Re: [PATCH RFC v2 00/18] mm/virtio: skip redundant zeroing of host-zeroed reported pages

David Hildenbrand (Arm)
2026-04-20T18:21:07Z

On 4/20/26 14:51, Michael S. Tsirkin wrote:
>

Hi!

>
> v2 - this is an attempt to address David Hildenbrand's comments:
> overloading GFP and using page->private, support for
> balloon deflate.
>
> I hope this one is acceptable, API wise.
>
> I also went ahead and implemented an alternative approach
> that David suggested:
> using GFP_ZERO to zero userspace pages.
> The issue is simple: on some architectures, one has to know the
> userspace fault address in order to flush the cache.
>
> So, I had to propagate the fault address everywhere.

As I said, that might not be necessary. vma_alloc_folio() is the
interface we mostly care about in that regard.

> A lot of churn, and my concern is, if we miss even one
> place, silent, subtle data corruption will result and only
> on some arches (x86 will be fine).

Which would *already* be the case of you use folio_alloc(GFP_ZERO)
instead of magical vma_alloc_folio() + folio_zero_user().

I don't really see how vma_alloc_folio_hints() -- that also consumes the
address -- is any better in that regard?

When we just do the right thing with vma_alloc_folio(GFP_ZERO), at least
vma_alloc_folio() users will not accidentally do the wrong thing by
forgetting to use folio_zero_user().

>
> Still, you can view that approach here:
> https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git gfp_zero
>
> David, if you still feel I should switch to that approach,
> let me know. Personally, I'd rather keep that as a separate
> project from this optimization.
I'd prefer if we extend vma_alloc_folio() to just handle GFP_ZERO for us.

But let's hear other opinions first.

--
Cheers,

David