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

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

On 09/09/2019 14:52, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2019-07-26 14:38:40)
>> On 17/07/2019 21:09, Tvrtko Ursulin wrote:
>>>
>>> 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
>>
>>
>> Just a reminder to not let this slip.
>> We run into userptr bugs in CI quite regularly.
> 
> Remind away. Revert or trylock, there doesn't seem to be a good answer.

Rock and a hard place. Data corruption for userptr users (with either 
trylock or no lock) or a deadlock (with the lock). I honestly can't 
decide what is worse. Tiny preference to deadlock rather than silent 
corruption. Misguided? Don't know really..

Regards,

Tvrtko