Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

Hello Sean,

On 1/11/23 20:05, Sean Christopherson wrote:
> On Thu, Aug 18, 2022, Christian König wrote:
>> Am 18.08.22 um 01:13 schrieb Dmitry Osipenko:
>>> On 8/18/22 01:57, Dmitry Osipenko wrote:
>>>> On 8/15/22 18:54, Dmitry Osipenko wrote:
>>>>> On 8/15/22 17:57, Dmitry Osipenko wrote:
>>>>>> On 8/15/22 16:53, Christian König wrote:
>>>>>>> Am 15.08.22 um 15:45 schrieb Dmitry Osipenko:
>>>>>>>> [SNIP]
>>>>>>>>> Well that comment sounds like KVM is doing the right thing, so I'm
>>>>>>>>> wondering what exactly is going on here.
>>>>>>>> KVM actually doesn't hold the page reference, it takes the temporal
>>>>>>>> reference during page fault and then drops the reference once page is
>>>>>>>> mapped, IIUC. Is it still illegal for TTM? Or there is a possibility for
>>>>>>>> a race condition here?
>>>>>>>>
>>>>>>> Well the question is why does KVM grab the page reference in the first
>>>>>>> place?
>>>>>>>
>>>>>>> If that is to prevent the mapping from changing then yes that's illegal
>>>>>>> and won't work. It can always happen that you grab the address, solve
>>>>>>> the fault and then immediately fault again because the address you just
>>>>>>> grabbed is invalidated.
>>>>>>>
>>>>>>> If it's for some other reason than we should probably investigate if we
>>>>>>> shouldn't stop doing this.
> 
> ...
> 
>>>>> If we need to bump the refcount only for VM_MIXEDMAP and not for
>>>>> VM_PFNMAP, then perhaps we could add a flag for that to the kvm_main
>>>>> code that will denote to kvm_release_page_clean whether it needs to put
>>>>> the page?
>>>> The other variant that kind of works is to mark TTM pages reserved using
>>>> SetPageReserved/ClearPageReserved, telling KVM not to mess with the page
>>>> struct. But the potential consequences of doing this are unclear to me.
>>>>
>>>> Christian, do you think we can do it?
>>> Although, no. It also doesn't work with KVM without additional changes
>>> to KVM.
>>
>> Well my fundamental problem is that I can't fit together why KVM is grabing
>> a page reference in the first place.
> 
> It's to workaround a deficiency in KVM.
> 
>> See the idea of the page reference is that you have one reference is that
>> you count the reference so that the memory is not reused while you access
>> it, e.g. for I/O or mapping it into different address spaces etc...
>>
>> But none of those use cases seem to apply to KVM. If I'm not totally
>> mistaken in KVM you want to make sure that the address space mapping, e.g.
>> the translation between virtual and physical address, don't change while you
>> handle it, but grabbing a page reference is the completely wrong approach
>> for that.
> 
> TL;DR: 100% agree, and we're working on fixing this in KVM, but were still months
> away from a full solution.
> 
> Yep.  KVM uses mmu_notifiers to react to mapping changes, with a few caveats that
> we are (slowly) fixing, though those caveats are only tangentially related.
> 
> The deficiency in KVM is that KVM's internal APIs to translate a virtual address
> to a physical address spit out only the resulting host PFN.  The details of _how_
> that PFN was acquired are not captured.  Specifically, KVM loses track of whether
> or not a PFN was acquired via gup() or follow_pte() (KVM is very permissive when
> it comes to backing guest memory).
> 
> Because gup() gifts the caller a reference, that means KVM also loses track of
> whether or not KVM holds a page refcount.  To avoid pinning guest memory, KVM does
> quickly put the reference gifted by gup(), but because KVM doesn't _know_ if it
> holds a reference, KVM uses a heuristic, which is essentially "is the PFN associated
> with a 'normal' struct page?".
> 
>    /*
>     * Returns a 'struct page' if the pfn is "valid" and backed by a refcounted
>     * page, NULL otherwise.  Note, the list of refcounted PG_reserved page types
>     * is likely incomplete, it has been compiled purely through people wanting to
>     * back guest with a certain type of memory and encountering issues.
>     */
>    struct page *kvm_pfn_to_refcounted_page(kvm_pfn_t pfn)
> 
> That heuristic also triggers if follow_pte() resolves to a PFN that is associated
> with a "struct page", and so to avoid putting a reference it doesn't own, KVM does
> the silly thing of manually getting a reference immediately after follow_pte().
> 
> And that in turn gets tripped up non-refcounted tail pages because KVM sees a
> normal, valid "struct page" and assumes it's refcounted.  To fudge around that
> issue, KVM requires "struct page" memory to be refcounted.
> 
> The long-term solution is to refactor KVM to precisely track whether or not KVM
> holds a reference.  Patches have been prosposed to do exactly that[1], but they
> were put on hold due to the aforementioned caveats with mmu_notifiers.  The
> caveats are that most flows where KVM plumbs a physical address into hardware
> structures aren't wired up to KVM's mmu_notifier.
> 
> KVM could support non-refcounted struct page memory without first fixing the
> mmu_notifier issues, but I was (and still am) concerned that that would create an
> even larger hole in KVM until the mmu_notifier issues are sorted out[2].
>  
> [1] https://lore.kernel.org/all/20211129034317.2964790-1-stevensd@google.com
> [2] https://lore.kernel.org/all/Ydhq5aHW+JFo15UF@google.com

Thanks for the summary! Indeed, it's the KVM side that needs to be
patched. Couple months ago I found that a non-TTM i915 driver also
suffers from the same problem because it uses huge pages that we want
map to a guest. So we definitely will need to fix the KVM side.

-- 
Best regards,
Dmitry