Re: [PATCH v10 00/13] khugepaged: mTHP support

Dev Jain
2025-08-21T15:14:07Z

On 21/08/25 8:31 pm, Lorenzo Stoakes wrote:
> OK so I noticed in patch 13/13 (!) where you change the documentation that you
> essentially state that the whole method used to determine the ratio of PTEs to
> collapse to mTHP is broken:
>
> khugepaged uses max_ptes_none scaled to the order of the enabled
> mTHP size to determine collapses. When using mTHPs it's recommended
> to set max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255
> on 4k page size). This will prevent undesired "creep" behavior that
> leads to continuously collapsing to the largest mTHP size; when we
> collapse, we are bringing in new non-zero pages that will, on a
> subsequent scan, cause the max_ptes_none check of the +1 order to
> always be satisfied. By limiting this to less than half the current
> order, we make sure we don't cause this feedback
> loop. max_ptes_shared and max_ptes_swap have no effect when
> collapsing to a mTHP, and mTHP collapse will fail on shared or
> swapped out pages.
>
> This seems to me to suggest that using
> /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none as some means
> of establishing a 'ratio' to do this calculation is fundamentally flawed.
>
> So surely we ought to introduce a new sysfs tunable for this? Perhaps
>
> /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio
>
> Or something like this?
>
> It's already questionable that we are taking a value that is expressed
> essentially in terms of PTE entries per PMD and then use it implicitly to
> determine the ratio for mTHP, but to then say 'oh but the default value is
> known-broken' is just a blocker for the series in my opinion.
>
> This really has to be done a different way I think.
>
> Cheers, Lorenzo

FWIW this was my version of the documentation patch:
https://lore.kernel.org/all/20250211111326.14295-18-dev.jain@arm.com/

The discussion about the creep problem started here:
https://lore.kernel.org/all/7098654a-776d-413b-8aca-28f811620df7@arm.com/

and the discussion continuing here:
https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com/

ending with a summary I gave here:
https://lore.kernel.org/all/8114d47b-b383-4d6e-ab65-a0e88b99c873@arm.com/

This should help you with the context.