Re: [PATCH RFC v2 11/18] mm: skip zeroing in vma_alloc_zeroed_movable_folio for pre-zeroed pages

David Hildenbrand (Arm)
2026-04-21T11:12:26Z

On 4/21/26 12:58, David Hildenbrand (Arm) wrote:
> On 4/20/26 14:50, Michael S. Tsirkin wrote:
>> Use vma_alloc_folio_hints() and check PGHINT_ZEROED to skip
>> clear_user_highpage() when the page is already zeroed.
>>
>> On x86, vma_alloc_zeroed_movable_folio is overridden by a macro
>> that uses __GFP_ZERO directly, so this change has no effect there.
>>
>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>> Assisted-by: Claude:claude-opus-4-6
>> Assisted-by: cursor-agent:GPT-5.4-xhigh
>> ---
>> include/linux/highmem.h | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index af03db851a1d..8bb67772c1cb 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -321,9 +321,11 @@ struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>> unsigned long vaddr)
>> {
>> struct folio *folio;
>> + pghint_t hints;
>>
>> - folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
>> - if (folio && user_alloc_needs_zeroing())
>> + folio = vma_alloc_folio_hints(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr,
>> + &hints);
>> + if (folio && user_alloc_needs_zeroing() && !(hints & PGHINT_ZEROED))
>> clear_user_highpage(&folio->page, vaddr);
>>
>> return folio;
>
>
> For others reading along, the variant on your other branch:
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index af03db851a1d9..ffa683f64f1d1 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -320,13 +320,8 @@ static inline
> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> unsigned long vaddr)
> {
> - struct folio *folio;
> -
> - folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
> - if (folio && user_alloc_needs_zeroing())
> - clear_user_highpage(&folio->page, vaddr);
> -
> - return folio;
> + return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO,
> + 0, vma, vaddr);
> }
> #endif
>
> Looks like an extremely clean interface.
>

And we could likely just do the following on top I assume.

diff --git a/arch/m68k/include/asm/page_no.h b/arch/m68k/include/asm/page_no.h
index d2532bc407ef..f511b763a235 100644
--- a/arch/m68k/include/asm/page_no.h
+++ b/arch/m68k/include/asm/page_no.h
@@ -12,9 +12,6 @@ extern unsigned long memory_end;

#define copy_user_page(to, from, vaddr, pg) copy_page(to, from)

-#define vma_alloc_zeroed_movable_folio(vma, vaddr) \
- vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr)
-
#define __pa(vaddr) ((unsigned long)(vaddr))
#define __va(paddr) ((void *)((unsigned long)(paddr)))

diff --git a/arch/s390/include/asm/page.h b/arch/s390/include/asm/page.h
index 56da819a79e6..e995d2a413f9 100644
--- a/arch/s390/include/asm/page.h
+++ b/arch/s390/include/asm/page.h
@@ -67,9 +67,6 @@ static inline void copy_page(void *to, void *from)

#define copy_user_page(to, from, vaddr, pg) copy_page(to, from)

-#define vma_alloc_zeroed_movable_folio(vma, vaddr) \
- vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr)
-
#ifdef CONFIG_STRICT_MM_TYPECHECKS
#define STRICT_MM_TYPECHECKS
#endif
diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
index 416dc88e35c1..92fa975b46f3 100644
--- a/arch/x86/include/asm/page.h
+++ b/arch/x86/include/asm/page.h
@@ -28,9 +28,6 @@ static inline void copy_user_page(void *to, void *from, unsigned long vaddr,
copy_page(to, from);
}

-#define vma_alloc_zeroed_movable_folio(vma, vaddr) \
- vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr)
-
#ifndef __pa
#define __pa(x) __phys_addr((unsigned long)(x))
#endif

--
Cheers,

David