Re: unused swap offset / bad page map.

[Hillf Danton <dhillf@gmail.com>]

Hello Dave

On Wed, Aug 7, 2013 at 1:51 PM, Dave Jones <davej@redhat.com> wrote:
> Seen while fuzzing with lots of child processes.
>
> swap_free: Unused swap offset entry 001263f5
> BUG: Bad page map in process trinity-child29  pte:24c7ea00 pmd:09fec067
> addr:00007f9db958d000 vm_flags:00100073 anon_vma:ffff88022c004ba0 mapping:          (null) index:f99
> Modules linked in: fuse ipt_ULOG snd_seq_dummy tun sctp scsi_transport_iscsi can_raw can_bcm rfcomm bnep nfnetlink hidp appletalk bluetooth rose can af_802154 phonet x25 af_rxrpc llc2 nfc rfkill af_key pppoe rds pppox ppp_generic slhc caif_socket caif irda crc_ccitt atm netrom ax25 ipx p8023 psnap p8022 llc snd_hda_codec_realtek pcspkr usb_debug snd_seq snd_seq_device snd_hda_intel snd_hda_codec snd_hwdep e1000e snd_pcm ptp pps_core snd_page_alloc snd_timer snd soundcore xfs libcrc32c
> CPU: 1 PID: 2624 Comm: trinity-child29 Not tainted 3.11.0-rc4+ #1
>  0000000000000000 ffff8801fd7ddc90 ffffffff81700f2c 00007f9db958d000
>  ffff8801fd7ddcd8 ffffffff8117cba7 0000000024c7ea00 0000000000000f99
>  00007f9db9600000 ffff880009fecc68 0000000024c7ea00 ffff8801fd7dde00
> Call Trace:
>  [<ffffffff81700f2c>] dump_stack+0x4e/0x82
>  [<ffffffff8117cba7>] print_bad_pte+0x187/0x220
>  [<ffffffff8117e415>] unmap_single_vma+0x535/0x890
>  [<ffffffff8117f719>] unmap_vmas+0x49/0x90
>  [<ffffffff81187ef1>] exit_mmap+0xc1/0x170
>  [<ffffffff810510ef>] mmput+0x6f/0x100
>  [<ffffffff81055818>] do_exit+0x288/0xcd0
>  [<ffffffff810c1da5>] ? trace_hardirqs_on_caller+0x115/0x1e0
>  [<ffffffff810c1e7d>] ? trace_hardirqs_on+0xd/0x10
>  [<ffffffff810575dc>] do_group_exit+0x4c/0xc0
>  [<ffffffff81057664>] SyS_exit_group+0x14/0x20
>  [<ffffffff81713dd4>] tracesys+0xdd/0xe2
>
> There were a slew of these. same trace, different addr/anon_vma/index.
> mapping always null.
>
Would you please run again with the debug info added?
---
--- a/mm/swapfile.c	Wed Aug  7 17:27:22 2013
+++ b/mm/swapfile.c	Wed Aug  7 17:57:20 2013
@@ -509,6 +509,7 @@ static struct swap_info_struct *swap_inf
 {
 	struct swap_info_struct *p;
 	unsigned long offset, type;
+	int race = 0;

 	if (!entry.val)
 		goto out;
@@ -524,10 +525,17 @@ static struct swap_info_struct *swap_inf
 	if (!p->swap_map[offset])
 		goto bad_free;
 	spin_lock(&p->lock);
+	if (!p->swap_map[offset]) {
+		race = 1;
+		spin_unlock(&p->lock);
+		goto bad_free;
+	}
 	return p;

 bad_free:
 	printk(KERN_ERR "swap_free: %s%08lx\n", Unused_offset, entry.val);
+	if (race)
+		printk(KERN_ERR "but due to race\n");
 	goto out;
 bad_offset:
 	printk(KERN_ERR "swap_free: %s%08lx\n", Bad_offset, entry.val);
--

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