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

[Vernon Yang <vernon2gm@gmail.com>]

On Tue, Feb 24, 2026 at 04:10:47AM +0800, Barry Song wrote:
> On Sat, Feb 21, 2026 at 9:39 PM Vernon Yang <vernon2gm@gmail.com> 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
            ^^
here

> > > > 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.

Hi Andrew, could you please squash the following fix into this patch?
also remove "if" in the changelog above.

---
From ab5060c7be655dd00bf3a9abc779915922b2f969 Mon Sep 17 00:00:00 2001
From: Vernon Yang <yanglincheng@kylinos.cn>
Date: Thu, 26 Feb 2026 13:18:39 +0800
Subject: [PATCH] fixup! mm: khugepaged: skip lazy-free folios

add comment about VM_DROPPABLE in code, make it clearer.

Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c85d7381adb5..7c1642fbe394 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -575,6 +575,10 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		folio = page_folio(page);
 		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);

+		/*
+		 * If the vma has the VM_DROPPABLE flag, the collapse will
+		 * preserve the lazyfree property without needing to skip.
+		 */
 		if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
 		    folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
 			result = SCAN_PAGE_LAZYFREE;
@@ -1333,6 +1337,10 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
 		}
 		folio = page_folio(page);

+		/*
+		 * If the vma has the VM_DROPPABLE flag, the collapse will
+		 * preserve the lazyfree property without needing to skip.
+		 */
 		if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
 		    folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
 			result = SCAN_PAGE_LAZYFREE;
--
2.51.0