Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default

David Hildenbrand
2025-09-05T15:11:54Z

On 05.09.25 16:11, David Hildenbrand wrote:
> We added an early exit in thp_underused(), probably to avoid scanning
> pages when there is no chance for success.
>
> However, assume we have max_ptes_none = 511 (default).
>
> Nothing should stop us from freeing all pages part of a THP that
> is completely zero (512) and khugepaged will for sure not try to
> instantiate a THP in that case (512 shared zeropages).
>
> This can just trivially happen if someone writes a single 0 byte into a
> PMD area, or of course, when data ends up being zero later.
>
> So let's remove that early exit.
>
> Do we want to CC stable? Hm, not sure. Probably not urgent.
>
> Note that, as default, the THP shrinker is active
> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all
> THPs are added to the deferred split lists. However, with the
> max_ptes_none default we would never scan them. We would not do that. If
> that's not desirable, we should just disable the shrinker as default,
> also not adding all THPs to the deferred split lists.
>
> Easy to reproduce:
>

Just a note that I will send a v2 with an updated patch description
after the discussions here settled.

--
Cheers

David / dhildenb