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

[Huang, Ying]

Zi Yan <ziy@xxxxxxxxxx> writes:

> 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.

I guess the optimization still applies now.  Although the size of the
per-core(thread) last level cache increases, it's still quite common for
it to be smaller than the size of THP.  And the sizes of L1/L2 are
significantly smaller, the likelihood for the accessed cache line to be
in L1/L2/LLC increases with the optimization.

--
Best Regards,
Huang, Ying