linux-kernel - Re: [PATCH] drm/vmwgfx: remove DRM_ERROR message, stops log spamming

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Date:   Tue, 12 Sep 2017 18:54:45 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Sinclair Yeh <syeh@...are.com>,
        David Airlie <airlied@...ux.ie>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        kernel-janitors@...r.kernel.org,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/vmwgfx: remove DRM_ERROR message, stops log spamming

On 12 September 2017 at 18:47, Colin Ian King <colin.king@...onical.com> wrote:
> On 12/09/17 18:42, Thomas Hellstrom wrote:
>> Hi, Colin,
>>
>> On 09/12/2017 07:35 PM, Colin King wrote:
>>> From: Colin Ian King <colin.king@...onical.com>
>>>
>>> mmap'ing the device multiple times will spam the kernel log with the
>>> DRM_ERROR message about illegal mmap'ing the old fifo space.
>> How are you hitting this? Multiple mappings should be fine as long as
>> mapping offsets are correct,
>> so hitting this message should indicate that the user-space app is doing
>> something seriously wrong, and
>> having it present in the log should probably help more than it hurts.
>>
>> /Thomas
>
> Good question.  I hit similar issues with the drm qxl driver when
> running some kernel regression tests with stress-ng [1]. I realize this
> is an artificial test scenario so it is definitely not a typical
> use-case, however, sync the illegal mmapping will return -EINVAL the
> application will pick up that this is an error without the need of
> spotting it in the kernel log. And a user space application can perform
> many millions of these invalid mmaps causing kernel log spamming.
>
FWIW I'm the one to "blame" here - pointing Colin to drop the message.

Two reasons come to mind:
 - there is a unwritten rule that roughly says "user input should not
cause kernel log spam"
 - out of all the DRM drivers only QXL and VMWGFX print a message,
with a patch addressing the former

HTH
Emil