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

Alex Williamson
2024-02-27T20:26:04Z

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!

Alex