Re: [PATCH] SLUB: clear c->freelist in __slab_alloc()/load_freelist:/SlabDebug path

[Pekka Enberg]

Hi Benny,

On Mon, May 12, 2008 at 11:32 PM, Benny Halevy <bhalevy@xxxxxxxxxxx> wrote:
> In the __slab_alloc()/load_freelist:/SlabDebug(c->page) path we only
>  use the object at the head of c->page->freelist
>  and the tail goes back to c->page->freelist.
>  We then set c->node = -1 to force __slab_alloc in next allocation.
>  c->freelist therefore needs to be cleared as it is invalid at this point.

But for debug pages, we never load c->page->freelist to c->freelist so
it should always be NULL.

>  Signed-off-by: Benny Halevy <bhalevy@xxxxxxxxxxx>
>  ---
>   mm/slub.c |    1 +
>   1 files changed, 1 insertions(+), 0 deletions(-)
>
>  Hit while running cthon04 test from an IBM AIX client
>  against my nfs41 tree.
>
>  Stack trace excerpt:
>
>  May 12 11:18:19 client kernel: general protection fault: 0000 [2] SMP
>  May 12 11:18:19 client kernel: CPU 3
>  May 12 11:18:19 client kernel: Modules linked in: panfs(P) nfsd auth_rpcgss exportfs autofs4 hidp nfs lockd nfs_acl fuse rfcomm l2cap bluetooth sunrpc nf_conntrack_netbios_ns nf_conntrack_ipv4 ipt_REJECT iptable_filter ip_tables nf_conntrack_ipv6 xt_state nf_conntrack xt_tcpudp ip6t_ipv6header ip6t_REJECT ip6table_filter ip6_tables x_tables ipv6 dm_multipath video output sbs sbshc battery ac e1000e i5000_edac iTCO_wdt iTCO_vendor_support i2c_i801 edac_core button sr_mod pcspkr i2c_core sg cdrom floppy dm_snapshot dm_zero dm_mirror dm_mod ata_piix libata shpchp pci_hotplug mptsas mptscsih mptbase scsi_transport_sas sd_mod scsi_mod ext3 jbd mbcache ehci_hcd ohci_hcd uhci_hcd [last unloaded: microcode]
>  May 12 11:18:19 client kernel: Pid: 2815, comm: nfsd Tainted: P      D  2.6.25-nfs41 #2
>  May 12 11:18:19 client kernel: RIP: 0010:[<ffffffff8108c0c8>]  [<ffffffff8108c0c8>] kmem_cache_alloc+0x3d/0x65
>  May 12 11:18:19 client kernel: RSP: 0018:ffff8104212c3de0  EFLAGS: 00010006
>  May 12 11:18:19 client kernel: RAX: 0000000000000000 RBX: 0000000000000246 RCX: ffffffff883546df
>  May 12 11:18:19 client kernel: RDX: 3200100010100000 RSI: 00000000000080d0 RDI: ffffffff813eadb8
>  May 12 11:18:19 client kernel: RBP: ffff810001029e60 R08: 0000000000000000 R09: ffff8103f118d130
>  May 12 11:18:19 client kernel: R10: ffff81041b076018 R11: ffffffff8826c313 R12: 00000000000080d0
>  May 12 11:18:19 client kernel: R13: ffff8104211aa000 R14: ffff81041b076000 R15: ffff8104239c8000
>  May 12 11:18:19 client kernel: FS:  00007f08fb8626f0(0000) GS:ffff81042fc02e80(0000) knlGS:0000000000000000
>  May 12 11:18:19 client kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>  May 12 11:18:19 client kernel: CR2: 00007fdaf41cf030 CR3: 0000000420827000 CR4: 00000000000006e0
>  May 12 11:18:19 client kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  May 12 11:18:19 client kernel: DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  May 12 11:18:19 client kernel: Process nfsd (pid: 2815, threadinfo ffff8104212c2000, task ffff81042381c940)
