On Thu, Aug 26, 2010 at 03:54:28AM -0700, Hugh Dickins wrote: > On Wed, Aug 25, 2010 at 11:41 PM, David Miller <davem@xxxxxxxxxxxxx> wrote: > > From: Hugh Dickins <hughd@xxxxxxxxxx> > > Date: Wed, 25 Aug 2010 23:12:54 -0700 (PDT) > > > >> After several hours, kbuild tests hang with anon_vma_prepare() spinning on > >> a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y > >> (which makes this very much more likely, but it could happen without). > >> > >> The ever-subtle page_lock_anon_vma() now needs a further twist: since > >> anon_vma_prepare() and anon_vma_fork() are liable to change the ->root > >> of a reused anon_vma structure at any moment, page_lock_anon_vma() > >> needs to check page_mapped() again before succeeding, otherwise > >> page_unlock_anon_vma() might address a different root->lock. > >> > >> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > > > > Interesting, is the condition which allows this to trigger specific > > to this merge window or was it always possible? > > Just specific to this merge window, which started using > anon_vma->root->lock in place of anon_vma->lock (anon_vma->root is > often anon_vma itself, but not always). I _think_ that change was > itself a simplification of the locking in 2.6.35, rather than plugging > a particular hole (it's not been backported to -stable), but I may be > wrong on that - Rik? rmap_walk_anon isn't stable without the anon_vma->root->lock. This is because it only locks the local anon_vma and but the "vma" can be still modified under it in vma_adjust, so vma_address can fail and migration will crash. swapping is not issue as it's ok to miss a pte once in a while if the vma is under vma_adjust by the time the VM tries to unmap the page. See the anon_vma_lock added as well to vma_adjust (without it, it'd be just an useless exercise, but instead it's really plugging a real hole thanks to the anon_vma_lock addition to vma_adjust). Again not an issue unless you use migration (incidentally what memory compaction uses, and you know who the primary user of memory compaction is :). Secondly ksm_does_need_to_copy can't be fixed to cow only pages that are nonlinear as the page->mapping != anon_vma can now generate false positives. It's now trivial to fix with page->mapping->root != anon_vma->root. About your patch, it's a noop in my view... A single page_mapping check after rcu_read_lock is enough. And "anon_vma->root" can't change if page->mapping points to "anon_vma" at any time after rcu_read_lock returns. rcu_read_lock works for all anon_vma including anon_vma->root. If you were running the page_mapped() check on a different "page" then it could make a difference, but repeating it on the same page in the same rcu_read_lock protected critical section won't make a difference as far as the anon_vma freeing is concerned. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>