Re: [RFC PATCH -v2] take all anon_vma locks in anon_vma_lock

[Mel Gorman]

On Wed, Apr 28, 2010 at 02:25:10PM -0400, Rik van Riel wrote:
> Take all the locks for all the anon_vmas in anon_vma_lock, this properly
> excludes migration and the transparent hugepage code from VMA changes done
> by mmap/munmap/mprotect/expand_stack/etc...
> 
> Also document the locking rules for the same_vma list in the anon_vma_chain
> and remove the anon_vma_lock call from expand_upwards, which does not need it.
> 
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> 
> --- 
> Posted quickly as an RFC patch, only compile tested so far.
> Andrea, Mel, does this look like a reasonable approach?
> 

Well, it looks nice but I am wary of making much in the way of comment
on its correctness. Every time I think I understand the new anon_vma
changes, I find something else to make me doubt myself :)

> v2:
>  - also change anon_vma_unlock to walk the loop
>  - add calls to anon_vma_lock & anon_vma_unlock to vma_adjust
>  - introduce a new lock for the vma->anon_vma_chain list, to prevent
>    the lock inversion that Andrea pointed out
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index b8bb9a6..a0679c6 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -239,6 +239,7 @@ struct mm_struct {
>  	int map_count;				/* number of VMAs */
>  	struct rw_semaphore mmap_sem;
>  	spinlock_t page_table_lock;		/* Protects page tables and some counters */
> +	spinlock_t anon_vma_chain_lock;		/* Protects vma->anon_vma_chain, with mmap_sem */
>  
>  	struct list_head mmlist;		/* List of maybe swapped mm's.	These are globally strung
>  						 * together off init_mm.mmlist, and are protected
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index d25bd22..492e7ca 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -52,11 +52,15 @@ struct anon_vma {
>   * all the anon_vmas associated with this VMA.
>   * The "same_anon_vma" list contains the anon_vma_chains
>   * which link all the VMAs associated with this anon_vma.
> + *
> + * The "same_vma" list is locked by either having mm->mmap_sem
> + * locked for writing, or having mm->mmap_sem locked for reading
> + * AND holding the mm->anon_vma_chain_lock.
>   */
>  struct anon_vma_chain {
>  	struct vm_area_struct *vma;
>  	struct anon_vma *anon_vma;
> -	struct list_head same_vma;   /* locked by mmap_sem & page_table_lock */
> +	struct list_head same_vma;	/* see above */
>  	struct list_head same_anon_vma;	/* locked by anon_vma->lock */
>  };
>  
> @@ -90,18 +94,24 @@ static inline struct anon_vma *page_anon_vma(struct page *page)
>  	return page_rmapping(page);
>  }
>  
> -static inline void anon_vma_lock(struct vm_area_struct *vma)
> +static inline void anon_vma_lock(struct vm_area_struct *vma, void *nest_lock)
>  {
>  	struct anon_vma *anon_vma = vma->anon_vma;
> -	if (anon_vma)
> -		spin_lock(&anon_vma->lock);
> +	if (anon_vma) {
> +		struct anon_vma_chain *avc;
> +		list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
> +			spin_lock_nest_lock(&avc->anon_vma->lock, nest_lock);
> +	}
>  }

This doesn't build with LOCKDEP enabled. As we discussed on IRC, I
changed nest_lock to spinlock_t * and always made it the
anon_vma_chain_lock. That vaguely makes more sense than sometimes
depending on the mmap_sem semaphore but I'm not 100% solid on it.

Walking along like this and taking the necessary locks seems reasonable
though and should be usable by transparent hugepage support where as my
approach probably wasn't.

>  
>  static inline void anon_vma_unlock(struct vm_area_struct *vma)
>  {
>  	struct anon_vma *anon_vma = vma->anon_vma;
> -	if (anon_vma)
> -		spin_unlock(&anon_vma->lock);
> +	if (anon_vma) {
> +		struct anon_vma_chain *avc;
> +		list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
> +			spin_unlock(&avc->anon_vma->lock);
> +	}
>  }
>  
>  /*
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 44b0791..83b1ba2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -468,6 +468,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
>  	mm->nr_ptes = 0;
>  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>  	spin_lock_init(&mm->page_table_lock);
> +	spin_lock_init(&mm->anon_vma_chain_lock);
>  	mm->free_area_cache = TASK_UNMAPPED_BASE;
>  	mm->cached_hole_size = ~0UL;
>  	mm_init_aio(mm);
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index 57aba0d..3ce8a1f 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -15,6 +15,7 @@ struct mm_struct init_mm = {
>  	.mm_count	= ATOMIC_INIT(1),
>  	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>  	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> +	.anon_vma_chain_lock =  __SPIN_LOCK_UNLOCKED(init_mm.anon_vma_chain_lock),
>  	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
>  	.cpu_vm_mask	= CPU_MASK_ALL,
>  };
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f90ea92..4602358 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -452,7 +452,7 @@ static void vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
>  		spin_lock(&mapping->i_mmap_lock);
>  		vma->vm_truncate_count = mapping->truncate_count;
>  	}
> -	anon_vma_lock(vma);
> +	anon_vma_lock(vma, &mm->mmap_sem);
>  

so I changed this and others like it to the anon_vma_chain_lock. I
haven't actually tested yet.

>  	__vma_link(mm, vma, prev, rb_link, rb_parent);
>  	__vma_link_file(vma);
> @@ -578,6 +578,7 @@ again:			remove_next = 1 + (end > next->vm_end);
>  		}
>  	}
>  
> +	anon_vma_lock(vma, &mm->mmap_sem);
>  	if (root) {
>  		flush_dcache_mmap_lock(mapping);
>  		vma_prio_tree_remove(vma, root);
> @@ -599,6 +600,7 @@ again:			remove_next = 1 + (end > next->vm_end);
>  		vma_prio_tree_insert(vma, root);
>  		flush_dcache_mmap_unlock(mapping);
>  	}
> +	anon_vma_unlock(vma);
>  
>  	if (remove_next) {
>  		/*
> @@ -1705,12 +1707,11 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  		return -EFAULT;
>  
>  	/*
> -	 * We must make sure the anon_vma is allocated
> -	 * so that the anon_vma locking is not a noop.
> +	 * Unlike expand_downwards, we do not need to take the anon_vma lock,
> +	 * because we leave vma->vm_start and vma->pgoff untouched. 
> +	 * This means rmap lookups of pages inside this VMA stay valid
> +	 * throughout the stack expansion.
>  	 */
> -	if (unlikely(anon_vma_prepare(vma)))
> -		return -ENOMEM;
> -	anon_vma_lock(vma);
>  
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
> @@ -1721,7 +1722,6 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  	if (address < PAGE_ALIGN(address+4))
>  		address = PAGE_ALIGN(address+4);
>  	else {
> -		anon_vma_unlock(vma);
>  		return -ENOMEM;
>  	}
>  	error = 0;
> @@ -1737,7 +1737,6 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  		if (!error)
>  			vma->vm_end = address;
>  	}
> -	anon_vma_unlock(vma);
>  	return error;
>  }
>  #endif /* CONFIG_STACK_GROWSUP || CONFIG_IA64 */
> @@ -1749,6 +1748,7 @@ static int expand_downwards(struct vm_area_struct *vma,
>  				   unsigned long address)
>  {
>  	int error;
> +	struct mm_struct *mm = vma->vm_mm;
>  
>  	/*
>  	 * We must make sure the anon_vma is allocated
> @@ -1762,7 +1762,8 @@ static int expand_downwards(struct vm_area_struct *vma,
>  	if (error)
>  		return error;
>  
> -	anon_vma_lock(vma);
> +	spin_lock(&mm->anon_vma_chain_lock);
> +	anon_vma_lock(vma, &mm->anon_vma_chain_lock);
>  
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
> @@ -1784,6 +1785,8 @@ static int expand_downwards(struct vm_area_struct *vma,
>  		}
>  	}
>  	anon_vma_unlock(vma);
> +	spin_unlock(&mm->anon_vma_chain_lock);
> +
>  	return error;
>  }
>  
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 526704e..aa27132 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -23,6 +23,7 @@
>   * inode->i_mutex	(while writing or truncating, not reading or faulting)
>   *   inode->i_alloc_sem (vmtruncate_range)
>   *   mm->mmap_sem
> + *   mm->anon_vma_chain_lock (mmap_sem for read, protects vma->anon_vma_chain)
>   *     page->flags PG_locked (lock_page)
>   *       mapping->i_mmap_lock
>   *         anon_vma->lock
> @@ -135,8 +136,8 @@ int anon_vma_prepare(struct vm_area_struct *vma)
>  		}
>  		spin_lock(&anon_vma->lock);
>  
> -		/* page_table_lock to protect against threads */
> -		spin_lock(&mm->page_table_lock);
> +		/* anon_vma_chain_lock to protect against threads */
> +		spin_lock(&mm->anon_vma_chain_lock);
>  		if (likely(!vma->anon_vma)) {
>  			vma->anon_vma = anon_vma;
>  			avc->anon_vma = anon_vma;
> @@ -145,7 +146,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
>  			list_add(&avc->same_anon_vma, &anon_vma->head);
>  			allocated = NULL;
>  		}
> -		spin_unlock(&mm->page_table_lock);
> +		spin_unlock(&mm->anon_vma_chain_lock);
>  
>  		spin_unlock(&anon_vma->lock);
>  		if (unlikely(allocated)) {
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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