Re: [PATCH] mm/huge_memory: skip huge_zero_pmd in zap_huge_pmd_folio()

[Lance Yang <lance.yang@linux.dev>]

On Thu, Apr 30, 2026 at 12:11:20PM +0800, Bibo Mao wrote:
>when executing command "make check" with qemu software, there is
>error report like this:
> BUG: Bad rss-counter state mm:00000000972846bc type:MM_FILEPAGES val:-4096 Comm:bios-tables-tes Pid:27802
> BUG: Bad rss-counter state mm:00000000752180c5 type:MM_FILEPAGES val:-2048 Comm:worker Pid:27815
> BUG: Bad rss-counter state mm:000000009c2f6a61 type:MM_FILEPAGES val:-2048 Comm:qom-test Pid:27825

Good catch!

>The problem is that when application exits, rss counter is calculated
>with huge_zero_pmd huge page, instead it should be skipped.

Looks like the same problem[1] we discussed recently.

[1] https://lore.kernel.org/linux-mm/74a75b59-2e13-3985-ee99-d5521f39df2a@google.com/

>Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>---
> mm/huge_memory.c | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 970e077019b7..3cbea344d4a2 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2423,6 +2423,9 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
> {
> 	const bool is_device_private = folio_is_device_private(folio);
> 
>+	if (is_huge_zero_pmd(pmdval))
>+		return;
>+

The huge zero PMD should not be returned by vm_normal_page_pmd() or
vm_normal_folio_pmd() as a normal folio. If it reaches
zap_huge_pmd_folio(), we already made the wrong normal-vs-special
decision ...

So I don't think we should special-case it in zap_huge_pmd_folio(). That
only avoids this RSS decrement :)

Could you please check whether the fix[2] also fixes your QEMU test?

[2] https://lore.kernel.org/linux-mm/ea1453a6-14c9-4334-ac7e-2758586393b2@kernel.org/

Thanks, Lance