Message-ID: <alpine.LSU.2.11.1404141952230.2980@eggly.anvils>
Date:	Mon, 14 Apr 2014 20:29:09 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dave Jones <davej@...hat.com>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [3.14+] kernel BUG at mm/filemap.c:1347!

On Mon, 14 Apr 2014, Dave Jones wrote:

> git tree from yesterday afternoon sometime, before Linus cut .15-rc1
> 
> kernel BUG at mm/filemap.c:1347!
> invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> Modules linked in: 8021q garp bridge stp dlci snd_seq_dummy tun fuse rfcomm ipt_ULOG nfnetlink llc2 af_key scsi_transport_iscsi hidp can_raw bnep can_bcm nfc caif_socket caif af_802154 ieee802154 phonet af_rxrpc can pppoe pppox ppp_generic slhc irda crc_ccitt rds rose x25 atm netrom appletalk ipx p8023 psnap p8022 llc ax25 cfg80211 coretemp hwmon x86_pkg_temp_thermal kvm_intel kvm snd_hda_codec_realtek snd_hda_codec_hdmi snd_hda_codec_generic snd_hda_intel snd_hda_controller btusb snd_hda_codec snd_hwdep bluetooth snd_seq snd_seq_device snd_pcm xfs e1000e snd_timer crct10dif_pclmul snd crc32c_intel libcrc32c ghash_clmulni_intel ptp 6lowpan_iphc rfkill usb_debug shpchp soundcore pps_core microcode pcspkr serio_raw
> CPU: 1 PID: 5440 Comm: trinity-c16 Not tainted 3.14.0+ #187
> task: ffff8801efe79ae0 ti: ffff8802082e4000 task.ti: ffff8802082e4000
> RIP: 0010:[<ffffffffb815aeab>]  [<ffffffffb815aeab>] find_get_pages_tag+0x1cb/0x220
> RSP: 0000:ffff8802082e5c70  EFLAGS: 00010246
> RAX: 7fffffffffffffff RBX: 000000000000000e RCX: 000000000000001d
> RDX: 000000000000001d RSI: ffff880041c7d4f0 RDI: 0000000000000000
> RBP: ffff8802082e5cd0 R08: 0000000000002600 R09: ffffea00075104dc
> R10: 0000000000000100 R11: 0000000000000228 R12: ffff8802082e5d08
> R13: 000000000000000a R14: 0000000000000101 R15: ffff8802082e5d20
> FS:  00007f97c44f2740(0000) GS:ffff880244200000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000000166d000 CR3: 000000016515d000 CR4: 00000000001407e0
> DR0: 00000000015e9000 DR1: 0000000000842000 DR2: 0000000001da3000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
> Stack:
>  ffffffffb815ad16 0000000000000000 ffff880067079da8 0000000000002681
>  00000000000026c0 7fffffffffffffff 000000004110b5bc ffff8802082e5d10
>  ffffea0007477a80 0000000000000000 ffff8802082e5d90 0000000000002fd6
> Call Trace:
>  [<ffffffffb815ad16>] ? find_get_pages_tag+0x36/0x220
>  [<ffffffffb8168511>] pagevec_lookup_tag+0x21/0x30
>  [<ffffffffb81595de>] filemap_fdatawait_range+0xbe/0x1e0
>  [<ffffffffb8159727>] filemap_fdatawait+0x27/0x30
>  [<ffffffffb81f2fa4>] sync_inodes_sb+0x204/0x2a0
>  [<ffffffffb874d98f>] ? wait_for_completion+0xff/0x130
>  [<ffffffffb81fa5b0>] ? vfs_fsync+0x40/0x40
>  [<ffffffffb81fa5c9>] sync_inodes_one_sb+0x19/0x20
>  [<ffffffffb81caab2>] iterate_supers+0xb2/0x110
>  [<ffffffffb81fa864>] sys_sync+0x44/0xb0
>  [<ffffffffb875c4a9>] ia32_do_call+0x13/0x13
> Code: 89 c1 85 c9 0f 84 ee fe ff ff 8d 51 01 89 c8 f0 41 0f b1 11 39 c1 0f 84 20 ff ff ff eb e2 66 90 0f 0b 83 e7 01 0f 85 af fe ff ff <0f> 0b 0f 1f 00 e8 ab 23 f1 ff 48 89 75 a8 e8 82 dd 00 00 48 8b 
> RIP  [<ffffffffb815aeab>] find_get_pages_tag+0x1cb/0x220
>  RSP <ffff8802082e5c70>
> ---[ end trace ea01792c1c61cb22 ]---
> 
> 
> 
> 1343                         /*
> 1344                          * This function is never used on a shmem/tmpfs
> 1345                          * mapping, so a swap entry won't be found here.
> 1346                          */
> 1347                         BUG();

Thanks for finding that, Dave.

Yes, it was me who put in that "shmem/tmpfs" comment and BUG();
but it's Hannes (Cc'ed) whom I'll blame for not removing the comment,
in extending the use of radix_tree exceptional entries way beyond
shmem/tmpfs in v3.15-rc1.  (Of course I should have noticed.)

As to the BUG(): at first I was aghast that it should have escaped
all our mmotm/next testing of the last couple of months; but now
realize that it is truly surprising for a PAGECACHE_TAG_WRITEBACK
(and probably any other PAGECACHE_TAG_*) to appear on an exceptional
entry.

I expect it comes down to an occasional race in RCU lookup of the
radix_tree: lacking absolute synchronization, we might sometimes
catch an exceptional entry, with the tag which really belongs
with the unexceptional entry which was there an instant before.

(That's actually one of the reasons why I introduced exceptional
entries, rather than tagging entries as exceptional: it's easier to
synchonize a word with a bit in, than a word with a bit elsewhere.)

Or I may be misreading it: whatever, Hannes will have a much surer
grasp of what to do about it.  It may be as simple as skipping over
any exceptional entry in find_get_pages_tag() - that would be easy
to provide as a quick fix if this BUG() starts to get in people's
way.  But I'd much prefer Hannes to consider the races, whether
there's more to worry about, and provide a more thoughtful fix.

(There are a few other "shmem/tmpfs" comments in mm/ that I put on
exceptional entries in v3.1: again, I'd prefer Hannes to check
through those, as he'll know best whether just to delete the
comments now, or rewrite them, or update the code a little.)

Hugh
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