Re: [PATCH] Repeated fork() causes SLAB to grow without bound

[Jerome Marchand]

On 11/26/2014 06:35 PM, Michal Hocko wrote:
> On Tue 25-11-14 16:00:06, Michal Hocko wrote:
>> On Tue 25-11-14 16:13:16, Konstantin Khlebnikov wrote:
>>> On Tue, Nov 25, 2014 at 1:59 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
>>>> On Mon 24-11-14 11:09:40, Konstantin Khlebnikov wrote:
>>>>> On Thu, Nov 20, 2014 at 6:03 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>>>>>> On Thu, Nov 20, 2014 at 5:50 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>>>>>>> -----BEGIN PGP SIGNED MESSAGE-----
>>>>>>> Hash: SHA1
>>>>>>>
>>>>>>> On 11/20/2014 09:42 AM, Konstantin Khlebnikov wrote:
>>>>>>>
>>>>>>>> I'm thinking about limitation for reusing anon_vmas which might
>>>>>>>> increase performance without breaking asymptotic estimation of
>>>>>>>> count anon_vma in the worst case. For example this heuristic: allow
>>>>>>>> to reuse only anon_vma with single direct descendant. It seems
>>>>>>>> there will be arount up to two times more anon_vmas but
>>>>>>>> false-aliasing must be much lower.
>>>>>
>>>>> Done. RFC patch in attachment.
> 
> Ok, finally managed to untagnle myself from vma chains and your patch
> makes sense to me, it is quite clever actually. Here is it including the
> fixup.
> ---
>> From 1d4b0b38198c69ecfeb37670cb1dda767a802c9a Mon Sep 17 00:00:00 2001
>> From: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
>> Date: Tue, 25 Nov 2014 10:54:44 +0100
>> Subject: [PATCH] mm: prevent endless growth of anon_vma hierarchy
>>
>> Constantly forking task causes unlimited grow of anon_vma chain.
>> Each next child allocate new level of anon_vmas and links vmas to all
>> previous levels because it inherits pages from them. None of anon_vmas
>> cannot be freed because there might be pages which points to them.
>>
>> This patch adds heuristic which decides to reuse existing anon_vma instead
>> of forking new one. It counts vmas and direct descendants for each anon_vma.
>> Anon_vma with degree lower than two will be reused at next fork.
>> As a result each anon_vma has either alive vma or at least two descendants,
>> endless chains are no longer possible and count of anon_vmas is no more than
>> two times more than count of vmas.
>>
>> Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
>> Link: http://lkml.kernel.org/r/20120816024610.GA5350@xxxxxxxxxxxxxxxxxxxxxxx
> 
> Tested-by: Michal Hocko <mhocko@xxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> 
> and I guess
> Reported-by: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx>

Tested-by: Jerome Marchand <jmarchan@xxxxxxxxxx>

Minor nitpicks below.

> 
> who somehow vanished from CC list (added back) would be appropriate as
> well.
> 
> plus
> 
> Fixes: 5beb49305251 (mm: change anon_vma linking to fix multi-process server scalability issue)
> and mark it for stable
> 
> Thanks!
> 
>> ---
>>  include/linux/rmap.h | 16 ++++++++++++++++
>>  mm/rmap.c            | 29 ++++++++++++++++++++++++++++-
>>  2 files changed, 44 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>> index c0c2bce6b0b7..b1d140c20b37 100644
>> --- a/include/linux/rmap.h
>> +++ b/include/linux/rmap.h
>> @@ -45,6 +45,22 @@ struct anon_vma {
>>  	 * mm_take_all_locks() (mm_all_locks_mutex).
>>  	 */
>>  	struct rb_root rb_root;	/* Interval tree of private "related" vmas */
>> +
>> +	/*
>> +	 * Count of child anon_vmas and VMAs which points to this anon_vma.
>> +	 *
>> +	 * This counter is used for making decision about reusing old anon_vma
>> +	 * instead of forking new one. It allows to detect anon_vmas which have
>> +	 * just one direct descendant and no vmas. Reusing such anon_vma not
>> +	 * leads to significant preformance regression but prevents degradation

Does it or does it not lead to significant performance issue? I can't tell.

