On Mon, Jan 10, 2011 at 11:08:04PM -0800, Hugh Dickins wrote: > Increased usage of page migration in mmotm reveals that the anon_vma > locking in unmap_and_move() has been deficient since 2.6.36 (or even > earlier). Hmm, a certain amount of the compaction work was spent fixing migration bugs. I wonder if there are mysterious bug reports out there related to the use of move_pages() that are only getting fixed now. > Review at the time of f18194275c39835cb84563500995e0d503a32d9a > "mm: fix hang on anon_vma->root->lock" missed the issue here: the anon_vma > to which we get a reference may already have been freed back to its slab > (it is in use when we check page_mapped, but that can change), and so its > anon_vma->root may be switched at any moment by reuse in anon_vma_prepare. > > Perhaps we could fix that with a get_anon_vma_unless_zero(), but let's not: > just rely on page_lock_anon_vma() to do all the hard thinking for us, then > we don't need any rcu read locking over here. > > In removing the rcu_unlock label: since PageAnon is a bit in page->mapping, > it's impossible for a !page->mapping page to be anon; but insert VM_BUG_ON > in case the implementation ever changes. > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: stable@xxxxxxxxxx [2.6.37, 2.6.36] Reasoning and patch look correct. Light testing did not show up any obvious problems. Reviewed-by: Mel Gorman <mel@xxxxxxxxx> -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>