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

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Message-ID: <7762D266-82C6-48E2-BF80-211413BDA1EB@nvidia.com>
Date: Mon, 21 Oct 2024 10:21:39 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "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

On 21 Oct 2024, at 8:23, David Hildenbrand wrote:

> 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.

I changed the wording a bit to fit the if statement and put the comment in both
call sites. Let me know how it looks. Thanks.

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 830d6aa5bf97..b304bb3ffcef 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1180,6 +1180,11 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma,
        }
        folio_throttle_swaprate(folio, gfp);

+       /*
+        * When a folio is not zeroed during allocation (__GFP_ZERO not used),
+        * folio_zero_user() is used to make sure that the page corresponding
+        * to the faulting address will be hot in the cache after zeroing.
+        */
        if (!alloc_zeroed())
                folio_zero_user(folio, addr);
        /*
diff --git a/mm/memory.c b/mm/memory.c
index 0f614523b9f4..42c8bb5fcd8e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4719,6 +4719,13 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
                                goto next;
                        }
                        folio_throttle_swaprate(folio, gfp);
+                       /*
+                        * When a folio is not zeroed during allocation
+                        * (__GFP_ZERO not used), folio_zero_user() is used
+                        * to make sure that the page corresponding to the
+                        * faulting address will be hot in the cache after
+                        * zeroing.
+                        */
                        if (!alloc_zeroed())
                                folio_zero_user(folio, vmf->address);
                        return folio;


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

Thanks.

--
Best Regards,
Yan, Zi

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