Re: [RFC PATCH] mm: avoid clearing user movable page twice with init_on_alloc=1

[David Hildenbrand <david@redhat.com>]

On 07.10.24 20:23, Zi Yan wrote:
> Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and
> init_on_free=1 boot options") forces allocated page to be cleared in
> post_alloc_hook() when init_on_alloc=1.
> 
> For non PMD folios, if arch does not define
> vma_alloc_zeroed_movable_folio(), the default implementation again clears
> the page return from the buddy allocator. So the page is cleared twice.
> Fix it by passing __GFP_ZERO instead to avoid double page clearing.
> At the moment, s390,arm64,x86,alpha,m68k are not impacted since they
> define their own vma_alloc_zeroed_movable_folio().
> 
> For PMD folios, folio_zero_user() is called to clear the folio again.
> Fix it by calling folio_zero_user() only if init_on_alloc is set.
> All arch are impacted.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   include/linux/highmem.h | 14 ++------------
>   mm/huge_memory.c        |  4 +++-
>   2 files changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 930a591b9b61..4b15224842e1 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -220,18 +220,8 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
>    * Return: A folio containing one allocated and zeroed page or NULL if
>    * we are out of memory.
>    */
> -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, false);
> -	if (folio)
> -		clear_user_highpage(&folio->page, vaddr);
> -
> -	return folio;
> -}
> +#define vma_alloc_zeroed_movable_folio(vma, vaddr) \
> +	vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr, false)
>   #endif
>   
>   static inline void clear_highpage(struct page *page)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index a7b05f4c2a5e..ff746151896f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1177,7 +1177,9 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>   		goto release;
>   	}
>   
> -	folio_zero_user(folio, vmf->address);
> +	if (!static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
> +				&init_on_alloc))
> +		folio_zero_user(folio, vmf->address);
>   	/*
>   	 * The memory barrier inside __folio_mark_uptodate makes sure that
>   	 * folio_zero_user writes become visible before the set_pmd_at()

I remember we discussed that in the past and that we do *not* want to 
sprinkle these CONFIG_INIT_ON_ALLOC_DEFAULT_ON checks all over the kernel.

Ideally, we'd use GFP_ZERO and have the buddy just do that for us? There 
is the slight chance that we zero-out when we're not going to use the 
allocated folio, but ... that can happen either way even with the 
current code?


-- 
Cheers,

David / dhildenb