Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page()

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 17/07/2019 14:17, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-07-17 14:09:00)
>>
>> On 16/07/2019 16:37, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-07-16 16:25:22)
>>>>
>>>> On 16/07/2019 13:49, Chris Wilson wrote:
>>>>> Following a try_to_unmap() we may want to remove the userptr and so call
>>>>> put_pages(). However, try_to_unmap() acquires the page lock and so we
>>>>> must avoid recursively locking the pages ourselves -- which means that
>>>>> we cannot safely acquire the lock around set_page_dirty(). Since we
>>>>> can't be sure of the lock, we have to risk skip dirtying the page, or
>>>>> else risk calling set_page_dirty() without a lock and so risk fs
>>>>> corruption.
>>>>
>>>> So if trylock randomly fail we get data corruption in whatever data set
>>>> application is working on, which is what the original patch was trying
>>>> to avoid? Are we able to detect the backing store type so at least we
>>>> don't risk skipping set_page_dirty with anonymous/shmemfs?
>>>
>>> page->mapping???
>>
>> Would page->mapping work? What is it telling us?
> 
> It basically tells us if there is a fs around; anything that is the most
> basic of malloc (even tmpfs/shmemfs has page->mapping).

Normal malloc so anonymous pages? Or you meant everything _apart_ from 
the most basic malloc?

>>> We still have the issue that if there is a mapping we should be taking
>>> the lock, and we may have both a mapping and be inside try_to_unmap().
>>
>> Is this a problem? On a path with mappings we trylock and so solve the
>> set_dirty_locked and recursive deadlock issues, and with no mappings
>> with always dirty the page and avoid data corruption.
> 
> The problem as I see it is !page->mapping are likely an insignificant
> minority of userptr; as I think even memfd are essentially shmemfs (or
> hugetlbfs) and so have mappings.

Better then nothing, no? If easy to do..

Regards,

Tvrtko