[2/2] drm/shmem-helper: Avoid vm_open error paths

[Guenter Roeck]

On 11/29/22 12:47, Rob Clark wrote:
> On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <linux at roeck-us.net> wrote:
>>
>> On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> vm_open() is not allowed to fail.  Fortunately we are guaranteed that
>>> the pages are already pinned, and only need to increment the refcnt.  So
>>> just increment it directly.
>>
>> I don't know anything about drm or gem, but I am wondering _how_
>> this would be guaranteed. Would it be through the pin function ?
>> Just wondering, because that function does not seem to be mandatory.
> 
> We've pinned the pages already in mmap.. vm->open() is perhaps not the
> best name for the callback function, but it is called for copying an
> existing vma into a new process (and for some other cases which do not
> apply here because VM_DONTEXPAND).
> 
> (Other drivers pin pages in the fault handler, where there is actually
> potential to return an error, but that change was a bit more like
> re-writing shmem helper ;-))
> 

Maybe add a bit of that (where the pinning happened) to the commit description
and to the patch itself ?

> BR,
> -R
> 
>>>
>>> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
>>> Cc: stable at vger.kernel.org
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>>   drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++++---
>>>   1 file changed, 11 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> index 110a9eac2af8..9885ba64127f 100644
>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>> @@ -571,12 +571,20 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
>>>   {
>>>        struct drm_gem_object *obj = vma->vm_private_data;
>>>        struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>>> -     int ret;
>>>
>>>        WARN_ON(shmem->base.import_attach);
>>>
>>> -     ret = drm_gem_shmem_get_pages(shmem);
>>> -     WARN_ON_ONCE(ret != 0);
>>> +     mutex_lock(&shmem->pages_lock);
>>> +
>>> +     /*
>>> +      * We should have already pinned the pages, vm_open() just grabs
>>
>> should or guaranteed ? This sounds a bit weaker than the commit
>> description.
>>
like ... the pages were already pinned in (mmap function).

>>> +      * an additional reference for the new mm the vma is getting
>>> +      * copied into.
>>> +      */
>>> +     WARN_ON_ONCE(!shmem->pages_use_count);

If the code can't be trusted and still needs the warning, how about
something like the following ?

	if (WARN_ON_ONCE(!shmem->pages_use_count)) {
		mutex_unlock(&shmem->pages_lock);
		return;
	}

Thanks,
Guenter

>>> +
>>> +     shmem->pages_use_count++;
>>> +     mutex_unlock(&shmem->pages_lock);
>>
>> The previous code, in that situation, would not increment pages_use_count,
>> and it would not set not set shmem->pages. Hopefully, it would not try to
>> do anything with the pages it was unable to get. The new code assumes that
>> shmem->pages is valid even if pages_use_count is 0, while at the same time
>> taking into account that this can possibly happen (or the WARN_ON_ONCE
>> would not be needed).
>>
>> Again, I don't know anything about gem and drm, but it seems to me that
>> there might now be a severe problem later on if the WARN_ON_ONCE()
>> ever triggers.
>>
>> Thanks,
>> Guenter
>>
>>>
>>>        drm_gem_vm_open(vma);
>>>   }