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

[Tvrtko Ursulin]

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?

Regards,

Tvrtko