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Message-ID: <ZLgEbalDPD38qHmm@google.com>
Date: Wed, 19 Jul 2023 08:42:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
pbonzini@...hat.com, stevensd@...omium.org
Subject: Re: [PATCH] KVM: allow mapping of compound tail pages for IO or
PFNMAP mapping
On Wed, Jul 19, 2023, Yan Zhao wrote:
> Allow mapping of tail pages of compound pages for IO or PFNMAP mapping
> by trying and getting ref count of its head page.
>
> For IO or PFNMAP mapping, sometimes it's backed by compound pages.
> KVM will just return error on mapping of tail pages of the compound pages,
> as ref count of the tail pages are always 0.
>
> So, rather than check and add ref count of a tail page, check and add ref
> count of its folio (head page) to allow mapping of the compound tail pages.
>
> This will not break the origial intention to disallow mapping of tail pages
> of non-compound higher order allocations as the folio of a non-compound
> tail page is the same as the page itself.
>
> On the other side, put_page() has already converted page to folio before
> putting page ref.
Is there an actual use case for this? It's not necessarily a strict requirement,
but it would be helpful to know if KVM supports this for a specific use case, or
just because it can.
Either way, this needs a selftest, KVM has had way too many bugs in this area.
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
> virt/kvm/kvm_main.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 138292a86174..6f2b51ef20f7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2551,7 +2551,7 @@ static int kvm_try_get_pfn(kvm_pfn_t pfn)
> if (!page)
> return 1;
>
> - return get_page_unless_zero(page);
> + return folio_try_get(page_folio(page));
> }
>
> static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>
> base-commit: 24ff4c08e5bbdd7399d45f940f10fed030dfadda
> --
> 2.17.1
>
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