Re: [PATCH] userfaultfd/shmem: fix minor fault page leak

From: Axel Rasmussen <axelrasmussen@google.com>
To: Peter Xu <peterx@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	 Andrea Arcangeli <aarcange@redhat.com>,
	Hugh Dickins <hughd@google.com>,
	 Jerome Glisse <jglisse@redhat.com>,
	Joe Perches <joe@perches.com>,
	 Lokesh Gidra <lokeshgidra@google.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	 Shaohua Li <shli@fb.com>, Shuah Khan <shuah@kernel.org>,
	Wang Qing <wangqing@vivo.com>,
	 LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org,  Linux MM <linux-mm@kvack.org>,
	linux-kselftest@vger.kernel.org,
	 Brian Geffon <bgeffon@google.com>,
	Cannon Matthews <cannonmatthews@google.com>,
	 "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	David Rientjes <rientjes@google.com>,
	 Michel Lespinasse <walken@google.com>,
	Mina Almasry <almasrymina@google.com>,
	 Oliver Upton <oupton@google.com>
Subject: Re: [PATCH] userfaultfd/shmem: fix minor fault page leak
Date: Wed, 24 Mar 2021 19:10:24 -0700	[thread overview]
Message-ID: <CAJHvVch7kH8JqbakfPgurfEMOGobw5LsEnbfmyT6HB+=cjgHCA@mail.gmail.com> (raw)
In-Reply-To: <20210325005234.GG219069@xz-x1>

On Wed, Mar 24, 2021 at 5:52 PM Peter Xu <peterx@redhat.com> wrote:
>
> Hi, Andrew,
>
> On Wed, Mar 24, 2021 at 04:20:27PM -0700, Andrew Morton wrote:
> > On Mon, 22 Mar 2021 13:48:35 -0700 Axel Rasmussen <axelrasmussen@google.com> wrote:
> >
> > > This fix is analogous to Peter Xu's fix for hugetlb [0]. If we don't
> > > put_page() after getting the page out of the page cache, we leak the
> > > reference.
> > >
> > > The fix can be verified by checking /proc/meminfo and running the
> > > userfaultfd selftest in shmem mode. Without the fix, we see MemFree /
> > > MemAvailable steadily decreasing with each run of the test. With the
> > > fix, memory is correctly freed after the test program exits.
> > >
> > > Fixes: 00da60b9d0a0 ("userfaultfd: support minor fault handling for shmem")
> >
> > Confused.  The affected code:
> >
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -1831,6 +1831,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> > >
> > >     if (page && vma && userfaultfd_minor(vma)) {
> > >             unlock_page(page);
> > > +           put_page(page);
> > >             *fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> > >             return 0;
> > >     }
> >
> > Is added by Peter's "page && vma && userfaultfd_minor".  I assume that
> > "Fixes:" is incorrect?
> >
>
> It seems to me the commit is correct as pointed to in "Fixes", but I do have a
> different commit ID here:
>
>     commit 63c826b1372c4930f89b8a55092699fa7f0d6f4e
>     Author: Axel Rasmussen <axelrasmussen@google.com>
>     Date:   Thu Mar 18 10:20:43 2021 -0400
>
>     userfaultfd: support minor fault handling for shmem
>
> Axel, did you fetched the commit ID from your local tree, perhaps?  Since I
> should have fetched from hnaz/linux-mm and I can see Andrew's sign-off too.
>
> Thanks,
>
> --
> Peter Xu
>

Ah, this is the SHA I see when I "git log --grep linux-next/akpm"
(where my repo's linux-next remote is [1]):

commit 00da60b9d0a03818c36a2fe862578309c27006ad
Author: Axel Rasmussen <axelrasmussen@google.com>
Date:   Thu Mar 18 17:01:51 2021 +1100

    userfaultfd: support minor fault handling for shmem

This is the commit that this new patch fixes. I'll admit I'm a bit
unsure which tree the "Fixes:" tag is meant to refer to before the
commits make it into Linus' tree, if I should look up the commit
another way just let me know. :) And, sorry for the confusion.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git