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Subject: Re: [PATCH v12 20/31] mm: introduce vma reference counter
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     akpm@linux-foundation.org, mhocko@kernel.org, peterz@infradead.org,
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        Thomas Gleixner <tglx@linutronix.de>,
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        Will Deacon <will.deacon@arm.com>,
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        Haiyan Song <haiyanx.song@intel.com>,
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        Michel Lespinasse <walken@google.com>,
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References: <20190416134522.17540-1-ldufour@linux.ibm.com>
 <20190416134522.17540-21-ldufour@linux.ibm.com>
 <20190422203647.GK14666@redhat.com>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Date:   Wed, 24 Apr 2019 16:26:06 +0200
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Le 22/04/2019 à 22:36, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:11PM +0200, Laurent Dufour wrote:
>> The final goal is to be able to use a VMA structure without holding the
>> mmap_sem and to be sure that the structure will not be freed in our back.
>>
>> The lockless use of the VMA will be done through RCU protection and thus a
>> dedicated freeing service is required to manage it asynchronously.
>>
>> As reported in a 2010's thread [1], this may impact file handling when a
>> file is still referenced while the mapping is no more there.  As the final
>> goal is to handle anonymous VMA in a speculative way and not file backed
>> mapping, we could close and free the file pointer in a synchronous way, as
>> soon as we are guaranteed to not use it without holding the mmap_sem. For
>> sanity reason, in a minimal effort, the vm_file file pointer is unset once
>> the file pointer is put.
>>
>> [1] https://lore.kernel.org/linux-mm/20100104182429.833180340@chello.nl/
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> 
> Using kref would have been better from my POV even with RCU freeing
> but anyway:
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

Thanks Jérôme,

I think kref is a good option here, I'll give it a try.


>> ---
>>   include/linux/mm.h       |  4 ++++
>>   include/linux/mm_types.h |  3 +++
>>   mm/internal.h            | 27 +++++++++++++++++++++++++++
>>   mm/mmap.c                | 13 +++++++++----
>>   4 files changed, 43 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index f14b2c9ddfd4..f761a9c65c74 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -529,6 +529,9 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
>>   	vma->vm_mm = mm;
>>   	vma->vm_ops = &dummy_vm_ops;
>>   	INIT_LIST_HEAD(&vma->anon_vma_chain);
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +	atomic_set(&vma->vm_ref_count, 1);
>> +#endif
>>   }
>>   
>>   static inline void vma_set_anonymous(struct vm_area_struct *vma)
>> @@ -1418,6 +1421,7 @@ static inline void INIT_VMA(struct vm_area_struct *vma)
>>   	INIT_LIST_HEAD(&vma->anon_vma_chain);
>>   #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>>   	seqcount_init(&vma->vm_sequence);
>> +	atomic_set(&vma->vm_ref_count, 1);
>>   #endif
>>   }
>>   
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 24b3f8ce9e42..6a6159e11a3f 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -285,6 +285,9 @@ struct vm_area_struct {
>>   	/* linked list of VM areas per task, sorted by address */
>>   	struct vm_area_struct *vm_next, *vm_prev;
>>   
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +	atomic_t vm_ref_count;
>> +#endif
>>   	struct rb_node vm_rb;
>>   
>>   	/*
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 9eeaf2b95166..302382bed406 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -40,6 +40,33 @@ void page_writeback_init(void);
>>   
>>   vm_fault_t do_swap_page(struct vm_fault *vmf);
>>   
>> +
>> +extern void __free_vma(struct vm_area_struct *vma);
>> +
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +static inline void get_vma(struct vm_area_struct *vma)
>> +{
>> +	atomic_inc(&vma->vm_ref_count);
>> +}
>> +
>> +static inline void put_vma(struct vm_area_struct *vma)
>> +{
>> +	if (atomic_dec_and_test(&vma->vm_ref_count))
>> +		__free_vma(vma);
>> +}
>> +
>> +#else
>> +
>> +static inline void get_vma(struct vm_area_struct *vma)
>> +{
>> +}
>> +
>> +static inline void put_vma(struct vm_area_struct *vma)
>> +{
>> +	__free_vma(vma);
>> +}
>> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>> +
>>   void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>>   		unsigned long floor, unsigned long ceiling);
>>   
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index f7f6027a7dff..c106440dcae7 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -188,6 +188,12 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>>   }
>>   #endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>>   
>> +void __free_vma(struct vm_area_struct *vma)
>> +{
>> +	mpol_put(vma_policy(vma));
>> +	vm_area_free(vma);
>> +}
>> +
>>   /*
>>    * Close a vm structure and free it, returning the next.
>>    */
>> @@ -200,8 +206,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>>   		vma->vm_ops->close(vma);
>>   	if (vma->vm_file)
>>   		fput(vma->vm_file);
>> -	mpol_put(vma_policy(vma));
>> -	vm_area_free(vma);
>> +	vma->vm_file = NULL;
>> +	put_vma(vma);
>>   	return next;
>>   }
>>   
>> @@ -990,8 +996,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   		if (next->anon_vma)
>>   			anon_vma_merge(vma, next);
>>   		mm->map_count--;
>> -		mpol_put(vma_policy(next));
>> -		vm_area_free(next);
>> +		put_vma(next);
>>   		/*
>>   		 * In mprotect's case 6 (see comments on vma_merge),
>>   		 * we must remove another next too. It would clutter
>> -- 
>> 2.21.0
>>
>