Re: [PATCH for 7.4-rc2+ 7/7] mm/rmap: re-enable discarding lazyfree THPs at PMD level

[Zi Yan]

On Thu Oct 15, 2026 at 2:13 PM EDT, Zi Yan wrote:
> The patch "mm/rmap: don't discard lazyfree THPs at PMD level" disabled
> the PMD-level discard, because it cleared the PMD before checking whether
> the folio could be discarded, and munmap() or mremap() could skip that
> transiently none PMD.
>
> __discard_anon_folio_pmd_locked() now keeps the PMD invalidated, not
> cleared, until the discard is committed, GUP-fast no longer follows an
> invalidated PMD on any architecture, powerpc page table check no longer
> counts an invalidated PMD as mapped, and sparc64 keeps thp_pte_count
> balanced when an invalidated PMD is set again or cleared. Discard lazyfree
> THPs at PMD level again.
>
> Assisted-by: LLM
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: David Hildenbrand <david@xxxxxxxxxx>
> To: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: Harry Yoo <harry@xxxxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Kyle Zeng <kylebot@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/rmap.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1131b76bbbc28..805db93fe0428 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2275,6 +2275,17 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  		}
>  
>  		if (!pvmw.pte) {
> +			if (folio_test_lazyfree(folio)) {
> +				if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, folio))

Sashiko mentioned that unmap_huge_pmd_locked() does add_mm_counter()
without calling update_hiwater_rss() first so that peak RSS can be
under-reported.

Answer:

yes, I fixed it locally by calling update_hiwater_rss() before
unmap_huge_pmd_locked().

> +					goto walk_done;
> +				/*
> +				 * unmap_huge_pmd_locked has either already marked
> +				 * the folio as swap-backed or decided to retain it
> +				 * due to GUP or speculative references.
> +				 */
> +				goto walk_abort;
> +			}
> +
>  			if (flags & TTU_SPLIT_HUGE_PMD) {
>  				/*
>  				 * We temporarily have to drop the PTL and




-- 
Best Regards,
Yan, Zi