Re: [RFC 1/3] mm: add large zero page for efficient zeroing of larger segments

[David Hildenbrand]

On 16.05.25 15:03, Pankaj Raghav (Samsung) wrote:
> On Fri, May 16, 2025 at 02:21:04PM +0200, David Hildenbrand wrote:
> > On 16.05.25 12:10, Pankaj Raghav wrote:
> > > Introduce LARGE_ZERO_PAGE of size 2M as an alternative to ZERO_PAGE of
> > > size PAGE_SIZE.
> > >
> > > There are many places in the kernel where we need to zeroout larger
> > > chunks but the maximum segment we can zeroout at a time is limited by
> > > PAGE_SIZE.
> > >
> > > This is especially annoying in block devices and filesystems where we
> > > attach multiple ZERO_PAGEs to the bio in different bvecs. With multipage
> > > bvec support in block layer, it is much more efficient to send out
> > > larger zero pages as a part of single bvec.
> > >
> > > While there are other options such as huge_zero_page, they can fail
> > > based on the system memory pressure requiring a fallback to ZERO_PAGE[3].
> >
> > Instead of adding another one, why not have a config option that will always
> > allocate the huge zeropage, and never free it?
> >
> > I mean, the whole thing about dynamically allocating/freeing it was for
> > memory-constrained systems. For large systems, we just don't care.
>
> That sounds like a good idea. I was just worried about wasting too much
> memory with a huge page in systems with 64k page size. But it can always be
> disabled by putting it behind a config.

Exactly. If the huge zero page is larger than 2M, we probably don't want 
it in any case.

On arm64k it could be 512 of MiBs. Full of zeroes.

I'm wondering why nobody ever complained about that before, and I don't 
see anything immediate that would disable the huge zero page in such 
environments. Well, we can just leave that as it is.

In any case, the idea would be to have a Kconfig where we statically 
allocate the huge zero page and disable all the refcounting / shrinking.

Then, we can make this Kconfig specific to sane environments (e.g., 4 
KiB page size).

From other MM code, we can then simply reuse that single huge zero page.

> Thanks, David. I will wait to see what others think but what you
> suggested sounds like a good idea on how to proceed.

In particular, it wouldn't be arch specific, and we wouldn't waste on 
x86 2x 2MB for storing zeroes ...

--
Cheers,

David / dhildenb