Re: [PATCH mm-new v8 4/4] mm: khugepaged: skip lazy-free folios

[Barry Song]

On Sat, Feb 21, 2026 at 9:39 PM Vernon Yang <vernon2gm@xxxxxxxxx> wrote:
>
> On Sat, Feb 21, 2026 at 06:27:36PM +0800, Barry Song wrote:
> > On Sat, Feb 21, 2026 at 5:40 PM Vernon Yang <vernon2gm@xxxxxxxxx> wrote:
> > >
> > > From: Vernon Yang <yanglincheng@xxxxxxxxxx>
> > >
> > > For example, create three task: hot1 -> cold -> hot2. After all three
> > > task are created, each allocate memory 128MB. the hot1/hot2 task
> > > continuously access 128 MB memory, while the cold task only accesses
> > > its memory briefly and then call madvise(MADV_FREE). However, khugepaged
> > > still prioritizes scanning the cold task and only scans the hot2 task
> > > after completing the scan of the cold task.
> > >
> > > And if all folios in VM_DROPPABLE are lazyfree, Collapsing maintains
> > > that property, so we can just collapse and memory pressure in the future
> >
> > I don’t think this is accurate. A VMA without VM_DROPPABLE
> > can still have all folios marked as lazyfree. Therefore, having
> > all folios lazyfree is not the reason why collapsing preserves
> > the property.
>
> In folio_add_new_anon_rmap(), we know that the vma has the VM_DROPPABLE
> attribute, which is the root reason why Collapsing maintains that property.
> The above commit log clearly states "all folios in VM_DROPPABLE are lazyfree"
>                                                 ^^^^^^^^^^^^^^^
> (the "if" is redundant and should be removed), not "all folios are lazyfree".

Yes, we should remove the if; otherwise, it’s misleading.

[...]

> >
> > I would prefer to add a comment about VM_DROPPABLE here
> > rather than only mentioning it in the changelog.
>
> Is the following comment clear?
>
> /*
>  * If the vma has the VM_DROPPABLE flag, the collapse will
>  * preserve the lazyfree property without needing to skip.
>  */

Looks good to me.

Best Regards
Barry