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 19:09:37 +0100 [thread overview]
Message-ID: <d867c0e8-e2e1-fff6-d073-3d5d98335712@linux.intel.com> (raw)
In-Reply-To: <156337241401.4375.2377981562987470090@skylake-alporthouse-com>
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
next prev parent reply other threads:[~2019-07-17 18:09 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
2019-07-17 14:06 ` Chris Wilson
2019-07-17 18:09 ` Tvrtko Ursulin [this message]
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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