Re: [PATCH v6] mm/pgmap: Use correct alignment when looking at first pfn from a region

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 12/3/19 6:20 AM, Dan Williams wrote:
> On Sat, Nov 30, 2019 at 3:13 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>>
>> On Wed, 25 Sep 2019 09:21:02 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:
>>
>>> Andrew Morton <akpm@linux-foundation.org> writes:
>>>
>>>> On Tue, 17 Sep 2019 21:01:29 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:
>>>>
>>>>> vmem_altmap_offset() adjust the section aligned base_pfn offset.
>>>>> So we need to make sure we account for the same when computing base_pfn.
>>>>>
>>>>> ie, for altmap_valid case, our pfn_first should be:
>>>>>
>>>>> pfn_first = altmap->base_pfn + vmem_altmap_offset(altmap);
>>>>
>>>> What are the user-visible runtime effects of this change?
>>>
>>> This was found by code inspection. If the pmem region is not correctly
>>> section aligned we can skip pfns while iterating device pfn using
>>>        for_each_device_pfn(pfn, pgmap)
>>>
>>>
>>> I still would want Dan to ack the change though.
>>>
>>
>> Dan?
>>
>>
>> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
>> Subject: mm/pgmap: use correct alignment when looking at first pfn from a region
>>
>> vmem_altmap_offset() adjusts the section aligned base_pfn offset.  So we
>> need to make sure we account for the same when computing base_pfn.
>>
>> ie, for altmap_valid case, our pfn_first should be:
>>
>> pfn_first = altmap->base_pfn + vmem_altmap_offset(altmap);
>>
>> This was found by code inspection. If the pmem region is not correctly
>> section aligned we can skip pfns while iterating device pfn using
>>
>>          for_each_device_pfn(pfn, pgmap)
>>
>> [akpm@linux-foundation.org: coding style fixes]
>> Link: http://lkml.kernel.org/r/20190917153129.12905-1-aneesh.kumar@linux.ibm.com
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>
>>   mm/memremap.c |   12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> --- a/mm/memremap.c~mm-pgmap-use-correct-alignment-when-looking-at-first-pfn-from-a-region
>> +++ a/mm/memremap.c
>> @@ -55,8 +55,16 @@ static void pgmap_array_delete(struct re
>>
>>   static unsigned long pfn_first(struct dev_pagemap *pgmap)
>>   {
>> -       return PHYS_PFN(pgmap->res.start) +
>> -               vmem_altmap_offset(pgmap_altmap(pgmap));
>> +       const struct resource *res = &pgmap->res;
>> +       struct vmem_altmap *altmap = pgmap_altmap(pgmap);
>> +       unsigned long pfn;
>> +
>> +       if (altmap)
>> +               pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
>> +       else
>> +               pfn = PHYS_PFN(res->start);
> 
> This would only be a problem if res->start is not subsection aligned.

Kernel is not enforcing this right? ie, If i create multiple namespace
as below

ndctl create-namespace -s 16908288 --align 64K

I can get base_pfn different from res->start PHYS_PFN

Yes that results in other error as below with the current upstream.

[   17.491097] memory add fail, invalid altmap



> Is that bug triggering in your case, or is this just inspection. Now
> that the subsections can be assumed as the minimum mapping granularity
> I'd rather see a cleanup  I'd rather cleanup the implementation to
> eliminate altmap->base_pfn or at least assert that
> PHYS_PFN(res->start) and altmap->base_pfn are always identical.
> 
> Otherwise ->base_pfn is supposed to be just a convenient way to recall
> the bounds of the memory hotplug operation deeper in the vmemmap
> setup.
> 

Is the right fix to ensure that we always make sure res->start is 
subsection aligned ? If so we may need the patch series
https://patchwork.kernel.org/project/linux-nvdimm/list/?series=209373

And enforce that to be multiple of subsection size?

-aneesh
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