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Subject: Re: [PATCH 0/7] ACPI HMAT memory sysfs representation
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Keith Busch <keith.busch@intel.com>,
        linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
        linux-mm@kvack.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rafael Wysocki <rafael@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <f5315662-5c1a-68a3-4d04-21b4b5ca94b1@intel.com>
Date:   Thu, 22 Nov 2018 10:01:53 -0800
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On 11/22/18 3:52 AM, Anshuman Khandual wrote:
>>
>> It sounds like the subset that's being exposed is insufficient for yo
>> We did that because we think doing anything but a subset in sysfs will
>> just blow up sysfs:  MAX_NUMNODES is as high as 1024, so if we have 4
>> attributes, that's at _least_ 1024*1024*4 files if we expose *all*
>> combinations.
> Each permutation need not be a separate file inside all possible NODE X
> (/sys/devices/system/node/nodeX) directories. It can be a top level file
> enumerating various attribute values for a given (X, Y) node pair based
> on an offset something like /proc/pid/pagemap.

My assumption has been that this kind of thing is too fancy for sysfs:

Documentation/filesystems/sysfs.txt:
> Attributes should be ASCII text files, preferably with only one value
> per file. It is noted that it may not be efficient to contain only one
> value per file, so it is socially acceptable to express an array of
> values of the same type. 
> 
> Mixing types, expressing multiple lines of data, and doing fancy
> formatting of data is heavily frowned upon. Doing these things may get
> you publicly humiliated and your code rewritten without notice. 

/proc/pid/pagemap is binary, not one-value-per-file and relatively
complicated to parse.

Do you really think following something like pagemap is the right model
for sysfs?

BTW, I'm not saying we don't need *some* interface like you propose.  We
almost certainly will at some point.  I just don't think it will be in
sysfs.