Re: [PATCH] x86/mm: Define the zapped-entry checks only with CONFIG_DEBUG_VM

[Jose A. Perez de Azpillaga]

On 11/2/26 3:39 PM, Kiryl Shutsemau (Meta) wrote:
> Zapping a range calls arch_check_zapped_pte() for every batch of PTEs,
> which is every page for small folios, and arch_check_zapped_pmd() or
> arch_check_zapped_pud() for every huge leaf. On x86 these are out-of-line
> functions whose only content is a VM_WARN_ON_ONCE(), so without
> CONFIG_DEBUG_VM they are empty, but the call remains. With the SRSO "Safe
> RET" mitigation the return of every such call mispredicts.
> 
> Declare and define the checks only with CONFIG_DEBUG_VM. Otherwise the
> empty inlines from <linux/pgtable.h> apply, as on every other
> architecture. They cannot be inlines in <asm/pgtable.h>: VM_SHADOW_STACK
> is defined in <linux/mm.h> after it includes <linux/pgtable.h>.
> 
> In a KVM guest on Zen 4 with Safe RET, zapping 4K pages takes one branch
> miss per page fewer. MADV_DONTNEED of 1 GiB gets 6% faster on anonymous
> memory and 8% on shmem, munmap() 5%. THP is unchanged.
> 
> Fixes: e5136e876581 ("mm: Warn on shadow stack memory in wrong vma")
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
LGTM:

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>

-- 
cheers,
jose a. p-a