>  May 12 11:18:19 client kernel: Stack:  ffff8104211ab000 ffff8103f118d000 0000000022270000 ffffffff883546df
>  May 12 11:18:19 client kernel:  ffffffff88373cb8 0000000000000000 ffff8103f118d130 ffff8104239c8000
>  May 12 11:18:19 client kernel:  ffffffff88373cb8 000000000000001c ffff81041b076018 ffff81041b076000
>  May 12 11:18:19 client kernel: Call Trace:
>  May 12 11:18:19 client kernel:  [<ffffffff883546df>] ? :nfsd:nfsd4_proc_compound+0xa9/0x3f6
>  May 12 11:18:19 client kernel:  [<ffffffff88346245>] ? :nfsd:nfsd_dispatch+0xde/0x1b6
>  May 12 11:18:19 client kernel:  [<ffffffff88268bb7>] ? :sunrpc:svc_process_common+0x2e8/0x5a9
>  May 12 11:18:19 client kernel:  [<ffffffff8834667c>] ? :nfsd:nfsd+0x0/0x2b4
>  May 12 11:18:19 client kernel:  [<ffffffff88269d16>] ? :sunrpc:svc_process+0x127/0x13d
>  May 12 11:18:19 client kernel:  [<ffffffff88346819>] ? :nfsd:nfsd+0x19d/0x2b4May 12 11:18:19 client kernel:  [<ffffffff8100cac8>] ? child_rip+0xa/0x12
>  May 12 11:18:19 client kernel:  [<ffffffff8834667c>] ? :nfsd:nfsd+0x0/0x2b4
>  May 12 11:18:19 client last message repeated 2 times
>  May 12 11:18:19 client kernel:  [<ffffffff8100cabe>] ? child_rip+0x0/0x12
>  May 12 11:18:19 client kernel:
>  May 12 11:18:19 client kernel:
>  May 12 11:18:19 client kernel: Code: 25 24 00 00 00 48 98 48 8b ac c7 d8 02 00 00 48 8b 55 00 48 85 d2 75 10 83 ca ff 49 89 e8 e8 7e f8 ff ff 48 89 c2 eb 0b 8b 45 14 <48> 8b 04 c2 48 89 45 00 53 9d 66 45 85 e4 79 10 48 85 d2 74 0b
>  May 12 11:18:19 client kernel: RIP  [<ffffffff8108c0c8>] kmem_cache_alloc+0x3d/0x65
>  May 12 11:18:19 client kernel:  RSP <ffff8104212c3de0>
>  May 12 11:18:19 client kernel: ---[ end trace 9b6f5806f68a2b8c ]---
>
>  $ grep SL.B .config
>  CONFIG_SLUB_DEBUG=y
>  # CONFIG_SLAB is not set
>  CONFIG_SLUB=y
>  # CONFIG_SLOB is not set
>  CONFIG_SLABINFO=y
>  # CONFIG_SLUB_DEBUG_ON is not set
>  # CONFIG_SLUB_STATS is not set
>
>  diff --git a/mm/slub.c b/mm/slub.c
>  index a505a82..0d1d820 100644
>  --- a/mm/slub.c
>  +++ b/mm/slub.c
>  @@ -1606,6 +1606,7 @@ debug:
>         if (!alloc_debug_processing(s, c->page, object, addr))
>                 goto another_slab;
>
>  +       c->freelist = NULL;
>         c->page->inuse++;
>         c->page->freelist = object[c->offset];
>         c->node = -1;

Looking at this, we're oopsing at:

   0:   48 8b 04 c2             mov    (%rdx,%rax,8),%rax

where rdx is c->freelist and rax c->offset. The the value for
c->freelist ("3200100010100000") doesn't make much sense. Furthermore,
we never if this really were a bug in __slab_alloc() shouldn't we be
hitting it more often?

How did you make SLUB hit the debug path since you have
CONFIG_SLUB_DEBUG_ON disabled?

                                    Pekka
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