linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Date:	Wed, 24 Oct 2012 21:36:27 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dave Jones <davej@...hat.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

On Wed, 24 Oct 2012, Dave Jones wrote:

> Machine under significant load (4gb memory used, swap usage fluctuating)
> triggered this...
> 
> WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
> Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49
> Call Trace:
>  [<ffffffff8107100f>] warn_slowpath_common+0x7f/0xc0
>  [<ffffffff8107106a>] warn_slowpath_null+0x1a/0x20
>  [<ffffffff811903fc>] shmem_getpage_gfp+0xa5c/0xa70
>  [<ffffffff8118fc3e>] ? shmem_getpage_gfp+0x29e/0xa70
>  [<ffffffff81190e4f>] shmem_fault+0x4f/0xa0
>  [<ffffffff8119f391>] __do_fault+0x71/0x5c0
>  [<ffffffff810e1ac6>] ? __lock_acquire+0x306/0x1ba0
>  [<ffffffff810b6ff9>] ? local_clock+0x89/0xa0
>  [<ffffffff811a2767>] handle_pte_fault+0x97/0xae0
>  [<ffffffff816d1069>] ? sub_preempt_count+0x79/0xd0
>  [<ffffffff8136d68e>] ? delay_tsc+0xae/0x120
>  [<ffffffff8136d578>] ? __const_udelay+0x28/0x30
>  [<ffffffff811a4a39>] handle_mm_fault+0x289/0x350
>  [<ffffffff816d091e>] __do_page_fault+0x18e/0x530
>  [<ffffffff810b6ff9>] ? local_clock+0x89/0xa0
>  [<ffffffff810b0e51>] ? get_parent_ip+0x11/0x50
>  [<ffffffff810b0e51>] ? get_parent_ip+0x11/0x50
>  [<ffffffff816d1069>] ? sub_preempt_count+0x79/0xd0
>  [<ffffffff8112d389>] ? rcu_user_exit+0xc9/0xf0
>  [<ffffffff816d0ceb>] do_page_fault+0x2b/0x50
>  [<ffffffff816cd3b8>] page_fault+0x28/0x30
>  [<ffffffff8136d259>] ? copy_user_enhanced_fast_string+0x9/0x20
>  [<ffffffff8121c181>] ? sys_futimesat+0x41/0xe0
>  [<ffffffff8102bf35>] ? syscall_trace_enter+0x25/0x2c0
>  [<ffffffff816d5625>] ? tracesys+0x7e/0xe6
>  [<ffffffff816d5688>] tracesys+0xe1/0xe6
> 
> 
> 
> 1148                         error = shmem_add_to_page_cache(page, mapping, index,
> 1149                                                 gfp, swp_to_radix_entry(swap));
> 1150                         /* We already confirmed swap, and make no allocation */
> 1151                         VM_BUG_ON(error);
> 1152                 }

That's very surprising.  Easy enough to handle an error there, but
of course I made it a VM_BUG_ON because it violates my assumptions:
I rather need to understand how this can be, and I've no idea.

Clutching at straws, I expect this is entirely irrelevant, but:
there isn't a warning on line 1151 of mm/shmem.c in 3.7.0-rc2 nor
in current linux.git; rather, there's a VM_BUG_ON on line 1149.

So you've inserted a couple of lines for some reason (more useful
trinity behaviour, perhaps)?  And have some config option I'm
unfamiliar with, that mutates a BUG_ON or VM_BUG_ON into a warning?

Hugh

> 
> 
>              total       used       free     shared    buffers     cached
> Mem:       3885528    2854064    1031464          0       9624      19208
> -/+ buffers/cache:    2825232    1060296
> Swap:      6029308      30656    5998652
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