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Date:   Fri, 14 Jun 2019 23:10:36 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>, Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH -next] mm/hotplug: skip bad PFNs from pfn_to_online_page()

On 6/14/19 10:38 PM, Jeff Moyer wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
> 
>> On 6/14/19 10:06 PM, Dan Williams wrote:
>>> On Fri, Jun 14, 2019 at 9:26 AM Aneesh Kumar K.V
>>> <aneesh.kumar@...ux.ibm.com> wrote:
>>
>>>> Why not let the arch
>>>> arch decide the SUBSECTION_SHIFT and default to one subsection per
>>>> section if arch is not enabled to work with subsection.
>>>
>>> Because that keeps the implementation from ever reaching a point where
>>> a namespace might be able to be moved from one arch to another. If we
>>> can squash these arch differences then we can have a common tool to
>>> initialize namespaces outside of the kernel. The one wrinkle is
>>> device-dax that wants to enforce the mapping size,
>>
>> The fsdax have a much bigger issue right? The file system block size
>> is the same as PAGE_SIZE and we can't make it portable across archs
>> that support different PAGE_SIZE?
> 
> File system blocks are not tied to page size.  They can't be *bigger*
> than the page size currently, but they can be smaller.
> 


ppc64 page size is 64K.

> Still, I don't see that as an arugment against trying to make the
> namespaces work across architectures.  Consider a user who only has
> sector mode namespaces.  We'd like that to work if at all possible.
> 

agreed. I was trying to list out the challenges here.

-aneesh

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