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

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Message-Id: <20210324162027.cc723b545ff62b1ad6f5223e@linux-foundation.org>
Date:   Wed, 24 Mar 2021 16:20:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     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>,
        Peter Xu <peterx@...hat.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

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?