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

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Date:   Tue, 12 Sep 2017 18:47:23 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Thomas Hellstrom <thellstrom@...are.com>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Sinclair Yeh <syeh@...are.com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vmwgfx: remove DRM_ERROR message, stops log spamming

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.

[1] http://kernel.ubuntu.com/~cking/stress-ng/

> 
> 
> 
>> Since
>> the mmap'ing will fail with an -EINVAL there is no need to emit this
>> message, so just remove it.
>>
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>   drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
>> b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
>> index e771091d2cd3..1e633c602fb1 100644
>> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
>> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
>> @@ -33,10 +33,8 @@ int vmw_mmap(struct file *filp, struct
>> vm_area_struct *vma)
>>       struct drm_file *file_priv;
>>       struct vmw_private *dev_priv;
>>   -    if (unlikely(vma->vm_pgoff < VMWGFX_FILE_PAGE_OFFSET)) {
>> -        DRM_ERROR("Illegal attempt to mmap old fifo space.\n");
>> +    if (unlikely(vma->vm_pgoff < VMWGFX_FILE_PAGE_OFFSET))
>>           return -EINVAL;
>> -    }
>>         file_priv = filp->private_data;
>>       dev_priv = vmw_priv(file_priv->minor->dev);
> 
>