Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs

[Lance Yang]

On 2025/9/18 15:37, Dev Jain wrote:
> On 18/09/25 10:34 am, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > Guard PTE markers are installed via MADV_GUARD_INSTALL to create
> > lightweight guard regions.
> >
> > Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when
> > encountering such a range.
> >
> > MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in
> > the special marker in __collapse_huge_page_swapin().
> >
> > hpage_collapse_scan_pmd()
> >   `- collapse_huge_page()
> >       `- __collapse_huge_page_swapin() -> fails!
> >
> > khugepaged's behavior is slightly different due to its max_ptes_swap 
> > limit
> > (default 64). It won't fail as deep, but it will still needlessly scan up
> > to 64 swap entries before bailing out.
> >
> > IMHO, we can and should detect this much earlier.
> >
> > This patch adds a check directly inside the PTE scan loop. If a guard
> > marker is found, the scan is aborted immediately with 
> > SCAN_PTE_NON_PRESENT,
> > avoiding wasted work.
> >
> > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >   mm/khugepaged.c | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 9ed1af2b5c38..70ebfc7c1f3e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct 
> > mm_struct *mm,
> >                       result = SCAN_PTE_UFFD_WP;
> >                       goto out_unmap;
> >                   }
> > +                /*
> > +                 * Guard PTE markers are installed by
> > +                 * MADV_GUARD_INSTALL. Any collapse path must
> > +                 * not touch them, so abort the scan immediately
> > +                 * if one is found.
> > +                 */
> > +                if (is_guard_pte_marker(pteval)) {
> > +                    result = SCAN_PTE_NON_PRESENT;
> > +                    goto out_unmap;
> > +                }
> >                   continue;
> >               } else {
> >                   result = SCAN_EXCEED_SWAP_PTE;
>
> I would like to hear everyone else's thoughts on
> https://lore.kernel.org/linux-mm/750a06dc-db3d-43c6- 
> b234-95efb393a9df@xxxxxxx/
> wherein I suggest that we should not continue to try collapsing other 
> regions
> but immediately exit. The SCAN_PTE_NON_PRESENT case does not exit.

Yes! Let's hear what other folks think on that[1].

[1] 
https://lore.kernel.org/linux-mm/c9d4d761-202f-48ce-8e3d-fb9075671ff3@xxxxxxxxx

Cheers,
Lance