On Tue, Mar 1, 2016 at 12:53 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > 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? That's a possibility although I also wanted to catch drivers that think they can use page->lru as long as they have a reference against the page. However, moving the safety mechanism to the individual call sites guarantees that we'll miss some. It trades one form of wack-a-mole for another, so I think just killing list_force_poison() is our best option. > There are probably quite a few possible places. (Why does the > comment say "slab"?). Yeah, it should say zone lru, I was referring to placing a ZONE_DEVICE page on a free list that would allow it to be allocated via alloc_page(). _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
