linux-kernel - Re: [PATCH] mm: avoid zeroing user movable page twice with init_on

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Message-ID: <a7279751-b9cd-4197-9c98-3aa70b1f5fe8@redhat.com>
Date: Mon, 21 Oct 2024 14:23:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Miaohe Lin <linmiaohe@...wei.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 John Hubbard <jhubbard@...dia.com>, "Huang, Ying" <ying.huang@...el.com>,
 Ryan Roberts <ryan.roberts@....com>, Alexander Potapenko
 <glider@...gle.com>, Kees Cook <keescook@...omium.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with
 init_on_alloc=1



Am 11.10.24 um 17:03 schrieb Zi Yan:
> Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and
> init_on_free=1 boot options") forces allocated page to be zeroed in
> post_alloc_hook() when init_on_alloc=1.
> 
> For order-0 folios, if arch does not define
> vma_alloc_zeroed_movable_folio(), the default implementation again zeros
> the page return from the buddy allocator. So the page is zeroed twice.
> Fix it by passing __GFP_ZERO instead to avoid double page zeroing.
> At the moment, s390,arm64,x86,alpha,m68k are not impacted since they
> define their own vma_alloc_zeroed_movable_folio().
> 
> For >0 order folios (mTHP and PMD THP), folio_zero_user() is called to
> zero the folio again. Fix it by calling folio_zero_user() only if
> init_on_alloc is set. All arch are impacted.
> 
> Added alloc_zeroed() helper to encapsulate the init_on_alloc check.
> 
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>   include/linux/highmem.h | 8 +-------
>   mm/huge_memory.c        | 3 ++-
>   mm/internal.h           | 6 ++++++
>   mm/memory.c             | 3 ++-
>   4 files changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index bec9bd715acf..6e452bd8e7e3 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -224,13 +224,7 @@ 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)
> -		clear_user_highpage(&folio->page, vaddr);
> -
> -	return folio;
> +	return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr);
>   }
>   #endif
>   
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 82f464865570..5dcbea96edb7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1176,7 +1176,8 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma,
>   	}
>   	folio_throttle_swaprate(folio, gfp);
>   
> -	folio_zero_user(folio, addr);
> +	if (!alloc_zeroed())
> +		folio_zero_user(folio, addr);



It might be reasonable to spell out why we are not using GFP_ZERO somewhere, 
something like

/*
  * We are not using __GFP_ZERO because folio_zero_user() will make sure that the
  * page corresponding to the faulting address will be hot in the cache.
  */

Sth. like that maybe.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb