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Date:   Wed, 20 Feb 2019 14:40:20 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH 10/10] mm/hmm: add helpers for driver to safely take the
 mmap_sem

On 2/20/19 2:19 PM, Jerome Glisse wrote:
> On Wed, Feb 20, 2019 at 01:59:13PM -0800, John Hubbard wrote:
>> On 1/29/19 8:54 AM, jglisse@...hat.com wrote:
>>> From: Jérôme Glisse <jglisse@...hat.com>
>>>
>>> The device driver context which holds reference to mirror and thus to
>>> core hmm struct might outlive the mm against which it was created. To
>>> avoid every driver to check for that case provide an helper that check
>>> if mm is still alive and take the mmap_sem in read mode if so. If the
>>> mm have been destroy (mmu_notifier release call back did happen) then
>>> we return -EINVAL so that calling code knows that it is trying to do
>>> something against a mm that is no longer valid.
>>>
>>> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Ralph Campbell <rcampbell@...dia.com>
>>> Cc: John Hubbard <jhubbard@...dia.com>
>>> ---
>>>    include/linux/hmm.h | 50 ++++++++++++++++++++++++++++++++++++++++++---
>>>    1 file changed, 47 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
>>> index b3850297352f..4a1454e3efba 100644
>>> --- a/include/linux/hmm.h
>>> +++ b/include/linux/hmm.h
>>> @@ -438,6 +438,50 @@ struct hmm_mirror {
>>>    int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
>>>    void hmm_mirror_unregister(struct hmm_mirror *mirror);
>>> +/*
>>> + * hmm_mirror_mm_down_read() - lock the mmap_sem in read mode
>>> + * @mirror: the HMM mm mirror for which we want to lock the mmap_sem
>>> + * Returns: -EINVAL if the mm is dead, 0 otherwise (lock taken).
>>> + *
>>> + * The device driver context which holds reference to mirror and thus to core
>>> + * hmm struct might outlive the mm against which it was created. To avoid every
>>> + * driver to check for that case provide an helper that check if mm is still
>>> + * alive and take the mmap_sem in read mode if so. If the mm have been destroy
>>> + * (mmu_notifier release call back did happen) then we return -EINVAL so that
>>> + * calling code knows that it is trying to do something against a mm that is
>>> + * no longer valid.
>>> + */
>>
>> Hi Jerome,
>>
>> Are you thinking that, throughout the HMM API, there is a problem that
>> the mm may have gone away, and so driver code needs to be littered with
>> checks to ensure that mm is non-NULL? If so, why doesn't HMM take a
>> reference on mm->count?
>>
>> This solution here cannot work. I think you'd need refcounting in order
>> to avoid this kind of problem. Just doing a check will always be open to
>> races (see below).
>>
>>
>>> +static inline int hmm_mirror_mm_down_read(struct hmm_mirror *mirror)
>>> +{
>>> +	struct mm_struct *mm;
>>> +
>>> +	/* Sanity check ... */
>>> +	if (!mirror || !mirror->hmm)
>>> +		return -EINVAL;
>>> +	/*
>>> +	 * Before trying to take the mmap_sem make sure the mm is still
>>> +	 * alive as device driver context might outlive the mm lifetime.
>>> +	 *
>>> +	 * FIXME: should we also check for mm that outlive its owning
>>> +	 * task ?
>>> +	 */
>>> +	mm = READ_ONCE(mirror->hmm->mm);
>>> +	if (mirror->hmm->dead || !mm)
>>> +		return -EINVAL;
>>> +
>>
>> Nothing really prevents mirror->hmm->mm from changing to NULL right here.
> 
> This is really just to catch driver mistake, if driver does not call
> hmm_mirror_unregister() then the !mm will never be true ie the
> mirror->hmm->mm can not go NULL until the last reference to hmm_mirror
> is gone.

In that case, then this again seems unnecessary, and in fact undesirable.
If the driver code has a bug, then let's let the backtrace from a NULL
dereference just happen, loud and clear.

This patch, at best, hides bugs. And it adds code that should simply be
unnecessary, so I don't like it. :)  Let's make it go away.

> 
>>
>>> +	down_read(&mm->mmap_sem);
>>> +	return 0;
>>> +}
>>> +
>>
>> ...maybe better to just drop this patch from the series, until we see a
>> pattern of uses in the calling code.
> 
> It use by nouveau now.

Maybe you'd have to remove that use case in a couple steps, depending on the
order that patches are going in.


thanks,
-- 
John Hubbard
NVIDIA

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