>> +	 * of anon_vma hierarchy to endless linear chain.
>> +	 *
>> +	 * Root anon_vma is never reused because it is its own parent and it has
>> +	 * at leat one vma or child, thus at fork it's degree is at least 2.

s/leat/least/

Thanks,
Jerome

>> +	 */
>> +	unsigned degree;
>> +
>> +	struct anon_vma *parent;	/* Parent of this anon_vma */
>>  };
>>  
>>  /*
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 19886fb2f13a..40ae8184a1e1 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -72,6 +72,8 @@ static inline struct anon_vma *anon_vma_alloc(void)
>>  	anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
>>  	if (anon_vma) {
>>  		atomic_set(&anon_vma->refcount, 1);
>> +		anon_vma->degree = 1;	/* Reference for first vma */
>> +		anon_vma->parent = anon_vma;
>>  		/*
>>  		 * Initialise the anon_vma root to point to itself. If called
>>  		 * from fork, the root will be reset to the parents anon_vma.
>> @@ -188,6 +190,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
>>  		if (likely(!vma->anon_vma)) {
>>  			vma->anon_vma = anon_vma;
>>  			anon_vma_chain_link(vma, avc, anon_vma);
>> +			anon_vma->degree++;
>>  			allocated = NULL;
>>  			avc = NULL;
>>  		}
>> @@ -256,7 +259,17 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>>  		anon_vma = pavc->anon_vma;
>>  		root = lock_anon_vma_root(root, anon_vma);
>>  		anon_vma_chain_link(dst, avc, anon_vma);
>> +
>> +		/*
>> +		 * Reuse existing anon_vma if its degree lower than two,
>> +		 * that means it has no vma and just one anon_vma child.
>> +		 */
>> +		if (!dst->anon_vma && anon_vma != src->anon_vma &&
>> +				anon_vma->degree < 2)
>> +			dst->anon_vma = anon_vma;
>>  	}
>> +	if (dst->anon_vma)
>> +		dst->anon_vma->degree++;
>>  	unlock_anon_vma_root(root);
>>  	return 0;
>>  
>> @@ -279,6 +292,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>  	if (!pvma->anon_vma)
>>  		return 0;
>>  
>> +	/* Drop inherited anon_vma, we'll reuse old one or allocate new. */
>> +	vma->anon_vma = NULL;
>> +
>>  	/*
>>  	 * First, attach the new VMA to the parent VMA's anon_vmas,
>>  	 * so rmap can find non-COWed pages in child processes.
>> @@ -286,6 +302,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>  	if (anon_vma_clone(vma, pvma))
>>  		return -ENOMEM;
>>  
>> +	/* An old anon_vma has been reused. */
>> +	if (vma->anon_vma)
>> +		return 0;
>> +
>>  	/* Then add our own anon_vma. */
>>  	anon_vma = anon_vma_alloc();
>>  	if (!anon_vma)
>> @@ -299,6 +319,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>  	 * lock any of the anon_vmas in this anon_vma tree.
>>  	 */
>>  	anon_vma->root = pvma->anon_vma->root;
>> +	anon_vma->parent = pvma->anon_vma;
>>  	/*
>>  	 * With refcounts, an anon_vma can stay around longer than the
>>  	 * process it belongs to. The root anon_vma needs to be pinned until
>> @@ -309,6 +330,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>  	vma->anon_vma = anon_vma;
>>  	anon_vma_lock_write(anon_vma);
>>  	anon_vma_chain_link(vma, avc, anon_vma);
>> +	anon_vma->parent->degree++;
>>  	anon_vma_unlock_write(anon_vma);
>>  
>>  	return 0;
>> @@ -339,12 +361,16 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
>>  		 * Leave empty anon_vmas on the list - we'll need
>>  		 * to free them outside the lock.
>>  		 */
>> -		if (RB_EMPTY_ROOT(&anon_vma->rb_root))
>> +		if (RB_EMPTY_ROOT(&anon_vma->rb_root)) {
>> +			anon_vma->parent->degree--;
>>  			continue;
>> +		}
>>  
>>  		list_del(&avc->same_vma);
>>  		anon_vma_chain_free(avc);
>>  	}
>> +	if (vma->anon_vma)
>> +		vma->anon_vma->degree--;
>>  	unlock_anon_vma_root(root);
>>  
>>  	/*
>> @@ -355,6 +381,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
>>  	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
>>  		struct anon_vma *anon_vma = avc->anon_vma;
>>  
>> +		BUG_ON(anon_vma->degree);
>>  		put_anon_vma(anon_vma);
>>  
>>  		list_del(&avc->same_vma);
>> -- 
>> 2.1.3
>

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