Subject: Re: [PATCH 1/2] libnvdimm/altmap: Track namespace boundaries in altmap

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 17 Sep 2019 13:09:50 +0530

On 9/16/19 11:28 PM, Dan Williams wrote:
> On Mon, Sep 9, 2019 at 11:29 PM Aneesh Kumar K.V
> <[email-masked]> wrote:
>>
>> With PFN_MODE_PMEM namespace, the memmap area is allocated from the device
>> area. Some architectures map the memmap area with large page size. On
>> architectures like ppc64, 16MB page for memap mapping can map 262144 pfns.
>> This maps a namespace size of 16G.
>>
>> When populating memmap region with 16MB page from the device area,
>> make sure the allocated space is not used to map resources outside this
>> namespace. Such usage of device area will prevent a namespace destroy.
>>
>> Add resource end pnf in altmap and use that to check if the memmap area
>> allocation can map pfn outside the namespace. On ppc64 in such case we fallback
>> to allocation from memory.
>>
>> This fix kernel crash reported below:
>>
>> [  132.034989] WARNING: CPU: 13 PID: 13719 at mm/memremap.c:133 devm_memremap_pages_release+0x2d8/0x2e0
>> [  133.464754] BUG: Unable to handle kernel data access at 0xc00c00010b204000
>> [  133.464760] Faulting instruction address: 0xc00000000007580c
>> [  133.464766] Oops: Kernel access of bad area, sig: 11 [#1]
>> [  133.464771] LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>> .....
>> [  133.464901] NIP [c00000000007580c] vmemmap_free+0x2ac/0x3d0
>> [  133.464906] LR [c0000000000757f8] vmemmap_free+0x298/0x3d0
>> [  133.464910] Call Trace:
>> [  133.464914] [c000007cbfd0f7b0] [c0000000000757f8] vmemmap_free+0x298/0x3d0 (unreliable)
>> [  133.464921] [c000007cbfd0f8d0] [c000000000370a44] section_deactivate+0x1a4/0x240
>> [  133.464928] [c000007cbfd0f980] [c000000000386270] __remove_pages+0x3a0/0x590
>> [  133.464935] [c000007cbfd0fa50] [c000000000074158] arch_remove_memory+0x88/0x160
>> [  133.464942] [c000007cbfd0fae0] [c0000000003be8c0] devm_memremap_pages_release+0x150/0x2e0
>> [  133.464949] [c000007cbfd0fb70] [c000000000738ea0] devm_action_release+0x30/0x50
>> [  133.464955] [c000007cbfd0fb90] [c00000000073a5a4] release_nodes+0x344/0x400
>> [  133.464961] [c000007cbfd0fc40] [c00000000073378c] device_release_driver_internal+0x15c/0x250
>> [  133.464968] [c000007cbfd0fc80] [c00000000072fd14] unbind_store+0x104/0x110
>> [  133.464973] [c000007cbfd0fcd0] [c00000000072ee24] drv_attr_store+0x44/0x70
>> [  133.464981] [c000007cbfd0fcf0] [c0000000004a32bc] sysfs_kf_write+0x6c/0xa0
>> [  133.464987] [c000007cbfd0fd10] [c0000000004a1dfc] kernfs_fop_write+0x17c/0x250
>> [  133.464993] [c000007cbfd0fd60] [c0000000003c348c] __vfs_write+0x3c/0x70
>> [  133.464999] [c000007cbfd0fd80] [c0000000003c75d0] vfs_write+0xd0/0x250
> 
> Question, does this crash only happen when the namespace is not 16MB
> aligned? In other words was this bug exposed by the subsection-hotplug
> changes and should it contain Fixes: tag for those commits?
> 

We are able to hit this crash even with older kernels. This happens when 
we have multiple namespaces from the same region of size 26G. In that 
case we need to make sure we don't end up using altmap from one 
namespace for mapping vmemmap of the adjacent namespace.

Considering this impacts ppc64 and we got the ppc64 SCM support in 4.20. 
may be  we can do just
Cc: <[email-masked]> # 4.20+

-aneesh