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SubjectRe: [PATCH v2 2/4] libnvdimm/namespace: Enforce memremap_compat_align()
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Dan Williams <dan.j.williams@intel.com> writes:

> On Thu, Feb 13, 2020 at 1:55 PM Jeff Moyer <jmoyer@redhat.com> wrote:
>>
>> Dan Williams <dan.j.williams@intel.com> writes:
>>
>> > The pmem driver on PowerPC crashes with the following signature when
>> > instantiating misaligned namespaces that map their capacity via
>> > memremap_pages().
>> >
>> > BUG: Unable to handle kernel data access at 0xc001000406000000
>> > Faulting instruction address: 0xc000000000090790
>> > NIP [c000000000090790] arch_add_memory+0xc0/0x130
>> > LR [c000000000090744] arch_add_memory+0x74/0x130
>> > Call Trace:
>> > arch_add_memory+0x74/0x130 (unreliable)
>> > memremap_pages+0x74c/0xa30
>> > devm_memremap_pages+0x3c/0xa0
>> > pmem_attach_disk+0x188/0x770
>> > nvdimm_bus_probe+0xd8/0x470
>> >
>> > With the assumption that only memremap_pages() has alignment
>> > constraints, enforce memremap_compat_align() for
>> > pmem_should_map_pages(), nd_pfn, or nd_dax cases.
>> >
>> > Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> > Cc: Jeff Moyer <jmoyer@redhat.com>
>> > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> > Link: https://lore.kernel.org/r/158041477336.3889308.4581652885008605170.stgit@dwillia2-desk3.amr.corp.intel.com
>> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> > ---
>> > drivers/nvdimm/namespace_devs.c | 10 ++++++++++
>> > 1 file changed, 10 insertions(+)
>> >
>> > diff --git a/drivers/nvdimm/namespace_devs.c b/drivers/nvdimm/namespace_devs.c
>> > index 032dc61725ff..aff1f32fdb4f 100644
>> > --- a/drivers/nvdimm/namespace_devs.c
>> > +++ b/drivers/nvdimm/namespace_devs.c
>> > @@ -1739,6 +1739,16 @@ struct nd_namespace_common *nvdimm_namespace_common_probe(struct device *dev)
>> > return ERR_PTR(-ENODEV);
>> > }
>> >
>> > + if (pmem_should_map_pages(dev) || nd_pfn || nd_dax) {
>> > + struct nd_namespace_io *nsio = to_nd_namespace_io(&ndns->dev);
>> > + resource_size_t start = nsio->res.start;
>> > +
>> > + if (!IS_ALIGNED(start | size, memremap_compat_align())) {
>> > + dev_dbg(&ndns->dev, "misaligned, unable to map\n");
>> > + return ERR_PTR(-EOPNOTSUPP);
>> > + }
>> > + }
>> > +
>> > if (is_namespace_pmem(&ndns->dev)) {
>> > struct nd_namespace_pmem *nspm;
>> >
>>
>> Actually, I take back my ack. :) This prevents a previously working
>> namespace from being successfully probed/setup.
>
> Do you have a test case handy? I can see a potential gap with a
> namespace that used internal padding to fix up the alignment.

# ndctl list -v -n namespace0.0
[
{
"dev":"namespace0.0",
"mode":"fsdax",
"map":"dev",
"size":52846133248,
"uuid":"b99f6f6a-2909-4189-9bfa-6eeebd95d40e",
"raw_uuid":"aff43777-015b-493f-bbf9-7c7b0fe33519",
"sector_size":512,
"align":4096,
"blockdev":"pmem0",
"numa_node":0
}
]

# cat /sys/bus/nd/devices/region0/mappings
6

# grep namespace0.0 /proc/iomem
1860000000-24e0003fff : namespace0.0

> The goal of this check is to catch cases that are just going to fail
> devm_memremap_pages(), and the expectation is that it could not have
> worked before unless it was ported from another platform, or someone
> flipped the page-size switch on PowerPC.

On x86, creation and probing of the namespace worked fine before this
patch. What *doesn't* work is creating another fsdax namespace after
this one. sector mode namespaces can still be created, though:

[
{
"dev":"namespace0.1",
"mode":"sector",
"size":53270768640,
"uuid":"67ea2c74-d4b1-4fc9-9c1a-a7d2a6c2a4a7",
"sector_size":512,
"blockdev":"pmem0.1s"
},

# grep namespace0.1 /proc/iomem
24e0004000-3160007fff : namespace0.1

>> I thought we were only going to enforce the alignment for a newly
>> created namespace? This should only check whether the alignment
>> works for the current platform.
>
> The model is a new default 16MB alignment is enforced at creation
> time, but if you need to support previously created namespaces then
> you can manually trim that alignment requirement to no less than
> memremap_compat_align() because that's the point at which
> devm_memremap_pages() will start failing or crashing.

The problem is that older kernels did not enforce alignment to
SUBSECTION_SIZE. We shouldn't prevent those namespaces from being
accessed. The probe itself will not cause the WARN_ON to trigger.
Creating new namespaces at misaligned addresses could, but you've
altered the free space allocation such that we won't hit that anymore.

If I drop this patch, the probe will still work, and allocating new
namespaces will also work:

# ndctl list
[
{
"dev":"namespace0.1",
"mode":"sector",
"size":53270768640,
"uuid":"67ea2c74-d4b1-4fc9-9c1a-a7d2a6c2a4a7",
"sector_size":512,
"blockdev":"pmem0.1s"
},
{
"dev":"namespace0.0",
"mode":"fsdax",
"map":"dev",
"size":52846133248,
"uuid":"b99f6f6a-2909-4189-9bfa-6eeebd95d40e",
"sector_size":512,
"align":4096,
"blockdev":"pmem0"
}
]
ndctl create-namespace -m fsdax -s 36g -r 0
{
"dev":"namespace0.2",
"mode":"fsdax",
"map":"dev",
"size":"35.44 GiB (38.05 GB)",
"uuid":"7893264c-c7ef-4cbe-95e1-ccf2aff041fb",
"sector_size":512,
"align":2097152,
"blockdev":"pmem0.2"
}

proc/iomem:

1860000000-d55fffffff : Persistent Memory
1860000000-24e0003fff : namespace0.0
24e0004000-3160007fff : namespace0.1
3162000000-3a61ffffff : namespace0.2

So, maybe the right thing is to make memremap_compat_align return
PAGE_SIZE for x86 instead of SUBSECTION_SIZE?

-Jeff

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