Re: [PATCH] mm/nvdimm: Use correct #defines instead of opencoding

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Linux MM <linux-mm@kvack.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH] mm/nvdimm: Use correct #defines instead of opencoding
Date: Tue, 14 May 2019 09:35:21 +0530	[thread overview]
Message-ID: <f99c4f11-a43d-c2d3-ab4f-b7072d090351@linux.ibm.com> (raw)
In-Reply-To: <CAPcyv4iNgFbSq0Hqb+CStRhGWMHfXx7tL3vrDaQ95DcBBY8QCQ@mail.gmail.com>

On 5/14/19 9:28 AM, Dan Williams wrote:
> On Mon, May 13, 2019 at 7:56 PM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> wrote:
>>
>> The nfpn related change is needed to fix the kernel message
>>
>> "number of pfns truncated from 2617344 to 163584"
>>
>> The change makes sure the nfpns stored in the superblock is right value.
>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>>   drivers/nvdimm/pfn_devs.c    | 6 +++---
>>   drivers/nvdimm/region_devs.c | 8 ++++----
>>   2 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
>> index 347cab166376..6751ff0296ef 100644
>> --- a/drivers/nvdimm/pfn_devs.c
>> +++ b/drivers/nvdimm/pfn_devs.c
>> @@ -777,8 +777,8 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn)
>>                   * when populating the vmemmap. This *should* be equal to
>>                   * PMD_SIZE for most architectures.
>>                   */
>> -               offset = ALIGN(start + reserve + 64 * npfns,
>> -                               max(nd_pfn->align, PMD_SIZE)) - start;
>> +               offset = ALIGN(start + reserve + sizeof(struct page) * npfns,
>> +                              max(nd_pfn->align, PMD_SIZE)) - start;
> 
> No, I think we need to record the page-size into the superblock format
> otherwise this breaks in debug builds where the struct-page size is
> extended.
> 
>>          } else if (nd_pfn->mode == PFN_MODE_RAM)
>>                  offset = ALIGN(start + reserve, nd_pfn->align) - start;
>>          else
>> @@ -790,7 +790,7 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn)
>>                  return -ENXIO;
>>          }
>>
>> -       npfns = (size - offset - start_pad - end_trunc) / SZ_4K;
>> +       npfns = (size - offset - start_pad - end_trunc) / PAGE_SIZE;
> 
> Similar comment, if the page size is variable then the superblock
> needs to explicitly account for it.
> 

PAGE_SIZE is not really variable. What we can run into is the issue you 
mentioned above. The size of struct page can change which means the 
reserved space for keeping vmemmap in device may not be sufficient for 
certain kernel builds.

I was planning to add another patch that fails namespace init if we 
don't have enough space to keep the struct page.

Why do you suggest we need to have PAGE_SIZE as part of pfn superblock?

-aneesh

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