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

[Pankaj Raghav (Samsung)]

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.

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

-- 
Pankaj Raghav