Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

[Lance Yang]

On Fri, Feb 2, 2024 at 4:37 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Thu, Feb 1, 2024 at 4:53 AM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >
> > The collapsing behavior of khugepaged with pages
> > marked using MADV_FREE might cause confusion
> > among users.
> >
> > For instance, allocate a 2MB chunk using mmap and
> > later release it by MADV_FREE. Khugepaged will not
> > collapse this chunk. From the user's perspective,
> > it treats lazyfree pages as pte_none. However,
> > for some pages marked as lazyfree with MADV_FREE,
> > khugepaged might collapse this chunk and copy
> > these pages to a new huge page. This inconsistency
> > in behavior could be confusing for users.
> >
> > After a successful MADV_FREE operation, if there is
> > no subsequent write, the kernel can free the pages
> > at any time. Therefore, in my opinion, counting
> > lazyfree pages in max_pte_none seems reasonable.
> >
> > Perhaps treating MADV_FREE like MADV_DONTNEED, not
> > copying lazyfree pages when khugepaged collapses
> > huge pages in the background better aligns with
> > user expectations.
> >
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2b219acb528e..6cbf46d42c6a 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -777,6 +777,7 @@ static int __collapse_huge_page_copy(pte_t *pte,
> >                                      pmd_t orig_pmd,
> >                                      struct vm_area_struct *vma,
> >                                      unsigned long address,
> > +                                    struct collapse_control *cc,
> >                                      spinlock_t *ptl,
> >                                      struct list_head *compound_pagelist)
> >  {
> > @@ -797,6 +798,13 @@ static int __collapse_huge_page_copy(pte_t *pte,
> >                         continue;
> >                 }
> >                 src_page = pte_page(pteval);
> > +
> > +               if (cc->is_khugepaged
> > +                               && !folio_test_swapbacked(page_folio(src_page))) {
> > +                       clear_user_highpage(page, _address);
> > +                       continue;
>
> If the page was written before khugepaged collapsed it, and khugepaged
> collapsed the page before memory reclaim kicked in, didn't this
> somehow cause data corruption?
>

Thanks a lot! Yang, you're correct; indeed, there is
a potential issue with data corruption.

I took a look at the check for lazyfree pages in
smaps_pte_entry.

Here's the modification:
if (cc->is_khugepaged && !PageSwapBacked(src_page)
        && !pte_dirty(pteval) && !PageDirty(src_page)) {
        clear_user_highpage(page, _address);
        continue;
}

Could you please take a look?

Thanks,
Lance

> > +               }
> > +
> >                 if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
> >                         result = SCAN_COPY_MC;
> >                         break;
> > @@ -1205,7 +1213,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >         anon_vma_unlock_write(vma->anon_vma);
> >
> >         result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> > -                                          vma, address, pte_ptl,
> > +                                          vma, address, cc, pte_ptl,
> >                                            &compound_pagelist);
> >         pte_unmap(pte);
> >         if (unlikely(result != SCAN_SUCCEED))
> > --
> > 2.33.1
> >