On Thu 27-08-15 10:23:51, Jorn Engel wrote:

On Thu, Aug 27, 2015 at 08:48:18AM +0200, Michal Hocko wrote:

quoted

quoted

On x86, HUGE_MAX_HSTATE == 2.  I don't consider that to be expensive.

If you are concerned about the memory allocation of struct hugetlb_usage, 
it could easily be embedded directly in struct mm_struct.

Yes I am concerned about that and
9 files changed, 112 insertions(+), 1 deletion(-)
for something that is even not clear to be really required. And I still
haven't heard any strong usecase to justify it.

Can we go with the single and much simpler cumulative number first and
only add the break down list if it is _really_ required? We can even
document that the future version of /proc/<pid>/status might add an
additional information to prepare all the parsers to be more careful.

I don't care much which way we decide.  But I find your reasoning a bit
worrying.  If someone asks for a by-size breakup of hugepages in a few
years, you might have existing binaries that depend on the _absence_ of
those extra characters on the line.

Compare:
  HugetlbPages:      18432 kB
  HugetlbPages:    1069056 kB (1*1048576kB 10*2048kB)

Once someone has written a script that greps for 'HugetlbPages:.*kB$',
you have lost the option of adding anything else to the line. 

If you think that an explicit note in the documentation is
not sufficient then I believe we can still handle it backward
compatible. Like separate entries for each existing hugetlb page:
HugetlbPages:	     1069056 kB
Hugetlb2MPages:	     20480 kB
Hugetlb1GPages:	     1048576 kB

or something similar. I would even argue this would be slightly easier
to parse. So it is not like we would be locked into anything.

You have
created yet another ABI compatibility headache today in order to save
112 lines of code.

That may be a worthwhile tradeoff, I don't know.  But at least I realize
there is a cost, while you seem to ignore that component.  There is
value in not painting yourself into a corner.

My primary point was that we are adding a code for a feature nobody
actually asked for just because somebody might ask for it in future.
-- 
Michal Hocko
SUSE Labs

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