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

[Tvrtko Ursulin]

On 17/07/2019 15:06, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-07-17 14:46:15)
>>
>> On 17/07/2019 14:35, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-07-17 14:23:55)
>>>>
>>>> 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?
>>>
>>> Aye missed the not.
>>>
>>>>>>> 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..
>>>
>>> Actually, I erring on the opposite side. Peeking at mm/ internals does
>>> not bode confidence and feels indefensible. I'd much rather throw my
>>> hands up and say "this is the best we can do with the API provided,
>>> please tell us what we should have done." To which the answer is
>>> probably to not have used gup in the first place :|
>>
>> """
>> /*
>>   * set_page_dirty() is racy if the caller has no reference against
>>   * page->mapping->host, and if the page is unlocked.  This is because another
>>   * CPU could truncate the page off the mapping and then free the mapping.
>>   *
>>   * Usually, the page _is_ locked, or the caller is a user-space process which
>>   * holds a reference on the inode by having an open file.
>>   *
>>   * In other cases, the page should be locked before running set_page_dirty().
>>   */
>> int set_page_dirty_lock(struct page *page)
>> """
>>
>> Could we hold a reference to page->mapping->host while having pages and then would be okay to call plain set_page_dirty?
> 
> We would then be hitting the warnings in ext4 for unlocked pages again.

Ah true..

> Essentially the argument is whether or not that warn is valid, to which I
> think requires inner knowledge of vfs + ext4. To hold a reference on the
> host would require us tracking page->mapping (reasonable since we
> already hooked into mmu and so will get an invalidate + fresh gup on
> any changes), plus iterating over all to acquire the extra reference if
> applicable -- and I have no idea what the side-effects of that would be.
> Could well be positive side-effects. Just feels like wandering even
> further off the beaten path without a map. Good news hmm is just around
> the corner (which will probably prohibit this use-case) :|

... can we reach out to someone more knowledgeable in mm matters to 
recommend us what to do?

Regards,

Tvrtko