On Thu, May 08, 2014 at 08:53:30AM -0700, Paul E. McKenney wrote: > On Thu, May 08, 2014 at 04:29:48PM +0100, Catalin Marinas wrote: > > On Thu, May 08, 2014 at 04:00:27PM +0100, Paul E. McKenney wrote: > > > On Thu, May 08, 2014 at 11:24:36AM +0100, Catalin Marinas wrote: > > > > On Thu, May 08, 2014 at 10:37:40AM +0100, Jaegeuk Kim wrote: > > > > > 2014-05-08 (목), 10:26 +0100, Catalin Marinas: > > > > > > On Thu, May 08, 2014 at 06:16:51PM +0900, Jaegeuk Kim wrote: > > > > > > > 2014-05-07 (수), 12:39 +0100, Catalin Marinas: > > > > > > > > On Wed, May 07, 2014 at 03:58:08AM +0100, Jaegeuk Kim wrote: > > > > > > > > > unreferenced object 0xffff880004226da0 (size 576): > > > > > > > > > comm "fsstress", pid 14590, jiffies 4295191259 (age 706.308s) > > > > > > > > > hex dump (first 32 bytes): > > > > > > > > > 01 00 00 00 81 ff ff ff 00 00 00 00 00 00 00 00 ................ > > > > > > > > > 50 89 34 81 ff ff ff ff b8 6d 22 04 00 88 ff ff P.4......m"..... > > > > > > > > > backtrace: > > > > > > > > > [<ffffffff816c02e8>] kmemleak_update_trace+0x58/0x80 > > > > > > > > > [<ffffffff81349517>] radix_tree_node_alloc+0x77/0xa0 > > > > > > > > > [<ffffffff81349718>] __radix_tree_create+0x1d8/0x230 > > > > > > > > > [<ffffffff8113286c>] __add_to_page_cache_locked+0x9c/0x1b0 > > > > > > > > > [<ffffffff811329a8>] add_to_page_cache_lru+0x28/0x80 > > > > > > > > > [<ffffffff81132f58>] grab_cache_page_write_begin+0x98/0xf0 > > > > > > > > > [<ffffffffa02e4bf4>] f2fs_write_begin+0xb4/0x3c0 [f2fs] > > > > > > > > > [<ffffffff81131b77>] generic_perform_write+0xc7/0x1c0 > > > > > > > > > [<ffffffff81133b7d>] __generic_file_aio_write+0x1cd/0x3f0 > > > > > > > > > [<ffffffff81133dfe>] generic_file_aio_write+0x5e/0xe0 > > > > > > > > > [<ffffffff81195c5a>] do_sync_write+0x5a/0x90 > > > > > > > > > [<ffffffff811968d2>] vfs_write+0xc2/0x1d0 > > > > > > > > > [<ffffffff81196daf>] SyS_write+0x4f/0xb0 > > > > > > > > > [<ffffffff816dead2>] system_call_fastpath+0x16/0x1b > > > > > > > > > [<ffffffffffffffff>] 0xffffffffffffffff > > > > > > > > > > > > > > > > OK, it shows that the allocation happens via add_to_page_cache_locked() > > > > > > > > and I guess it's page_cache_tree_insert() which calls > > > > > > > > __radix_tree_create() (the latter reusing the preloaded node). I'm not > > > > > > > > familiar enough to this code (radix-tree.c and filemap.c) to tell where > > > > > > > > the node should have been freed, who keeps track of it. > > > > > > > > > > > > > > > > At a quick look at the hex dump (assuming that the above leak is struct > > > > > > > > radix_tree_node): > > > > > > > > > > > > > > > > .path = 1 > > > > > > > > .count = -0x7f (or 0xffffff81 as unsigned int) > > > > > > > > union { > > > > > > > > { > > > > > > > > .parent = NULL > > > > > > > > .private_data = 0xffffffff81348950 > > > > > > > > } > > > > > > > > { > > > > > > > > .rcu_head.next = NULL > > > > > > > > .rcu_head.func = 0xffffffff81348950 > > > > > > > > } > > > > > > > > } > > > > > > > > > > > > > > > > The count is a bit suspicious. > > > > > > > > > > > > > > > > From the union, it looks most likely like rcu_head information. Is > > > > > > > > radix_tree_node_rcu_free() function at the above rcu_head.func? > > > > My summary so far: > > > > > > > > - radix_tree_node reported by kmemleak as it cannot find any trace of it > > > > when scanning the memory > > > > - at allocation time, radix_tree_node is memzero'ed by > > > > radix_tree_node_ctor(). Given that node->rcu_head.func == > > > > radix_tree_node_rcu_free, my guess is that radix_tree_node_free() has > > > > been called The constructor is called once when the slab is initially allocated, not on every object allocation. The user is expected to return objects in a pristine form or overwrite fields on reallocation, so it's possible that the RCU values are left over from the previous allocation. > > > > - some time later, kmemleak still hasn't received any callback for > > > > kmem_cache_free(node). Possibly radix_tree_node_rcu_free() hasn't been > > > > called either since node->count is not NULL. > > > > > > > > For RCU queued objects, kmemleak should still track references to them > > > > via rcu_sched_state and rcu_head members. But even if this went wrong, I > > > > would expect the object to be freed eventually and kmemleak notified (so > > > > just a temporary leak report which doesn't seem to be the case here). > > > > > > OK, so you are saying that this memory has been in this state for quite > > > some time? > > > > These leaks don't seem to disappear (time lapsed to be confirmed) and > > the object checksum not changed either (otherwise kmemleak would not > > report it). > > > > > If the system is responsive during this time, I recommend building with > > > CONFIG_RCU_TRACE=y, then polling the debugfs rcu/*/rcugp files. The value > > > of "*" will be "rcu_sched" for kernels built with CONFIG_PREEMPT=n and > > > "rcu_preempt" for kernels built with CONFIG_PREEMPT=y. > > > > > > If the number printed does not advance, then the RCU grace period is > > > stalled, which will prevent memory waiting for that grace period from > > > ever being freed. > > > > Thanks for the suggestions > > > > > Of course, if the value of node->count is preventing call_rcu() from > > > being invoked in the first place, then the needed grace period won't > > > start, much less finish. ;-) > > > > Given the rcu_head.func value, my assumption is that call_rcu() has > > already been called. > > Fair point -- given that it is a union, you would expect this field to > be overwritten upon reuse. .parent is overwritten immediately on reuse, but .private_data is actually unlikely to be used during the lifetime of the node. This could explain why .rcu.head.next is NULL like parent, and .private_data/.rcu.head.func is untouched and retains RCU stuff: to me it doesn't look like the node is lost in RCU-freeing, rather it was previously RCU freed and then lost somewhere after reallocation. > > BTW, is it safe to have a union overlapping node->parent and > > node->rcu_head.next? I'm still staring at the radix-tree code but a > > scenario I have in mind is that call_rcu() has been raised for a few > > nodes, other CPU may have some reference to one of them and set > > node->parent to NULL (e.g. concurrent calls to radix_tree_shrink()), > > breaking the RCU linking. I can't confirm this theory yet ;) Only writers shrink the tree and free nodes, and they have to be properly serialized. > If this were reproducible, I would suggest retrying with non-overlapping > node->parent and node->rcu_head.next, but you knew that already. ;-) > > But the usual practice would be to make node removal exclude shrinking. > And the radix-tree code seems to delegate locking to the caller. > > So, is the correct locking present in the page cache? The radix-tree > code seems to assume that all update operations for a given tree are > protected by a lock global to that tree. Yep, mapping->tree_lock protects all mapping->page_tree modifications. -- 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>