linux-kernel - Re: [PATCH v2 05/27] drm/i915/gvt: Verify VFIO-pinned page is THP when shadowing 2M gtt entry

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Message-ID: <ZFX1PaKIa44WtSNX@yzhao56-desk.sh.intel.com>
Date:   Sat, 6 May 2023 14:35:41 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <kvm@...r.kernel.org>, <intel-gfx@...ts.freedesktop.org>,
        <linux-kernel@...r.kernel.org>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        "Ben Gardon" <bgardon@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        <intel-gvt-dev@...ts.freedesktop.org>,
        Zhi Wang <zhi.a.wang@...el.com>
Subject: Re: [PATCH v2 05/27] drm/i915/gvt: Verify VFIO-pinned page is THP
 when shadowing 2M gtt entry

> > Maybe the checking of PageTransHuge(cur_page) and bailing out is not necessary.
> > If a page is not transparent huge, but there are 512 contigous 4K
> > pages, I think it's still good to map them in IOMMU in 2M.
> > See vfio_pin_map_dma() who does similar things.
> 
> I agree that bailing isn't strictly necessary, and processing "blindly" should
> Just Work for HugeTLB and other hugepage types.  I was going to argue that it
> would be safer to add this and then drop it at the end, but I think that's a
> specious argument.  If not checking the page type is unsafe, then the existing
> code is buggy, and this changelog literally states that the check for contiguous
> pages guards against any such problems.
> 
> I do think there's a (very, very theoretical) issue though.  For "CONFIG_SPARSEMEM=y
> && CONFIG_SPARSEMEM_VMEMMAP=n", struct pages aren't virtually contiguous with respect
> to their pfns, i.e. it's possible (again, very theoretically) that two struct pages
> could be virtually contiguous but physically discontiguous.  I suspect I'm being
> ridiculously paranoid, but for the efficient cases where pages are guaranteed to
> be contiguous, the extra page_to_pfn() checks should be optimized away by the
> compiler, i.e. there's no meaningful downside to the paranoia.
To make sure I understand it correctly:
There are 3 conditions:
(1) Two struct pages aren't virtually contiguous, but there PFNs are contiguous.
(2) Two struct pages are virtually contiguous but their PFNs aren't contiguous.
    (Looks this will not happen?)
(3) Two struct pages are virtually contiguous, and their PFNs are contiguous, too.
    But they have different backends, e.g.
    PFN 1 and PFN 2 are contiguous, while PFN 1 belongs to RAM, and PFN 2
    belongs to DEVMEM.

I think you mean condition (3) is problematic, am I right?
> 
> TL;DR: My plan is to drop this patch and instead harden the continuity check.

So you want to check page zone?