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Subject: Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu()
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     akpm@linux-foundation.org, mhocko@kernel.org, peterz@infradead.org,
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        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
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        Daniel Jordan <daniel.m.jordan@oracle.com>,
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        Ganesh Mahendran <opensource.ganesh@gmail.com>,
        Minchan Kim <minchan@kernel.org>,
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        Haiyan Song <haiyanx.song@intel.com>,
        Balbir Singh <bsingharora@gmail.com>, sj38.park@gmail.com,
        Michel Lespinasse <walken@google.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
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References: <20190416134522.17540-1-ldufour@linux.ibm.com>
 <20190416134522.17540-22-ldufour@linux.ibm.com>
 <20190422205721.GL14666@redhat.com>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Date:   Wed, 24 Apr 2019 16:39:01 +0200
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Le 22/04/2019 à 22:57, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote:
>> This allows to search for a VMA structure without holding the mmap_sem.
>>
>> The search is repeated while the mm seqlock is changing and until we found
>> a valid VMA.
>>
>> While under the RCU protection, a reference is taken on the VMA, so the
>> caller must call put_vma() once it not more need the VMA structure.
>>
>> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
>> reference is taken to the VMA by calling get_vma().
>>
>> When removing a VMA from the MM RB tree, the VMA is not release immediately
>> but at the end of the RCU grace period through vm_rcu_put(). This ensures
>> that the VMA remains allocated until the end the RCU grace period.
>>
>> Since the vm_file pointer, if valid, is released in put_vma(), there is no
>> guarantee that the file pointer will be valid on the returned VMA.
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> 
> Minor comments about comment (i love recursion :)) see below.
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

Thanks Jérôme, see my comments to your comments on my comments below ;)

>> ---
>>   include/linux/mm_types.h |  1 +
>>   mm/internal.h            |  5 ++-
>>   mm/mmap.c                | 76 ++++++++++++++++++++++++++++++++++++++--
>>   3 files changed, 78 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 6a6159e11a3f..9af6694cb95d 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -287,6 +287,7 @@ struct vm_area_struct {
>>   
>>   #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>>   	atomic_t vm_ref_count;
>> +	struct rcu_head vm_rcu;
>>   #endif
>>   	struct rb_node vm_rb;
>>   
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 302382bed406..1e368e4afe3c 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma)
>>   		__free_vma(vma);
>>   }
>>   
>> -#else
>> +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm,
>> +					   unsigned long addr);
>> +
>> +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */
>>   
>>   static inline void get_vma(struct vm_area_struct *vma)
>>   {
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index c106440dcae7..34bf261dc2c8 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>>   {
>>   	write_sequnlock(&mm->mm_seq);
>>   }
>> +
>> +static void __vm_rcu_put(struct rcu_head *head)
>> +{
>> +	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
>> +						  vm_rcu);
>> +	put_vma(vma);
>> +}
>> +static void vm_rcu_put(struct vm_area_struct *vma)
>> +{
>> +	VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>> +	call_rcu(&vma->vm_rcu, __vm_rcu_put);
>> +}
>>   #else
>>   static inline void mm_write_seqlock(struct mm_struct *mm)
>>   {
>> @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>>   
>>   void __free_vma(struct vm_area_struct *vma)
>>   {
>> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
>> +		VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>>   	mpol_put(vma_policy(vma));
>>   	vm_area_free(vma);
>>   }
>> @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma)
>>   /*
>>    * Close a vm structure and free it, returning the next.
>>    */
>> -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma)
>>   {
>>   	struct vm_area_struct *next = vma->vm_next;
>>   
>>   	might_sleep();
>> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) &&
>> +	    !RB_EMPTY_NODE(&vma->vm_rb)) {
>> +		/*
>> +		 * If the VMA is still linked in the RB tree, we must release
>> +		 * that reference by calling put_vma().
>> +		 * This should only happen when called from exit_mmap().
>> +		 * We forcely clear the node to satisfy the chec in
>                                                          ^
> Typo: chec -> check

Yep

> 
>> +		 * __free_vma(). This is safe since the RB tree is not walked
>> +		 * anymore.
>> +		 */
>> +		RB_CLEAR_NODE(&vma->vm_rb);
>> +		put_vma(vma);
>> +	}
>>   	if (vma->vm_ops && vma->vm_ops->close)
>>   		vma->vm_ops->close(vma);
>>   	if (vma->vm_file)
>> @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>>   	return next;
>>   }
>>   
>> +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> +{
>> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
>> +		VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
> 
> Adding a comment here explaining the BUG_ON so people can understand
> what is wrong if that happens. For instance:
> 
> /*
>   * remove_vma() should be call only once a vma have been remove from the rbtree
>   * at which point the vma->vm_rb is an empty node. The exception is when vmas
>   * are destroy through exit_mmap() in which case we do not bother updating the
>   * rbtree (see comment in __remove_vma()).
>   */

I agree !


>> +	return __remove_vma(vma);
>> +}
>> +
>>   static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
>>   		struct list_head *uf);
>>   SYSCALL_DEFINE1(brk, unsigned long, brk)
>> @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma,
>>   
>>   	/* All rb_subtree_gap values must be consistent prior to insertion */
>>   	validate_mm_rb(root, NULL);
>> -
>> +	get_vma(vma);
>>   	rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>>   }
>>   
>> @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
>>   	mm_write_seqlock(mm);
>>   	rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>>   	mm_write_sequnlock(mm);	/* wmb */
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +	/*
>> +	 * Ensure the removal is complete before clearing the node.
>> +	 * Matched by vma_has_changed()/handle_speculative_fault().
>> +	 */
>> +	RB_CLEAR_NODE(&vma->vm_rb);
>> +	vm_rcu_put(vma);
>> +#endif
>>   }
>>   
>>   static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
>> @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>>   
>>   EXPORT_SYMBOL(find_vma);
>>   
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +/*
>> + * Like find_vma() but under the protection of RCU and the mm sequence counter.
>> + * The vma returned has to be relaesed by the caller through the call to
>> + * put_vma()
>> + */
>> +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr)
>> +{
>> +	struct vm_area_struct *vma = NULL;
>> +	unsigned int seq;
>> +
>> +	do {
>> +		if (vma)
>> +			put_vma(vma);
>> +
>> +		seq = read_seqbegin(&mm->mm_seq);
>> +
>> +		rcu_read_lock();
>> +		vma = find_vma(mm, addr);
>> +		if (vma)
>> +			get_vma(vma);
>> +		rcu_read_unlock();
>> +	} while (read_seqretry(&mm->mm_seq, seq));
>> +
>> +	return vma;
>> +}
>> +#endif
>> +
>>   /*
>>    * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
>>    */
>> @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm)
>>   	while (vma) {
>>   		if (vma->vm_flags & VM_ACCOUNT)
>>   			nr_accounted += vma_pages(vma);
>> -		vma = remove_vma(vma);
>> +		vma = __remove_vma(vma);
>>   	}
>>   	vm_unacct_memory(nr_accounted);
>>   }
>> -- 
>> 2.21.0
>>
>