Re: [PATCH v3] mm: prevent endless growth of anon_vma hierarchy

[Michal Hocko]

What happened to this patch? I do not see it merged for 3.19 and
nor in the current mmotm tree (2014-12-15-17-05)

On Wed 26-11-14 22:11:45, Konstantin Khlebnikov wrote:
> 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>
> Reported-by: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx>
> Tested-by: Michal Hocko <mhocko@xxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> Link: http://lkml.kernel.org/r/20120816024610.GA5350@xxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 5beb49305251 ("mm: change anon_vma linking to fix multi-process server scalability issue")
> Cc: Stable <stable@xxxxxxxxxxxxxxx> (2.6.34+)
> 
> ---
> 
> v2: update degree in anon_vma_prepare for merged anon_vma
> v3: update comment and tags
> ---
>  include/linux/rmap.h |   16 ++++++++++++++++
>  mm/rmap.c            |   30 +++++++++++++++++++++++++++++-
>  2 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index c0c2bce..b1d140c 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
> +	 * 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.
> +	 */
> +	unsigned degree;
> +
> +	struct anon_vma *parent;	/* Parent of this anon_vma */
>  };
>  
>  /*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 19886fb..df5c44e 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 +190,8 @@ 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);
> +			/* vma link if merged or child link for new root */
> +			anon_vma->degree++;
>  			allocated = NULL;
>  			avc = NULL;
>  		}
> @@ -256,7 +260,17 @@ 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 +293,9 @@ 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 +303,10 @@ 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 +320,7 @@ 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 +331,7 @@ 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 +362,16 @@ 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 +382,7 @@ 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);
> 
> --
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-- 
Michal Hocko
SUSE Labs
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