Re: [PATCH] vfio/type1: unpin PageReserved page

David Hildenbrand
2024-03-01T10:57:26Z

On 29.02.24 23:04, Alex Williamson wrote:
> On Tue, 27 Feb 2024 13:25:56 -0700
> Alex Williamson <alex.williamson@redhat.com> wrote:
>
>> On Tue, 27 Feb 2024 11:27:08 +0100
>> David Hildenbrand <david@redhat.com> wrote:
>>
>>> On 26.02.24 18:32, Alex Williamson wrote:
>>>> On Tue, 27 Feb 2024 01:14:54 +0800
>>>> Yisheng Xie <ethan.xys@linux.alibaba.com> wrote:
>>>>
>>>>> 在 2024/2/27 00:14, Alex Williamson 写道:
>>>>>> On Tue, 27 Feb 2024 00:01:06 +0800
>>>>>> Yisheng Xie<ethan.xys@linux.alibaba.com> wrote:
>>>>>>
>>>>>>> We meet a warning as following:
>>>>>>> WARNING: CPU: 99 PID: 1766859 at mm/gup.c:209 try_grab_page.part.0+0xe8/0x1b0
>>>>>>> CPU: 99 PID: 1766859 Comm: qemu-kvm Kdump: loaded Tainted: GOE 5.10.134-008.2.x86_64 #1
>>>>>> ^^^^^^^^
>>>>>>
>>>>>> Does this issue reproduce on mainline? Thanks,
>>>>>
>>>>> I have check the code of mainline, the logical seems the same as my
>>>>> version.
>>>>>
>>>>> so I think it can reproduce if i understand correctly.
>>>>
>>>> I obviously can't speak to what's in your 5.10.134-008.2 kernel, but I
>>>> do know there's a very similar issue resolved in v6.0 mainline and
>>>> included in v5.10.146 of the stable tree. Please test. Thanks,
>>>
>>> This commit, to be precise:
>>>
>>> commit 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4
>>> Author: Alex Williamson <alex.williamson@redhat.com>
>>> Date: Mon Aug 29 21:05:40 2022 -0600
>>>
>>> vfio/type1: Unpin zero pages
>>>
>>> There's currently a reference count leak on the zero page. We increment
>>> the reference via pin_user_pages_remote(), but the page is later handled
>>> as an invalid/reserved page, therefore it's not accounted against the
>>> user and not unpinned by our put_pfn().
>>>
>>> Introducing special zero page handling in put_pfn() would resolve the
>>> leak, but without accounting of the zero page, a single user could
>>> still create enough mappings to generate a reference count overflow.
>>>
>>> The zero page is always resident, so for our purposes there's no reason
>>> to keep it pinned. Therefore, add a loop to walk pages returned from
>>> pin_user_pages_remote() and unpin any zero pages.
>>>
>>>
>>> BUT
>>>
>>> in the meantime, we also have
>>>
>>> commit c8070b78751955e59b42457b974bea4a4fe00187
>>> Author: David Howells <dhowells@redhat.com>
>>> Date: Fri May 26 22:41:40 2023 +0100
>>>
>>> mm: Don't pin ZERO_PAGE in pin_user_pages()
>>>
>>> Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
>>> to it from the page tables and make unpin_user_page*() correspondingly
>>> ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a
>>> zero page's refcount as we're only allowed ~2 million pins on it -
>>> something that userspace can conceivably trigger.
>>>
>>> Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.
>>>
>>>
>>> So the unpin_user_page_* won't do anything with the shared zeropage.
>>>
>>> (likely, we could revert 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4)
>>
>>
>> Yes, according to the commit log it seems like the unpin is now just
>> wasted work since v6.5. Thanks!
>
> I dusted off an old unit test for mapping the zeropage through vfio and
> started working on posting a revert for 873aefb376bb but I actually
> found that this appears to be resolved even before c8070b787519. I
> bisected it to:
>
> commit 84209e87c6963f928194a890399e24e8ad299db1
> Author: David Hildenbrand <david@redhat.com>
> Date: Wed Nov 16 11:26:48 2022 +0100
>
> mm/gup: reliable R/O long-term pinning in COW mappings
>
> We already support reliable R/O pinning of anonymous memory.
> However, assume we end up pinning (R/O long-term) a pagecache page
> or the shared zeropage inside a writable private ("COW") mapping.
> The next write access will trigger a write-fault and replace the
> pinned page by an exclusive anonymous page in the process page
> tables to break COW: the pinned page no longer corresponds to the
> page mapped into the process' page table.
> Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a
> COW mapping, let's properly break COW first before R/O long-term
> pinning something that's not an exclusive anon page inside a COW
> mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive
> anon page instead that can get pinned safely.
>
> With this change, we can stop using FOLL_FORCE|FOLL_WRITE for
> reliable R/O long-term pinning in COW mappings.
>
> [...]
>
> Note 3: For users that use FOLL_LONGTERM right now without
> FOLL_WRITE, such as VFIO, we'd now no longer pin the shared
> zeropage. Instead, we'd populate exclusive anon pages that we can
> pin. There was a concern that this could affect the memlock limit
> of existing setups.
>
> For example, a VM running with VFIO could run into the memlock
> limit and fail to run. However, we essentially had the same
> behavior already in commit 17839856fd58 ("gup: document and work
> around "COW can break either way" issue") which got merged into
> some enterprise distros, and there were not any such complaints. So
> most probably, we're fine.
>

Oh, I almost forgot about that one :)

Indeed, 84209e87c696 was v6.2 and c8070b787519 was v6.5.

... and c8070b787519 was primarily concerned about !FOLL_LONGTERM usage,
so that makes sense that they would still run into zeropages.

For vfio, 84209e87c696 did the trick.

--
Cheers,

David / dhildenb