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Subject: Re: [PATCH 3/5] mm/hmm: Use mm_get_hmm() in hmm_range_register()
To:     Jason Gunthorpe <jgg@ziepe.ca>, <rcampbell@nvidia.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20190506232942.12623-1-rcampbell@nvidia.com>
 <20190506232942.12623-4-rcampbell@nvidia.com>
 <20190523125108.GA14013@ziepe.ca>
From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Thu, 23 May 2019 11:46:48 -0700
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On 5/23/19 5:51 AM, Jason Gunthorpe wrote:
> On Mon, May 06, 2019 at 04:29:40PM -0700, rcampbell@nvidia.com wrote:
>> From: Ralph Campbell <rcampbell@nvidia.com>
>>
>> In hmm_range_register(), the call to hmm_get_or_create() implies that
>> hmm_range_register() could be called before hmm_mirror_register() when
>> in fact, that would violate the HMM API.
>>
>> Use mm_get_hmm() instead of hmm_get_or_create() to get the HMM structure.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: John Hubbard <jhubbard@nvidia.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Balbir Singh <bsingharora@gmail.com>
>> Cc: Dan Carpenter <dan.carpenter@oracle.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Souptick Joarder <jrdr.linux@gmail.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>   mm/hmm.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index f6c4c8633db9..2aa75dbed04a 100644
>> +++ b/mm/hmm.c
>> @@ -936,7 +936,7 @@ int hmm_range_register(struct hmm_range *range,
>>   	range->start = start;
>>   	range->end = end;
>>   
>> -	range->hmm = hmm_get_or_create(mm);
>> +	range->hmm = mm_get_hmm(mm);
>>   	if (!range->hmm)
>>   		return -EFAULT;
> 
> I looked for documentation saying that hmm_range_register should only
> be done inside a hmm_mirror_register and didn't see it. Did I miss it?
> Can you add a comment?
> 
> It is really good to fix this because it means we can rely on mmap sem
> to manage mm->hmm!
> 
> If this is true then I also think we should change the signature of
> the function to make this dependency relationship clear, and remove
> some possible confusing edge cases.
> 
> What do you think about something like this? (unfinished)

I like it...

> 
> commit 29098bd59cf481ad1915db40aefc8435dabb8b28
> Author: Jason Gunthorpe <jgg@mellanox.com>
> Date:   Thu May 23 09:41:19 2019 -0300
> 
>      mm/hmm: Use hmm_mirror not mm as an argument for hmm_register_range
>      
>      Ralf observes that hmm_register_range() can only be called by a driver

       ^Ralph
:)

>      while a mirror is registered. Make this clear in the API by passing
>      in the mirror structure as a parameter.
>      
>      This also simplifies understanding the lifetime model for struct hmm,
>      as the hmm pointer must be valid as part of a registered mirror
>      so all we need in hmm_register_range() is a simple kref_get.
>      
>      Suggested-by: Ralph Campbell <rcampbell@nvidia.com>
>      Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 8b91c90d3b88cb..87d29e085a69f7 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -503,7 +503,7 @@ static inline bool hmm_mirror_mm_is_alive(struct hmm_mirror *mirror)
>    * Please see Documentation/vm/hmm.rst for how to use the range API.
>    */
>   int hmm_range_register(struct hmm_range *range,
> -		       struct mm_struct *mm,
> +		       struct hmm_mirror *mirror,
>   		       unsigned long start,
>   		       unsigned long end,
>   		       unsigned page_shift);
> @@ -539,7 +539,8 @@ static inline bool hmm_vma_range_done(struct hmm_range *range)
>   }
>   
>   /* This is a temporary helper to avoid merge conflict between trees. */
> -static inline int hmm_vma_fault(struct hmm_range *range, bool block)
> +static inline int hmm_vma_fault(struct hmm_mirror *mirror,
> +				struct hmm_range *range, bool block)
>   {
>   	long ret;
>   
> @@ -552,7 +553,7 @@ static inline int hmm_vma_fault(struct hmm_range *range, bool block)
>   	range->default_flags = 0;
>   	range->pfn_flags_mask = -1UL;
>   
> -	ret = hmm_range_register(range, range->vma->vm_mm,
> +	ret = hmm_range_register(range, mirror,
>   				 range->start, range->end,
>   				 PAGE_SHIFT);
>   	if (ret)
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 824e7e160d8167..fa1b04fcfc2549 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -927,7 +927,7 @@ static void hmm_pfns_clear(struct hmm_range *range,
>    * Track updates to the CPU page table see include/linux/hmm.h
>    */
>   int hmm_range_register(struct hmm_range *range,
> -		       struct mm_struct *mm,
> +		       struct hmm_mirror *mirror,
>   		       unsigned long start,
>   		       unsigned long end,
>   		       unsigned page_shift)
> @@ -935,7 +935,6 @@ int hmm_range_register(struct hmm_range *range,
>   	unsigned long mask = ((1UL << page_shift) - 1UL);
>   
>   	range->valid = false;
> -	range->hmm = NULL;
>   
>   	if ((start & mask) || (end & mask))
>   		return -EINVAL;
> @@ -946,15 +945,12 @@ int hmm_range_register(struct hmm_range *range,
>   	range->start = start;
>   	range->end = end;
>   
> -	range->hmm = hmm_get_or_create(mm);
> -	if (!range->hmm)
> -		return -EFAULT;
> -
>   	/* Check if hmm_mm_destroy() was call. */
> -	if (range->hmm->mm == NULL || range->hmm->dead) {
> -		hmm_put(range->hmm);
> +	if (mirror->hmm->mm == NULL || mirror->hmm->dead)
>   		return -EFAULT;
> -	}
> +
> +	range->hmm = mirror->hmm;
> +	kref_get(&range->hmm->kref);
>   
>   	/* Initialize range to track CPU page table update */
>   	mutex_lock(&range->hmm->lock);
> 

So far, this looks very good to me. Passing the mirror around is an
elegant API solution to the "we must have a valid mirror in order to
call this function" constraint.


thanks,
-- 
John Hubbard
NVIDIA