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

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.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Hugh Dickins and committed by
Linus Torvalds 5dbe0af4 826267cf

+2 -2
+2 -2
mm/rmap.c
··· 1014 1014 return; 1015 1015 1016 1016 VM_BUG_ON(!PageLocked(page)); 1017 - VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end); 1017 + /* address might be in next vma when migration races vma_adjust */ 1018 1018 if (first) 1019 1019 __page_set_anon_rmap(page, vma, address, exclusive); 1020 1020 else ··· 1709 1709 1710 1710 BUG_ON(!PageLocked(page)); 1711 1711 BUG_ON(!anon_vma); 1712 - BUG_ON(address < vma->vm_start || address >= vma->vm_end); 1712 + /* address might be in next vma when migration races vma_adjust */ 1713 1713 first = atomic_inc_and_test(&page->_mapcount); 1714 1714 if (first) 1715 1715 __hugepage_set_anon_rmap(page, vma, address, 0);