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

[Vernon Yang]

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".

> This raises a question: if a VMA without VM_DROPPABLE has
> many contiguous lazyfree folios that can be collapsed, and
> none of those folios are non-lazyfree, should we collapse
> them and pass the lazyfree state to the new folio?
>
> Currently, our approach skips the collapse, which also feels
> a bit inconsistent.

Yes, they are inconsistent, because this question need to scan all folios
to make a decision, and it cannot solve the hot1->cold->hot2 scenario.

> > will free it up. In contrast, collapsing in !VM_DROPPABLE does not
> > maintain that property, the collapsed folio will not be lazyfree and
> > memory pressure in the future will not be able to free it up.
> >
> > So if the user has explicitly informed us via MADV_FREE that this memory
> > will be freed, and this vma does not have VM_DROPPABLE flags, it is
> > appropriate for khugepaged to skip it only, thereby avoiding unnecessary
> > scan and collapse operations to reducing CPU wastage.
> >
> > Here are the performance test results:
> > (Throughput bigger is better, other smaller is better)
> >
> > Testing on x86_64 machine:
> >
> > | task hot2           | without patch | with patch    |  delta  |
> > |---------------------|---------------|---------------|---------|
> > | total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  |
> > | cycles per access   |  4.96         |  2.21         | -55.44% |
> > | Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  |
> > | dTLB-load-misses    |  284814532    |  69597236     | -75.56% |
> >
> > Testing on qemu-system-x86_64 -enable-kvm:
> >
> > | task hot2           | without patch | with patch    |  delta  |
> > |---------------------|---------------|---------------|---------|
> > | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
> > | cycles per access   |  7.29         |  2.07         | -71.60% |
> > | Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% |
> > | dTLB-load-misses    |  241600871    |  3216108      | -98.67% |
> >
> > Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
> > Acked-by: David Hildenbrand (arm) <david@xxxxxxxxxx>
> > Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
>
> Overall, LGTM,
>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

Thank you for review.

> >  include/trace/events/huge_memory.h |  1 +
> >  mm/khugepaged.c                    | 13 +++++++++++++
> >  2 files changed, 14 insertions(+)
> >
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index 384e29f6bef0..bcdc57eea270 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -25,6 +25,7 @@
> >         EM( SCAN_PAGE_LRU,              "page_not_in_lru")              \
> >         EM( SCAN_PAGE_LOCK,             "page_locked")                  \
> >         EM( SCAN_PAGE_ANON,             "page_not_anon")                \
> > +       EM( SCAN_PAGE_LAZYFREE,         "page_lazyfree")                \
> >         EM( SCAN_PAGE_COMPOUND,         "page_compound")                \
> >         EM( SCAN_ANY_PROCESS,           "no_process_for_page")          \
> >         EM( SCAN_VMA_NULL,              "vma_null")                     \
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 61e25cf5424b..e792e9074b48 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -46,6 +46,7 @@ enum scan_result {
> >         SCAN_PAGE_LRU,
> >         SCAN_PAGE_LOCK,
> >         SCAN_PAGE_ANON,
> > +       SCAN_PAGE_LAZYFREE,
> >         SCAN_PAGE_COMPOUND,
> >         SCAN_ANY_PROCESS,
> >         SCAN_VMA_NULL,
> > @@ -574,6 +575,12 @@ 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 (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
> > +                   folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
>
> 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.
 */

> > +                       result = SCAN_PAGE_LAZYFREE;
> > +                       goto out;
> > +               }
> > +
> >                 /* See hpage_collapse_scan_pmd(). */
> >                 if (folio_maybe_mapped_shared(folio)) {
> >                         ++shared;
> > @@ -1326,6 +1333,12 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >                 }
> >                 folio = page_folio(page);
> >
> > +               if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
> > +                   folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
> > +                       result = SCAN_PAGE_LAZYFREE;
> > +                       goto out_unmap;
> > +               }
>
> As above.
>
> > +
> >                 if (!folio_test_anon(folio)) {
> >                         result = SCAN_PAGE_ANON;
> >                         goto out_unmap;
> > --
> > 2.51.0
> >
>
> Thanks
> Barry
>