On Wed, Dec 23, 2015 at 02:22:28PM +0900, Minchan Kim wrote: > During my compaction-related stuff, I encountered some problems with > ballooning. > > Firstly, with repeated inflating and deflating cycle, guest memory(ie, > cat /proc/meminfo | grep MemTotal) decreased and couldn't recover. > > When I review source code, balloon_lock should cover release_pages_balloon. > Otherwise, struct virtio_balloon fields could be overwritten by race > of fill_balloon(e,g, vb->*pfns could be critical). > Below patch fixed the problem. > > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c > index 7efc32945810..7d3e5d0e9aa4 100644 > --- a/drivers/virtio/virtio_balloon.c > +++ b/drivers/virtio/virtio_balloon.c > @@ -209,8 +209,8 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num) > */ > if (vb->num_pfns != 0) > tell_host(vb, vb->deflate_vq); > - mutex_unlock(&vb->balloon_lock); > release_pages_balloon(vb); > + mutex_unlock(&vb->balloon_lock); > return num_freed_pages; > } > > Secondly, in balloon_page_dequeue, pages_lock should cover > list_for_each_entry_safe loop. Otherwise, the cursor page > could be isolated by compaction and then list_del by isolation > could poison the page->lru so the loop could access wrong address > like this. > > general protection fault: 0000 [#1] SMP > Dumping ftrace buffer: > (ftrace buffer empty) > Modules linked in: > CPU: 2 PID: 82 Comm: vballoon Not tainted 4.4.0-rc5-mm1+ #1906 > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > task: ffff8800a7ff0000 ti: ffff8800a7fec000 task.ti: ffff8800a7fec000 > RIP: 0010:[<ffffffff8115e754>] [<ffffffff8115e754>] balloon_page_dequeue+0x54/0x130 > RSP: 0018:ffff8800a7fefdc0 EFLAGS: 00010246 > RAX: ffff88013fff9a70 RBX: ffffea000056fe00 RCX: 0000000000002b7d > RDX: ffff88013fff9a70 RSI: ffffea000056fe00 RDI: ffff88013fff9a68 > RBP: ffff8800a7fefde8 R08: ffffea000056fda0 R09: 0000000000000000 > R10: ffff8800a7fefd90 R11: 0000000000000001 R12: dead0000000000e0 > R13: ffffea000056fe20 R14: ffff880138809070 R15: ffff880138809060 > FS: 0000000000000000(0000) GS:ffff88013fc40000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b > CR2: 00007f229c10e000 CR3: 00000000b8b53000 CR4: 00000000000006a0 > Stack: > 0000000000000100 ffff880138809088 ffff880138809000 ffff880138809060 > 0000000000000046 ffff8800a7fefe28 ffffffff812c86d3 ffff880138809020 > ffff880138809000 fffffffffff91900 0000000000000100 ffff880138809060 > Call Trace: > [<ffffffff812c86d3>] leak_balloon+0x93/0x1a0 > [<ffffffff812c8bc7>] balloon+0x217/0x2a0 > [<ffffffff8143739e>] ? __schedule+0x31e/0x8b0 > [<ffffffff81078160>] ? abort_exclusive_wait+0xb0/0xb0 > [<ffffffff812c89b0>] ? update_balloon_stats+0xf0/0xf0 > [<ffffffff8105b6e9>] kthread+0xc9/0xe0 > [<ffffffff8105b620>] ? kthread_park+0x60/0x60 > [<ffffffff8143b4af>] ret_from_fork+0x3f/0x70 > [<ffffffff8105b620>] ? kthread_park+0x60/0x60 > Code: 8d 60 e0 0f 84 af 00 00 00 48 8b 43 20 a8 01 75 3b 48 89 d8 f0 0f ba 28 00 72 10 48 8b 03 f6 c4 08 75 2f 48 89 df e8 8c 83 f9 ff <49> 8b 44 24 20 4d 8d 6c 24 20 48 83 e8 20 4d 39 f5 74 7a 4c 89 > RIP [<ffffffff8115e754>] balloon_page_dequeue+0x54/0x130 > RSP <ffff8800a7fefdc0> > ---[ end trace 43cf28060d708d5f ]--- > Kernel panic - not syncing: Fatal exception > Dumping ftrace buffer: > (ftrace buffer empty) > Kernel Offset: disabled > > We could fix it by protecting the entire loop by pages_lock but > problem is irq latency during walking the list. > But I doubt how often such worst scenario happens because > in normal situation, the loop would exit easily via succeeding > trylock_page. > > Any comments? Nope, I think the simplest way to address both cases you stumbled across is by replacing the locking to extend those critical sections as you suggested. Merry Xmas! -- Rafael -- 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>