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

[Zi Yan]

On 8 Oct 2024, at 4:26, David Hildenbrand wrote:

> 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@xxxxxxxxxx>
>> ---
>>   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?

I agree that putting CONFIG_INIT_ON_ALLOC_DEFAULT_ON here is not ideal, but
folio_zero_user() uses vmf->address to improve cache performance by changing
subpage clearing order. See commit c79b57e462b5 ("mm: hugetlb: clear target
sub-page last when clearing huge page”). If we use GFP_ZERO, we lose this
optimization. To keep it, vmf->address will need to be passed to allocation
code. Maybe that is acceptable?

Best Regards,
Yan, Zi

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