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Date: Mon, 9 Jun 2008 15:37:49 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       Rik van Riel <riel@redhat.com>, Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: 2.6.26-rc5-mm1: kernel BUG at mm/filemap.c:575!
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On Tue, 10 Jun 2008 00:45:59 +0400
Alexey Dobriyan <adobriyan@gmail.com> wrote:

> This happened after LTP run finished.
> 
> ------------[ cut here ]------------
> kernel BUG at mm/filemap.c:575!
> invalid opcode: 0000 [1] PREEMPT SMP DEBUG_PAGEALLOC
> last sysfs file: /sys/kernel/uevent_seqnum
> CPU 1 
> Modules linked in: ext2 nf_conntrack_irc xt_state iptable_filter ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack ip_tables x_tables usblp uhci_hcd ehci_hcd usbcore sr_mod cdrom
> Pid: 19327, comm: pdflush Not tainted 2.6.26-rc5-mm1 #4
> RIP: 0010:[<ffffffff80266a37>]  [<ffffffff80266a37>] unlock_page+0x17/0x40
> RSP: 0018:ffff81015c697540  EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffffe20000e38c08 RCX: 0000000000000034
> RDX: 0000000000000000 RSI: ffffe20000e38c08 RDI: ffffe20000e38c08
> RBP: ffff81015c697550 R08: 0000000000000002 R09: 000000000007794e
> R10: ffffffff8028b6f1 R11: 0000000000000001 R12: ffffe20000e38c08
> R13: 0000000000000000 R14: ffff81015c6977a0 R15: ffff81015c6978c0
> FS:  0000000000000000(0000) GS:ffff81017f845320(0000) knlGS:0000000000000000
> CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
> CR2: 00007fb64f84f020 CR3: 0000000000201000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process pdflush (pid: 19327, threadinfo ffff81015c696000, task ffff81015c795690)
> Stack:  ffff81015c697550 0000000000000000 ffff81015c697680 ffffffff802722ce
>  ffffe20005367df8 ffff81015c697640 0000000000000000 0000000000000001
>  0000000000000001 0000000000000001 ffffe20000b02d38 ffffe20001274970
> Call Trace:
>  [<ffffffff802722ce>] shrink_page_list+0x2ce/0x6d0
>  [<ffffffff80254f37>] ? mark_held_locks+0x47/0x90
>  [<ffffffff8025517d>] ? trace_hardirqs_on+0xd/0x10
>  [<ffffffff802550e9>] ? trace_hardirqs_on_caller+0xc9/0x150
>  [<ffffffff802728f4>] shrink_list+0x224/0x590
>  [<ffffffff80272eab>] shrink_zone+0x24b/0x330
>  [<ffffffff80273407>] try_to_free_pages+0x267/0x3e0
>
> ...
>

We unlocked an already-unlocked page.

Although pretty straightforward, shrink_page_list() is, umm, large.

This part:

		if (PagePrivate(page)) {
			if (!try_to_release_page(page, sc->gfp_mask))
				goto activate_locked;
			if (!mapping && page_count(page) == 1) {
				unlock_page(page);
				if (put_page_testzero(page))
					goto free_it;
				else {
					nr_reclaimed++;
					continue;
				}
			}
		}

		if (!mapping || !__remove_mapping(mapping, page))
			goto keep_locked;

free_it:
		unlock_page(page);

has a very obvious double-unlock.  It was added by the obviously-buggy,
reviewed-by-everyone mm-speculative-page-references.patch - part of
Nick's lockless pagecache work.

argh.  This means that I need to a) stop merging anything and b) be
sent a fix really fast or drop them all and fix up all the fallout and
c) get -mm2 out asap to that someone can test all the other
page-reclaim changes.  argh.

Also, what's up with that "continue" which got added there?  We just
leave the page floating about without reattaching it to any LRU? 
Where's the code comment explaining wth is going on in there?

More argh.  
--
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