Re: [PATCH] KVM: x86: Fix rmap allocation for very large memslots

[Maciej S. Szmigiero]

On 28.01.2022 22:47, Sean Christopherson wrote:
> On Fri, Jan 28, 2022, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> >
> > Commit 7661809d493b ("mm: don't allow oversized kvmalloc() calls") has
> > forbidden using kvmalloc() to make allocations larger than INT_MAX (2 GiB).
> >
> > Unfortunately, adding a memslot exceeding 1 TiB in size will result in rmap
> > code trying to make an allocation exceeding this limit.
> > Besides failing this allocation, such operation will also trigger a
> > WARN_ON_ONCE() added by the aforementioned commit.
> >
> > Since we probably still want to use kernel slab for small rmap allocations
> > let's only redirect such oversized allocations to vmalloc.
> >
> > A possible alternative would be to add some kind of a __GFP_LARGE flag to
> > skip the INT_MAX check behind kvmalloc(), however this will impact the
> > common kernel memory allocation code, not just KVM.
>
> Paolo has a cleaner fix for this[1][2], but it appears to have stalled out somewhere.
>
> Paolo???
>
> [1] https://lore.kernel.org/all/20211015165519.135670-1-pbonzini@xxxxxxxxxx
> [2] https://lore.kernel.org/all/20211016064302.165220-1-pbonzini@xxxxxxxxxx

So, what we do here?

Apparently the cleaner fix at [2] wasn't picked up despite Kees giving
it his "Reviewed-by".

Thanks,
Maciej