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

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Message-ID: <20240227132556.17e87767.alex.williamson@redhat.com>
Date: Tue, 27 Feb 2024 13:25:56 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Yisheng Xie <ethan.xys@...ux.alibaba.com>, akpm@...ux-foundation.org,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] vfio/type1: unpin PageReserved page

On Tue, 27 Feb 2024 11:27:08 +0100
David Hildenbrand <david@...hat.com> wrote:

> 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  510.134-008.2.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)


Yes, according to the commit log it seems like the unpin is now just
wasted work since v6.5.  Thanks!

Alex