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

[Zi Yan <ziy@nvidia.com>]

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On 21 Oct 2024, at 10:21, Zi Yan wrote:

> 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@nvidia.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.

Hi Andrew,

Do you mind folding the changes below into the original patch?

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@redhat.com>
>
> Thanks.

--
Best Regards,
Yan, Zi

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