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SubjectRe: [PATCH 0/8] v5 De-couple sysfs memory directories from memory sections
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On Mon, 2010-08-16 at 09:34 -0500, Nathan Fontenot wrote:
> > It's not an unresolvable issue, as this is a must-fix problem. But you
> > should tell us what your proposal is to prevent breakage of existing
> > installations. A Kconfig option would be good, but a boot-time kernel
> > command line option which selects the new format would be much better.
>
> This shouldn't break existing installations, unless an architecture chooses
> to do so. With my patch only the powerpc/pseries arch is updated such that
> what is seen in userspace is different.

Even if an arch defines the override for the sysfs dir size, I still
don't think this breaks anything (it shouldn't). We move _all_ of the
directories over, all at once, to a single, uniform size. The only
apparent change to a user moving kernels would be a larger
block_size_bytes (which is certainly not changing the ABI) and a new
sysfs file for the end of the section. The new sysfs file is
_completely_ redundant at this point.

The architecture is only supposed to bump up the directory size when it
*KNOWS* that all operations will be done at the larger section size,
such as if the specific hardware has physical DIMMs which are much
larger than SECTION_SIZE.

Let's say we have a system with 20MB of memory, SECTION_SIZE of 1MB and
a sysfs dir size of 4MB.

Before the patch, we have 20 directories: one for each section. After
this patch, we have 5 directories.

The thing that I think is the next step, but that we _will_ probably
need eventually is this, take the 5 sysfs dirs in the above case:

0->3, 4->7, 8->11, 12->15, 16->19

and turn that into a single one:

0->19

*That* will require changing the ABI, but we could certainly have some
bloated and slow, but backward-compatible mode.

-- Dave



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