On Sat, 20 May 2017, Xishi Qiu wrote: > On 2017/5/20 6:00, Hugh Dickins wrote: > > > > You're ignoring the rcu_read_lock() on entry to page_lock_anon_vma_read(), > > and the SLAB_DESTROY_BY_RCU (recently renamed SLAB_TYPESAFE_BY_RCU) nature > > of the anon_vma_cachep kmem cache. It is not safe to muck with anon_vma-> > > root in anon_vma_free(), others could still be looking at it. > > > > Hugh > > > > Hi Hugh, > > Thanks for your reply. > > SLAB_DESTROY_BY_RCU will let it call call_rcu() in free_slab(), but if the > anon_vma *reuse* by someone again, access root_anon_vma is not safe, right? That is safe, on reuse it is still a struct anon_vma; then the test for !page_mapped(page) will show that it's no longer a reliable anon_vma for this page, so page_lock_anon_vma_read() returns NULL. But of course, if page->_mapcount has been corrupted or misaccounted, it may think page_mapped(page) when actually page is not mapped, and the anon_vma is not good for it. > > e.g. if I clean the root pointer before free it, then access root_anon_vma > in page_lock_anon_vma_read() is NULL pointer access, right? Yes, cleaning root pointer before free may result in NULL pointer access. Hugh > > anon_vma_free() > ... > anon_vma->root = NULL; > kmem_cache_free(anon_vma_cachep, anon_vma); > ... > > Thanks, > Xishi Qiu -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
