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mm: fix kernel BUG at mm/rmap.c:1017!

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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 5f70b962cc "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.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2011-05-28 13:17:04 -07:00 committed by Linus Torvalds
parent 826267cf1e
commit 5dbe0af47f
1 changed files with 2 additions and 2 deletions
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4
mm/rmap.c
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@ -1014,7 +1014,7 @@ void do_page_add_anon_rmap(struct page *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 *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);