linux-kernel - Re: [PATCH] vfio/type1: unpin PageReserved page

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Date: Tue, 27 Feb 2024 11:27:08 +0100
From: David Hildenbrand <david@...hat.com>
To: Alex Williamson <alex.williamson@...hat.com>,
 Yisheng Xie <ethan.xys@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] vfio/type1: unpin PageReserved page

On 26.02.24 18:32, Alex Williamson wrote:
> On Tue, 27 Feb 2024 01:14:54 +0800
> Yisheng Xie <ethan.xys@...ux.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@...ux.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@...hat.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@...hat.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)

-- 
Cheers,

David / dhildenb