linux-kernel - Re: [PATCH] userfaultfd_zeropage: return -ENOSPC in case mm has gone

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Message-ID: <20170731122204.GB4878@dhcp22.suse.cz>
Date:   Mon, 31 Jul 2017 14:22:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] userfaultfd_zeropage: return -ENOSPC in case mm has gone

On Thu 27-07-17 09:26:59, Mike Rapoport wrote:
> In the non-cooperative userfaultfd case, the process exit may race with
> outstanding mcopy_atomic called by the uffd monitor.  Returning -ENOSPC
> instead of -EINVAL when mm is already gone will allow uffd monitor to
> distinguish this case from other error conditions.

Normally we tend to return ESRCH in such case. ENOSPC sounds rather
confusing...
 
> Cc: stable@...r.kernel.org
> Fixes: 96333187ab162 ("userfaultfd_copy: return -ENOSPC in case mm has gone")
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> ---
> 
> Unfortunately, I've overlooked userfaultfd_zeropage when I updated
> userfaultd_copy :(
> 
>  fs/userfaultfd.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index cadcd12a3d35..2d8c2d848668 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1643,6 +1643,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
>  		ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
>  				     uffdio_zeropage.range.len);
>  		mmput(ctx->mm);
> +	} else {
> +		return -ENOSPC;
>  	}
>  	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
>  		return -EFAULT;
> -- 
> 2.7.4
> 
> --
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-- 
Michal Hocko
SUSE Labs