Re: [PATCH v2 05/12] mm/collapse: state what a collapse may do in the policy

[Baolin Wang]

On 9/10/26 8:02 PM, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
>
> Tests scattered through the collapse path decide what a collapse is
> allowed to do by asking whether khugepaged started it.  Between them they
> settle:
>
>   - which VMAs are eligible, and how hard to try for a folio;
>   - how many empty, swapped-out or shared PTEs a window may contain, and
>     whether a sub-PMD window is held to a stricter rule than a PMD;
>   - whether a range has to look used, and whether a MADV_FREE'd page is
>     left alone;
>   - whether the PMD is mapped as part of the request, and whether dirty
>     pages are worth writing back and retrying.
>
> None of those is a fact about khugepaged.  Each is something the caller
> decided before asking, and the collapse code should not have to look up
> who called to find out.
>
> Add struct collapse_policy for the caller to fill: khugepaged from its
> own settings, MADV_COLLAPSE from the fact that a user asked explicitly.
> Every test becomes a read of a field, and cc->is_khugepaged goes, having
> no reader left.
>
> khugepaged fills the policy once per scan pass, MADV_COLLAPSE once per
> call.  That is the one change in behaviour.  The max_ptes_* limits and the
> defrag setting behind the allocation mask are sampled once per pass rather
> than on every table.  A table scanned early in a pass and one scanned late
> are then judged alike.
>
> collapse_file() also drops a NULL check on the collapse_control.  It has
> one call site, reached only from collapse_single_pmd(), which dereferences
> cc unconditionally, so the check was already dead.
>
> Assisted-by: LLM
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> ---

Thanks. The logic is much clearer now.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>