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

[Zi Yan <ziy@nvidia.com>]

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On 8 Oct 2024, at 9:06, David Hildenbrand wrote:

> On 08.10.24 14:57, Vlastimil Babka wrote:
>> On 10/8/24 13:52, Zi Yan wrote:
>>> On 8 Oct 2024, at 4:26, David Hildenbrand wrote:
>>>
>>>>
>>>> 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
>>
>> Create some nice inline wrapper for the test and it will look less ugly? :)

something like?

static inline bool alloc_zeroed()
{
	return static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
			&init_on_alloc);
}


I missed another folio_zero_user() caller in alloc_anon_folio() for mTHP.
So both PMD THP and mTHP are zeroed twice for all arch.

Adding Ryan for mTHP.

>>
>>> 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?
>>
>> I'd rather not change the page allocation code for this...
>
> Although I'm curious if that optimization from 2017 is still valuable :)

Maybe Ying can give some insight on this.


Do we need some general guidance on who is responsible for zeroing allocated
folios? Should people use GFP_ZERO instead of zeroing by themselves if possible?


Best Regards,
Yan, Zi

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