From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Cc: stable@vger.kernel.org
Subject: Re: [Intel-gfx] [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page()
Date: Wed, 17 Jul 2019 14:46:15 +0100 [thread overview]
Message-ID: <951e2751-15d7-9ca8-ef6f-299ba59c47a6@linux.intel.com> (raw)
In-Reply-To: <156337053617.4375.13675276970408492219@skylake-alporthouse-com>
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
next prev parent reply other threads:[~2019-07-17 13:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-16 12:49 [PATCH 1/5] drm/i915/userptr: Beware recursive lock_page() Chris Wilson
2019-07-16 15:25 ` [Intel-gfx] " Tvrtko Ursulin
2019-07-16 15:37 ` Chris Wilson
2019-07-17 13:09 ` Tvrtko Ursulin
2019-07-17 13:17 ` Chris Wilson
2019-07-17 13:23 ` Tvrtko Ursulin
2019-07-17 13:35 ` Chris Wilson
2019-07-17 13:46 ` Tvrtko Ursulin [this message]
2019-07-17 14:06 ` Chris Wilson
2019-07-17 18:09 ` Tvrtko Ursulin
2019-07-26 13:38 ` Lionel Landwerlin
2019-09-09 13:52 ` Chris Wilson
2019-09-11 11:31 ` Tvrtko Ursulin
2019-09-11 11:38 ` Chris Wilson
2019-09-11 12:10 ` Tvrtko Ursulin
2019-11-06 7:22 ` Chris Wilson
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