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

[Baolin Wang]

On 9/7/26 6:56 PM, Kiryl Shutsemau wrote:
> On Mon, Sep 07, 2026 at 05:05:19PM +0800, Baolin Wang wrote:
> > > +/* What a collapse is allowed to do, decided by the caller that asks for it */
> > > +struct collapse_policy {
> > > +	/* Limits, stated per PMD; HPAGE_PMD_NR means "no limit" */
> > > +	unsigned int max_ptes_none;
> > > +	unsigned int max_ptes_swap;
> > > +	unsigned int max_ptes_shared;
> > > +
> > > +	/*
> > > +	 * Hold a sub-PMD window to a stricter rule than a PMD: no swapped-out
> > > +	 * and no shared PTEs at all, and max_ptes_none as
> > > +	 * collapse_max_ptes_none() scales it.
> > > +	 */
> > > +	bool strict_sub_pmd;
> >
> > This is a bit confusing to me. Actually, the check for mTHP collapse is
> > stricter.
> >
> > How about naming it 'allow_mthp_collapse'? That way we can keep the most
> > original comments for the collapse_max_ptes_xxx() functions, which is
> > clearer to me.
> >
> > If others have a better name, please ignore my comment.
>
> I would rather keep strict_sub_pmd.
>
> Which orders a caller asks for is already decided elsewhere:
> collapse_possible_orders() hands khugepaged every anonymous order and
> MADV_COLLAPSE the PMD order only, from tva_type.
>
> A flag called allow_mthp_collapse next to that would read as
> a second place deciding the same thing, and flipping it would not change
> which orders get collapsed.  What it does change is how a sub-PMD window
> is judged once one is asked for, and it is read at exactly the three
> places that judge one.
>
> I will do:
>
>        /* Take no swapped-out or shared PTE into a sub-PMD collapse */
>        bool strict_sub_pmd;

OK. Fair enough. Thanks.