Linux Virtualization - Sep 2017 - [PATCH] drm: qxl: ratelimit pr_info message, reduce log spamming

From: Colin Ian King <colin.king at canonical.com>

Simply mmap'ing /dev/dri/card0 repeatedly will spam the kernel
log with qxl_mmap information messages. The following example code
illustrates this:

int main(void)
{
	int fd = open("/dev/dri/card0", O_RDONLY);
	if (fd == -1)
		err(1, "open failed");

	for (;;) {
		void *m = mmap(NULL, 4096, PROT_READ,
			MAP_SHARED, fd, 0);
		if (m != MAP_FAILED)
			munmap(m, 4096);
	}
}

Reduce the spamming by ratelimiting the pr_info messages.

Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 drivers/gpu/drm/qxl/qxl_ttm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/qxl/qxl_ttm.c b/drivers/gpu/drm/qxl/qxl_ttm.c
index 7ecf8a4b9fe6..6502e699f462 100644
--- a/drivers/gpu/drm/qxl/qxl_ttm.c
+++ b/drivers/gpu/drm/qxl/qxl_ttm.c
@@ -124,7 +124,7 @@ int qxl_mmap(struct file *filp, struct vm_area_struct *vma)
 	int r;
 
 	if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET)) {
-		pr_info("%s: vma->vm_pgoff (%ld) < DRM_FILE_PAGE_OFFSET\n",
+		pr_info_ratelimited("%s: vma->vm_pgoff (%ld) <
DRM_FILE_PAGE_OFFSET\n",
 			__func__, vma->vm_pgoff);
 		return -EINVAL;
 	}
-- 
2.14.1

Hi Colin,

On 12 September 2017 at 10:45, Colin King <colin.king at canonical.com>
wrote:
> From: Colin Ian King <colin.king at canonical.com>
>
> Simply mmap'ing /dev/dri/card0 repeatedly will spam the kernel
> log with qxl_mmap information messages. The following example code
> illustrates this:
>
> int main(void)
> {
>         int fd = open("/dev/dri/card0", O_RDONLY);
>         if (fd == -1)
>                 err(1, "open failed");
>
>         for (;;) {
>                 void *m = mmap(NULL, 4096, PROT_READ,
>                         MAP_SHARED, fd, 0);
>                 if (m != MAP_FAILED)
>                         munmap(m, 4096);
>         }
> }
>
> Reduce the spamming by ratelimiting the pr_info messages.
>
> Signed-off-by: Colin Ian King <colin.king at canonical.com>
> ---
>  drivers/gpu/drm/qxl/qxl_ttm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/qxl/qxl_ttm.c b/drivers/gpu/drm/qxl/qxl_ttm.c
> index 7ecf8a4b9fe6..6502e699f462 100644
> --- a/drivers/gpu/drm/qxl/qxl_ttm.c
> +++ b/drivers/gpu/drm/qxl/qxl_ttm.c
> @@ -124,7 +124,7 @@ int qxl_mmap(struct file *filp, struct vm_area_struct
*vma)
>         int r;
>
>         if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET)) {
> -               pr_info("%s: vma->vm_pgoff (%ld) <
DRM_FILE_PAGE_OFFSET\n",
> +               pr_info_ratelimited("%s: vma->vm_pgoff (%ld) <
DRM_FILE_PAGE_OFFSET\n",
Quick grep suggests that only qxl and vmwgfx produce any output in
this case, likely for the reasons you mentioned.

That said, I'm not sure how useful the information is - perhaps it's
better to drop it all together? Regardless of the route taken, vmwgfx
could use a similar fix.

HTH
Emil

On Tue, Sep 12, 2017 at 03:02:04PM +0100, Emil Velikov
wrote:
> That said, I'm not sure how useful the information is - perhaps
it's
> better to drop it all together?
Or a WARN_ONCE().

regards,
dan carpenter