Re: [PATCH] mm: fix kernel BUG at mm/rmap.c:1017!

[Nai Xia <nai.xia@gmail.com>]

On Sunday 29 May 2011 04:17:04 Hugh Dickins wrote:
> I've hit the "address >= vma->vm_end" check in do_page_add_anon_rmap()
> just once.  The stack showed khugepaged allocation trying to compact
> pages: the call to page_add_anon_rmap() coming from remove_migration_pte().
> 
> That path holds anon_vma lock, but does not hold mmap_sem: it can
> therefore race with a split_vma(), and in commit 5f70b962ccc2 "mmap:
> avoid unnecessary anon_vma lock" we just took away the anon_vma lock
> protection when adjusting vma->vm_end.
> 
> I don't think that particular BUG_ON ever caught anything interesting,
> so better replace it by a comment, than reinstate the anon_vma locking.

Is there another racing between "vma->vm_pgoff = pgoff;" in 
vma_adjust() and linear_page_index() in __page_set_anon_rmap() ?


Nai Xia

> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  mm/rmap.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- linux.orig/mm/rmap.c	2011-05-27 19:05:27.000000000 -0700
> +++ linux/mm/rmap.c	2011-05-27 20:07:44.601361236 -0700
> @@ -1014,7 +1014,7 @@ void do_page_add_anon_rmap(struct page *
>  		return;
>  
>  	VM_BUG_ON(!PageLocked(page));
> -	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
> +	/* address might be in next vma when migration races vma_adjust */
>  	if (first)
>  		__page_set_anon_rmap(page, vma, address, exclusive);
>  	else
> @@ -1709,7 +1709,7 @@ void hugepage_add_anon_rmap(struct page
>  
>  	BUG_ON(!PageLocked(page));
>  	BUG_ON(!anon_vma);
> -	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
> +	/* address might be in next vma when migration races vma_adjust */
>  	first = atomic_inc_and_test(&page->_mapcount);
>  	if (first)
>  		__hugepage_set_anon_rmap(page, vma, address, 0);
> 
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