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To: Laura Abbott <labbott@redhat.com>,
        "Greg KH" <gregkh@linuxfoundation.org>,
        "Sumit Semwal" <sumit.semwal@linaro.org>
From: Chris Wilson <chris@chris-wilson.co.uk>
In-Reply-To: <20180105191409.17807-1-labbott@redhat.com>
Cc: "Laura Abbott" <labbott@redhat.com>, devel@driverdev.osuosl.org,
        "LKML" <linux-kernel@vger.kernel.org>,
        "Dan Carpenter" <dan.carpenter@oracle.com>
References: <20180105191409.17807-1-labbott@redhat.com>
Message-ID: <151518101521.6838.4541956658403978617@mail.alporthouse.com>
User-Agent: alot/0.3.6
Subject: Re: [PATCH 1/2] staging: android: ion: Add __GFP_NOWARN for system contig
 heap
Date: Fri, 05 Jan 2018 19:36:55 +0000
Sender: linux-kernel-owner@vger.kernel.org

Quoting Laura Abbott (2018-01-05 19:14:08)
> syzbot reported a warning from Ion:
> 
>   WARNING: CPU: 1 PID: 3485 at mm/page_alloc.c:3926
> 
>   ...
>    __alloc_pages_nodemask+0x9fb/0xd80 mm/page_alloc.c:4252
>   alloc_pages_current+0xb6/0x1e0 mm/mempolicy.c:2036
>   alloc_pages include/linux/gfp.h:492 [inline]
>   ion_system_contig_heap_allocate+0x40/0x2c0
>   drivers/staging/android/ion/ion_system_heap.c:374
>   ion_buffer_create drivers/staging/android/ion/ion.c:93 [inline]
>   ion_alloc+0x2c1/0x9e0 drivers/staging/android/ion/ion.c:420
>   ion_ioctl+0x26d/0x380 drivers/staging/android/ion/ion-ioctl.c:84
>   vfs_ioctl fs/ioctl.c:46 [inline]
>   do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
>   SYSC_ioctl fs/ioctl.c:701 [inline]
>   SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
> 
> This is a warning about attempting to allocate order > MAX_ORDER. This
> is coming from a userspace Ion allocation request. Since userspace is
> free to request however much memory it wants (and the kernel is free to
> deny its allocation), silence the allocation attempt with __GFP_NOWARN
> in case it fails.
> 
> Reported-by: syzbot+76e7efc4748495855a4d@syzkaller.appspotmail.com
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
>  drivers/staging/android/ion/ion_system_heap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/android/ion/ion_system_heap.c b/drivers/staging/android/ion/ion_system_heap.c
> index 71c4228f8238..bc19cdd30637 100644
> --- a/drivers/staging/android/ion/ion_system_heap.c
> +++ b/drivers/staging/android/ion/ion_system_heap.c
> @@ -362,7 +362,7 @@ static int ion_system_contig_heap_allocate(struct ion_heap *heap,
>         unsigned long i;
>         int ret;
>  
> -       page = alloc_pages(low_order_gfp_flags, order);
> +       page = alloc_pages(low_order_gfp_flags | __GFP_NOWARN, order);

There's both high_order_gfp and low_order_gfp. The former includes
NOWARN and NORETRY.

Interesting, ion_system_heap_create_pools() tries to mix low_order and
high_order, but it only ever uses high_order flags. (orders[0] == 8
forcing a permanent switch from low_order_gfp to high_order_gfp).

There's no good reason for low_order_gfp, high_order_gfp to be static
rewritable variables.

For this instance, I would go farther and suggest you may want
__GFP_RETRY_MAYFAIL | __GFP_NOWARN to prevent userspace from triggering
the lowmemkiller/oomkiller here.
 
(I would kill low_order_gfp_flags / high_order_gfp_flags and avoid the
obfuscation.)
-Chris