On Tue, 01 Mar 2016 11:55:04 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > list_force_poison is a debug mechanism to make sure that ZONE_DEVICE > pages never appear on an lru. Those pages only exist for enabling DMA > to device discovered memory ranges and are not suitable for general > purpose allocations. list_force_poison() explicitly initializes a > list_head with a poison value that list_add() can use to detect mistaken > use of page->lru. > > Unfortunately, it seems calling list_add() leads to the poison value > leaking on to the stack and occasionally cause stack-allocated > list_heads to be inadvertently "force poisoned". > > list_add attempted on force-poisoned entry > WARNING: at lib/list_debug.c:34 > [..] > NIP [c00000000043c390] __list_add+0xb0/0x150 > LR [c00000000043c38c] __list_add+0xac/0x150 > Call Trace: > [c000000fb5fc3320] [c00000000043c38c] __list_add+0xac/0x150 (unreliable) > [c000000fb5fc33a0] [c00000000081b454] __down+0x4c/0xf8 > [c000000fb5fc3410] [c00000000010b6f8] down+0x68/0x70 > [c000000fb5fc3450] [d0000000201ebf4c] xfs_buf_lock+0x4c/0x150 [xfs] > > list_add attempted on force-poisoned entry(0000000000000500), > new->next == d0000000059ecdb0, new->prev == 0000000000000500 > WARNING: at lib/list_debug.c:33 > [..] > NIP [c00000000042db78] __list_add+0xa8/0x140 > LR [c00000000042db74] __list_add+0xa4/0x140 > Call Trace: > [c0000004c749f620] [c00000000042db74] __list_add+0xa4/0x140 (unreliable) > [c0000004c749f6b0] [c0000000008010ec] rwsem_down_read_failed+0x6c/0x1a0 > [c0000004c749f760] [c000000000800828] down_read+0x58/0x60 > [c0000004c749f7e0] [d000000005a1a6bc] xfs_log_commit_cil+0x7c/0x600 [xfs] > > We can squash these uninitialized list_heads as they pop-up as this > patch does, or maybe need to rethink how to implement the > list_force_poison() safety mechanism. Yes, problem. > kernel/locking/rwsem-xadd.c | 4 +++- > kernel/locking/semaphore.c | 4 +++- The patch adds slight overhead and there will be other uninitialized list_heads around the place and more will turn up in the future. I don't see how list_force_poison is fixable, really - we're relying upon some uninitialized word of memory not having some particular value. Good luck with that. Maybe we simply remove list_force_poison() - it isn't terribly important? /* ZONE_DEVICE pages must never appear on a slab lru */ Can we instead add a check of page_zone(page) into the lru-addition sites? There are probably quite a few possible places. (Why does the comment say "slab"?). -- 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>