Re: [PATCH] mm/hugetlb: prevent hugetlb VMA to be misaligned

[Laurent Dufour]

On 20/03/2018 22:26, Mike Kravetz wrote:
> On 03/20/2018 10:25 AM, Laurent Dufour wrote:
>> When running the sampler detailed below, the kernel, if built with the VM
>> debug option turned on (as many distro do), is panicing with the following
>> message :
>> kernel BUG at /build/linux-jWa1Fv/linux-4.15.0/mm/hugetlb.c:3310!
>> Oops: Exception in kernel mode, sig: 5 [#1]
>> LE SMP NR_CPUS=2048 NUMA PowerNV
>> Modules linked in: kcm nfc af_alg caif_socket caif phonet fcrypt
>> 		8<--8<--8<--8< snip 8<--8<--8<--8<
>> CPU: 18 PID: 43243 Comm: trinity-subchil Tainted: G         C  E
>> 4.15.0-10-generic #11-Ubuntu
>> NIP:  c00000000036e764 LR: c00000000036ee48 CTR: 0000000000000009
>> REGS: c000003fbcdcf810 TRAP: 0700   Tainted: G         C  E
>> (4.15.0-10-generic)
>> MSR:  9000000000029033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 24002222  XER:
>> 20040000
>> CFAR: c00000000036ee44 SOFTE: 1
>> GPR00: c00000000036ee48 c000003fbcdcfa90 c0000000016ea600 c000003fbcdcfc40
>> GPR04: c000003fd9858950 00007115e4e00000 00007115e4e10000 0000000000000000
>> GPR08: 0000000000000010 0000000000010000 0000000000000000 0000000000000000
>> GPR12: 0000000000002000 c000000007a2c600 00000fe3985954d0 00007115e4e00000
>> GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> GPR20: 00000fe398595a94 000000000000a6fc c000003fd9858950 0000000000018554
>> GPR24: c000003fdcd84500 c0000000019acd00 00007115e4e10000 c000003fbcdcfc40
>> GPR28: 0000000000200000 00007115e4e00000 c000003fbc9ac600 c000003fd9858950
>> NIP [c00000000036e764] __unmap_hugepage_range+0xa4/0x760
>> LR [c00000000036ee48] __unmap_hugepage_range_final+0x28/0x50
>> Call Trace:
>> [c000003fbcdcfa90] [00007115e4e00000] 0x7115e4e00000 (unreliable)
>> [c000003fbcdcfb50] [c00000000036ee48]
>> __unmap_hugepage_range_final+0x28/0x50
>> [c000003fbcdcfb80] [c00000000033497c] unmap_single_vma+0x11c/0x190
>> [c000003fbcdcfbd0] [c000000000334e14] unmap_vmas+0x94/0x140
>> [c000003fbcdcfc20] [c00000000034265c] exit_mmap+0x9c/0x1d0
>> [c000003fbcdcfce0] [c000000000105448] mmput+0xa8/0x1d0
>> [c000003fbcdcfd10] [c00000000010fad0] do_exit+0x360/0xc80
>> [c000003fbcdcfdd0] [c0000000001104c0] do_group_exit+0x60/0x100
>> [c000003fbcdcfe10] [c000000000110584] SyS_exit_group+0x24/0x30
>> [c000003fbcdcfe30] [c00000000000b184] system_call+0x58/0x6c
>> Instruction dump:
>> 552907fe e94a0028 e94a0408 eb2a0018 81590008 7f9c5036 0b090000 e9390010
>> 7d2948f8 7d2a2838 0b0a0000 7d293038 <0b090000> e9230086 2fa90000 419e0468
>> ---[ end trace ee88f958a1c62605 ]---
>>
>> The panic is due to a VMA pointing to a hugetlb area while the
>> vma->vm_start or vma->vm_end field are not aligned to the huge page
>> boundaries. The sampler is just unmapping a part of the hugetlb area,
>> leading to 2 VMAs which are not well aligned.  The same could be achieved
>> by calling madvise() situation, as it is when running:
>> stress-ng --shm-sysv 1
>>
>> The hugetlb code is assuming that the VMA will be well aligned when it is
>> unmapped, so we must prevent such a VMA to be split or shrink to a
>> misaligned address.
>>
>> This patch is preventing this by checking the new VMA's boundaries when a
>> VMA is modified by calling vma_adjust().
>>
>> If this patch is applied, stable should be Cced.
> 
> Thanks Laurent!
> 
> This bug was introduced by 31383c6865a5.  Dan's changes for 31383c6865a5
> seem pretty straight forward.  It simply replaces an explicit check when
> splitting a vma to a new vm_ops split callout.  Unfortunately, mappings
> created via shmget/shmat have their vm_ops replaced.  Therefore, this
> split callout is never made.
> 
> The shm vm_ops do indirectly call the original vm_ops routines as needed.
> Therefore, I would suggest a patch something like the following instead.
> If we move forward with the patch, we should include Laurent's BUG output
> and perhaps test program in the commit message.

Hi Mike,

That's definitively smarter ! I missed that split() new vm ops...

Cheers,
Laurent.