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

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

On 2026/4/30 12:28, Lance Yang wrote:
> 
> 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?

In addition, like x86-32, 64-bit LoongArch selects ARCH_HAS_PTE_SPECIAL,
but not ARCH_SUPPORTS_HUGE_PFNMAP. So CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
is not enabled, and pmd_special() falls back to the generic stub that
always returns false.

So I guess the fix should do the trick :)

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