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Date: Thu, 25 Oct 2012 16:52:13 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Hugh Dickins <hughd@google.com>
Cc: Dave Jones <davej@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
Message-ID: <20121025205213.GB4771@cmpxchg.org>
References: <20121025023738.GA27001@redhat.com>
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On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:
> 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.

Could it be concurrent truncation clearing out the entry between
shmem_confirm_swap() and shmem_add_to_page_cache()?  I don't see
anything preventing that.

The empty slot would not match the expected swap entry this call
passes in and the returned error would be -ENOENT.
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