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

[David Hildenbrand]

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? :)
>
> > 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 :)

--
Cheers,

David / dhildenb