Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

[Ni zhan Chen]

On 10/25/2012 12:36 PM, 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.
>
> 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?

Hi Hugh,

I think it maybe caused by your commit [d189922862e03ce: shmem: fix 
negative rss in memcg memory.stat], one question:

if function shmem_confirm_swap confirm the entry has already brought 
back from swap by a racing thread, then why call shmem_add_to_page_cache 
to add page from swapcache to pagecache again? otherwise, will goto 
unlock and then go to repeat? where I miss?

Regards,
Chen

> 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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