
>     pointing this out.

LGTM, thank you.