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

[Barry Song <21cnbao@gmail.com>]

On Mon, Feb 23, 2026 at 9:16 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 2/21/26 14:38, Vernon Yang 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@gmail.com> wrote:
> >>>
> >>> From: Vernon Yang <yanglincheng@kylinos.cn>
> >>>
> >>> 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".
>
>
> Exactly. folio_add_new_anon_rmap() makes sure that all folios (except
> the shared zero folios ;) ) in VM_DROPPABLE are lazyfree.
>
> In fact, MADV_FREE should be a NOP on VM_DROPPABLE, as
> folio_mark_lazyfree() doesn't do anything.
>

Maybe we could do something like the following?

diff --git a/mm/madvise.c b/mm/madvise.c
index c0370d9b4e23..173b0e5308b5 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -817,6 +817,11 @@ static int madvise_free_single_vma(struct
madvise_behavior *madv_behavior)
        range.end = min(vma->vm_end, end_addr);
        if (range.end <= vma->vm_start)
                return -EINVAL;
+
+       /* All folios in the VM_DROPPABLE VMA are already lazyfree */
+       if (vma->vm_flags & VM_DROPPABLE)
+               return 0;
+
        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
                                range.start, range.end);

Thanks
Barry