Date:   Wed, 18 Dec 2019 13:06:30 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't free usage map when removing
 a re-added early section

On 18.12.19 09:53, David Hildenbrand wrote:
> On 18.12.19 04:21, Dan Williams wrote:
>> On Tue, Dec 17, 2019 at 2:47 AM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> When we remove an early section, we don't free the usage map, as the
>>> usage maps of other sections are placed into the same page. Once the
>>> section is removed, it is no longer an early section (especially, the
>>> memmap is freed). When we re-add that section, the usage map is reused,
>>> however, it is no longer an early section. When removing that section
>>> again, we try to kfree() a usage map that was allocated during early
>>> boot - bad.
>>>
>>> Let's check against PageReserved() to see if we are dealing with an usage
>>> map that was allocated during boot. We could also check against
>>> !(PageSlab(usage_page) || PageCompound(usage_page)), but PageReserved()
>>> is cleaner.
>>>
>>> Can be triggered using memtrace under ppc64/powernv:
>>>
>>> $ mount -t debugfs none /sys/kernel/debug/
>>> $ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
>>> $ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
>>> [   12.093442] ------------[ cut here ]------------
>>> [   12.093469] kernel BUG at mm/slub.c:3969!
>>> [   12.093656] Oops: Exception in kernel mode, sig: 5 [#1]
>>> [   12.093961] LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
>>> [   12.094320] Modules linked in:
>>> [   12.094615] CPU: 0 PID: 154 Comm: sh Not tainted 5.5.0-rc2-next-20191216-00005-g0be1dba7b7c0 #61
>>> [   12.095066] NIP:  c000000000396b38 LR: c000000000385848 CTR: c000000000143d30
>>> [   12.095427] REGS: c000000073077680 TRAP: 0700   Not tainted  (5.5.0-rc2-next-20191216-00005-g0be1dba7b7c0)
>>> [   12.095886] MSR:  900000000282b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 28004828  XER: 20000000
>>> [   12.096395] CFAR: c000000000396b9c IRQMASK: 0
>>> [   12.096395] GPR00: c000000000385848 c000000073077910 c00000000110f300 c00000007ffffc00
>>> [   12.096395] GPR04: 0000000000000000 ffffffffffffffff 0000000000000000 0000000000000000
>>> [   12.096395] GPR08: 0000000000000000 0000000000000001 0000000000000000 ffffffffffffffc8
>>> [   12.096395] GPR12: 0000000000004000 c0000000012d0000 0000000000001000 c000000000d33c78
>>> [   12.096395] GPR16: 0000000000000000 c0000000011bfeb0 ffffffffffffe000 c0000000000b6370
>>> [   12.096395] GPR20: ffffffffe0000000 c0000000011411c0 0000000000006000 c0000000000b6390
>>> [   12.096395] GPR24: 0000000010000000 0000000000000040 0000000000000000 0000000000000000
>>> [   12.096395] GPR28: c000000000385848 c00c0000001fffc0 0000000000004000 0000000000000000
>>> [   12.099882] NIP [c000000000396b38] kfree+0x338/0x3b0
>>> [   12.100135] LR [c000000000385848] section_deactivate+0x138/0x200
>>> [   12.100508] Call Trace:
>>> [   12.100927] [c000000073077910] [c0000000010599a8] 0xc0000000010599a8 (unreliable)
>>> [   12.101338] [c000000073077960] [c000000000385848] section_deactivate+0x138/0x200
>>> [   12.101696] [c000000073077a10] [c00000000039b9f4] __remove_pages+0x114/0x150
>>> [   12.102025] [c000000073077a60] [c00000000006793c] arch_remove_memory+0x3c/0x160
>>> [   12.102381] [c000000073077ae0] [c00000000039c154] try_remove_memory+0x114/0x1a0
>>> [   12.102715] [c000000073077b90] [c00000000039c020] __remove_memory+0x20/0x40
>>> [   12.103041] [c000000073077bb0] [c0000000000b6714] memtrace_enable_set+0x254/0x850
>>> [   12.103402] [c000000073077cb0] [c0000000004197e8] simple_attr_write+0x138/0x160
>>> [   12.103751] [c000000073077d10] [c000000000558c9c] full_proxy_write+0x8c/0x110
>>> [   12.104100] [c000000073077d60] [c0000000003d02a8] __vfs_write+0x38/0x70
>>> [   12.104409] [c000000073077d80] [c0000000003d3c5c] vfs_write+0x11c/0x2a0
>>> [   12.104711] [c000000073077dd0] [c0000000003d4054] ksys_write+0x84/0x140
>>> [   12.105011] [c000000073077e20] [c00000000000b594] system_call+0x5c/0x68
>>> [   12.105357] Instruction dump:
>>> [   12.105606] e93d0000 75290001 41820090 8bfd0051 38a0ffff 7ca5f830 7bff0020 7ca507b4
>>> [   12.105993] e95d0000 39200000 754a0001 4182005c <0b090000> 893d0007 3d42000b 38800006
>>> [   12.106583] ---[ end trace 4b053cbd84e0db62 ]---
>>>
>>> The first invocation will offline+remove memory blocks. The second
>>> invocation will first add+online them again, in order to offline+remove
>>> them again (usually we are lucky and the exact same memory blocks will
>>> get "reallocated").
>>>
>>> Tested on powernv with boot memory: The usage map will not get freed.
>>> Tested on x86-64 with DIMMs: The usage map will get freed.
>>>
>>> Fixes: 326e1b8f83a4 ("mm/sparsemem: introduce a SECTION_IS_EARLY flag")
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Dan Williams <dan.j.williams@...el.com>
>>> Cc: Oscar Salvador <osalvador@...e.de>
>>> Cc: Michal Hocko <mhocko@...nel.org>
>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>> ---
>>>  mm/sparse.c | 9 ++++++++-
>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index b20ab7cdac86..3822ecbd8a1f 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -777,7 +777,14 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>>>         if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION)) {
>>>                 unsigned long section_nr = pfn_to_section_nr(pfn);
>>>
>>> -               if (!section_is_early) {
>>
>> With this change this function no longer needs a local
>> section_is_early variable...
> 
> Right. I'll send a cleanup patch and we can decide if we want to squash
> them in Andrews tree.

Ah, just realized that we cannot do that.

ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr)

already clears the early flag, so inlining that condition would break it.

-- 
Thanks,

David / dhildenb

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