linux-kernel - Re: [PATCH] mm: initialize variable for mem_cgroup_end_page

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Date:	Thu, 30 Oct 2014 10:24:47 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Michal Hocko <mhocko@...e.cz>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	peterz@...radead.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: initialize variable for mem_cgroup_end_page_stat

On 10/30/2014 10:14 AM, Johannes Weiner wrote:
>> The problem is that you are attempting to read 'locked' when you call
>> > mem_cgroup_end_page_stat(), so it gets used even before you enter the
>> > function - and using uninitialized variables is undefined.
> We are not using that value anywhere if !memcg.  What path are you
> referring to?

You're using that value as soon as you are passing it to a function, it
doesn't matter what happens inside that function.

>> > Yes, it's a compiler warning.
> Could you provide that please, including arch, and gcc version?

On x86,

$ gcc --version
gcc (GCC) 5.0.0 20141029 (experimental)

[   26.868116] ================================================================================
[   26.870376] UBSan: Undefined behaviour in mm/rmap.c:1084:2
[   26.871792] load of value 255 is not a valid value for type '_Bool'
[   26.873256] CPU: 4 PID: 8304 Comm: rngd Not tainted 3.18.0-rc2-next-20141029-sasha-00039-g77ed13d-dirty #1427
[   26.875636]  ffff8800cac17ff0 0000000000000000 0000000000000000 ffff880069ffbb28
[   26.877611]  ffffffffaf010c16 0000000000000037 ffffffffb1c0d050 ffff880069ffbb38
[   26.879140]  ffffffffa6e97899 ffff880069ffbbb8 ffffffffa6e97cc7 ffff880069ffbbb8
[   26.880765] Call Trace:
[   26.881185] dump_stack (lib/dump_stack.c:52)
[   26.882755] ubsan_epilogue (lib/ubsan.c:159)
[   26.883555] __ubsan_handle_load_invalid_value (lib/ubsan.c:482)
[   26.884492] ? mem_cgroup_begin_page_stat (mm/memcontrol.c:1962)
[   26.885441] ? unmap_page_range (./arch/x86/include/asm/paravirt.h:694 mm/memory.c:1091 mm/memory.c:1258 mm/memory.c:1279 mm/memory.c:1303)
[   26.886242] page_remove_rmap (mm/rmap.c:1084 mm/rmap.c:1096)
[   26.886922] unmap_page_range (./arch/x86/include/asm/atomic.h:27 include/linux/mm.h:463 mm/memory.c:1146 mm/memory.c:1258 mm/memory.c:1279 mm/memory.c:1303)
[   26.887824] unmap_single_vma (mm/memory.c:1348)
[   26.888582] unmap_vmas (mm/memory.c:1377 (discriminator 3))
[   26.889430] exit_mmap (mm/mmap.c:2837)
[   26.890060] mmput (kernel/fork.c:659)
[   26.890656] do_exit (./arch/x86/include/asm/thread_info.h:168 kernel/exit.c:462 kernel/exit.c:747)
[   26.891359] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[   26.892287] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2559 kernel/locking/lockdep.c:2601)
[   26.893107] ? syscall_trace_enter_phase2 (arch/x86/kernel/ptrace.c:1598 (discriminator 2))
[   26.893974] do_group_exit (include/linux/sched.h:775 kernel/exit.c:873)
[   26.894695] SyS_exit_group (kernel/exit.c:901)
[   26.895433] tracesys_phase2 (arch/x86/kernel/entry_64.S:529)
[   26.896134] ================================================================================


Thanks,
Sasha
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