Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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

On Thu, Oct 9, 2025 at 3:56 AM Lance Yang <lance.yang@linux.dev> wrote:
>
> From: Lance Yang <lance.yang@linux.dev>
>
> Currently, special non-swap entries (like PTE markers) are not caught
> early in hpage_collapse_scan_pmd(), leading to failures deep in the
> swap-in logic.
>
> A function that is called __collapse_huge_page_swapin() and documented
> to "Bring missing pages in from swap" will handle other types as well.
>
> As analyzed by David[1], we could have ended up with the following
> entry types right before do_swap_page():
>
>   (1) Migration entries. We would have waited.
>       -> Maybe worth it to wait, maybe not. We suspect we don't stumble
>          into that frequently such that we don't care. We could always
>          unlock this separately later.
>

On phones, we’ve actually observed user-visible stutters when the
migration process gets preempted and a foreground app is waiting for
do_swap_page() to complete. Anyway, that’s a separate issue we might
address later.

>   (2) Device-exclusive entries. We would have converted to non-exclusive.
>       -> See make_device_exclusive(), we cannot tolerate PMD entries and
>          have to split them through FOLL_SPLIT_PMD. As popped up during
>          a recent discussion, collapsing here is actually
>          counter-productive, because the next conversion will PTE-map
>          it again.
>       -> Ok to not collapse.
>
>   (3) Device-private entries. We would have migrated to RAM.
>       -> Device-private still does not support THPs, so collapsing right
>          now just means that the next device access would split the
>          folio again.
>       -> Ok to not collapse.
>
>   (4) HWPoison entries
>       -> Cannot collapse
>
>   (5) Markers
>       -> Cannot collapse
>
> First, this patch adds an early check for these non-swap entries. If
> any one is found, the scan is aborted immediately with the
> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
> work. While at it, convert pte_swp_uffd_wp_any() to pte_swp_uffd_wp()
> since we are in the swap pte branch.
>
> Second, as Wei pointed out[3], we may have a chance to get a non-swap
> entry, since we will drop and re-acquire the mmap lock before
> __collapse_huge_page_swapin(). To handle this, we also add a
> non_swap_entry() check there.
>
> Note that we can unlock later what we really need, and not account it
> towards max_swap_ptes.
>
> [1] https://lore.kernel.org/linux-mm/09eaca7b-9988-41c7-8d6e-4802055b3f1e@redhat.com
> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
> [3] https://lore.kernel.org/linux-mm/20251005010511.ysek2nqojebqngf3@master
>
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---

Reviewed-by: Barry Song <baohua@kernel.org>

Thanks
Barry