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Message-ID: <ZcrfQS883NfOso4r@google.com>
Date: Mon, 12 Feb 2024 19:17:21 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, pbonzini@...hat.com, shuah@...nel.org,
stevensd@...omium.org
Subject: Re: [RFC PATCH v2 1/3] KVM: allow mapping of compound tail pages for
IO or PFNMAP mapping
On Wed, Jan 03, 2024, 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.
Can you add a blurb to call out that this is effectively what gup() does in
try_get_folio()? That knowledge give me a _lot_ more confidence that this is
correct (I didn't think too deeply about what this patch was doing when I looked
at v1).
> 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.
>
> 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 acd67fb40183..f53b58446ac7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2892,7 +2892,7 @@ 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));
This seems like it needs retry logic, a la try_get_folio(), to guard against a
race with the folio being split. From page_folio():
If the caller* does not hold a reference, this call may race with a folio split,
so it should re-check the folio still contains this page after gaining a
reference on the folio.
I assume that splitting one of these folios is extremely unlikely, but I don't
see any harm in being paranoid (unless this really truly cannot race).
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