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

[Kiryl Shutsemau]

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;

> 
> > +	/*
> > +	 * Collapse only where it looks worth doing: require some sign the
> > +	 * range is in use, and leave clean lazyfree folios for reclaim rather
> > +	 * than collapsing them into a folio that is not lazyfree.
> > +	 */
> > +	bool skip_lazyfree;
> > +	bool require_referenced;
> > +
> > +	/*
> > +	 * Finish the job rather than leaving it half done for a fault to pick
> > +	 * up: map the PMD over a file collapse before returning, and write
> > +	 * dirty pages back and retry once instead of refusing them.  Both cost
> > +	 * latency the caller has to be willing to pay.
> > +	 */
> 
> Can you simplify theses comments? I think the 'install_pmd' is easy to
> expalain. :)

Done for v2.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov