Re: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 26 Apr 2010 23:37:58 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> vma_adjust() is updating anon VMA information without any locks taken.
> In contrast, file-backed mappings use the i_mmap_lock and this lack of
> locking can result in races with page migration. During rmap_walk(),
> vma_address() can return -EFAULT for an address that will soon be valid.
> This leaves a dangling migration PTE behind which can later cause a BUG_ON
> to trigger when the page is faulted in.
> 
> With the recent anon_vma changes, there can be more than one anon_vma->lock
> that can be taken in a anon_vma_chain but a second lock cannot be spinned
> upon in case of deadlock. Instead, the rmap walker tries to take locks of
> different anon_vma's. If the attempt fails, the operation is restarted.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>

Ok, acquiring vma->anon_vma->spin_lock always sounds very safe.
(but slow.)

I'll test this, too.

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>



> ---
>  mm/ksm.c  |   13 +++++++++++++
>  mm/mmap.c |    6 ++++++
>  mm/rmap.c |   22 +++++++++++++++++++---
>  3 files changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 3666d43..baa5b4d 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1674,9 +1674,22 @@ again:
>  		spin_lock(&anon_vma->lock);
>  		list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
>  			vma = vmac->vma;
> +
> +			/* See comment in mm/rmap.c#rmap_walk_anon on locking */
> +			if (anon_vma != vma->anon_vma) {
> +				if (!spin_trylock(&vma->anon_vma->lock)) {
> +					spin_unlock(&anon_vma->lock);
> +					goto again;
> +				}
> +			}
> +
>  			if (rmap_item->address < vma->vm_start ||
>  			    rmap_item->address >= vma->vm_end)
>  				continue;
> +
> +			if (anon_vma != vma->anon_vma)
> +				spin_unlock(&vma->anon_vma->lock);
> +
>  			/*
>  			 * Initially we examine only the vma which covers this
>  			 * rmap_item; but later, if there is still work to do,
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f90ea92..61d6f1d 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -578,6 +578,9 @@ again:			remove_next = 1 + (end > next->vm_end);
>  		}
>  	}
>  
> +	if (vma->anon_vma)
> +		spin_lock(&vma->anon_vma->lock);
> +
>  	if (root) {
>  		flush_dcache_mmap_lock(mapping);
>  		vma_prio_tree_remove(vma, root);
> @@ -620,6 +623,9 @@ again:			remove_next = 1 + (end > next->vm_end);
>  	if (mapping)
>  		spin_unlock(&mapping->i_mmap_lock);
>  
> +	if (vma->anon_vma)
> +		spin_unlock(&vma->anon_vma->lock);
> +
>  	if (remove_next) {
>  		if (file) {
>  			fput(file);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 85f203e..bc313a6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1368,15 +1368,31 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
>  	 * are holding mmap_sem. Users without mmap_sem are required to
>  	 * take a reference count to prevent the anon_vma disappearing
>  	 */
> +retry:
>  	anon_vma = page_anon_vma(page);
>  	if (!anon_vma)
>  		return ret;
>  	spin_lock(&anon_vma->lock);
>  	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
>  		struct vm_area_struct *vma = avc->vma;
> -		unsigned long address = vma_address(page, vma);
> -		if (address == -EFAULT)
> -			continue;
> +		unsigned long address;
> +
> +		/*
> +		 * Guard against deadlocks by not spinning against
> +		 * vma->anon_vma->lock. If contention is found, release our
> +		 * lock and try again until VMA list can be traversed without
> +		 * contention.
> +		 */
> +		if (anon_vma != vma->anon_vma) {
> +			if (!spin_trylock(&vma->anon_vma->lock)) {
> +				spin_unlock(&anon_vma->lock);
> +				goto retry;
> +			}
> +		}
> +		address = vma_address(page, vma);
> +		if (anon_vma != vma->anon_vma)
> +			spin_unlock(&vma->anon_vma->lock);
> +
>  		ret = rmap_one(page, vma, address, arg);
>  		if (ret != SWAP_AGAIN)
>  			break;
> -- 
> 1.6.5
> 
> 

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