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

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Date:   Wed, 24 Mar 2021 20:52:34 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Shaohua Li <shli@...com>, Shuah Khan <shuah@...nel.org>,
        Wang Qing <wangqing@...o.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, Brian Geffon <bgeffon@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH] userfaultfd/shmem: fix minor fault page leak

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@...gle.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@...gle.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