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

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

On Sunday 29 May 2011 18:23:46 Nai Xia wrote:
> 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() ?

Oh, sorry, please ignore this, this _is_ protected by anon_vma lock.

Nai Xia

> 
> 
> 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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> 

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