Re: [PATCH] ipc,shm: increase default size for shmmax

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

>> Our middleware engineers has been complaining about this sysctl limit.
>> System administrator need to calculate required sysctl value by making sum
>> of all planned middlewares, and middleware provider needs to write "please
>> calculate systcl param by....." in their installation manuals.
>
> Why aren't people just setting the sysctl to a petabyte?  What problems
> would that lead to?

I don't have much Fujitsu middleware knowledges. But I'd like to explain
very funny bug I saw.

1. middleware-A suggest to set SHMMAX to very large value (maybe
LONG_MAX, but my memory was flushed)
2. middleware-B suggest to set SHMMAX to increase some dozen mega byte.

Finally, it was overflow and didn't work at all.

Let's demonstrate.

# echo 18446744073709551615 > /proc/sys/kernel/shmmax
# cat /proc/sys/kernel/shmmax
18446744073709551615
# echo 18446744073709551616 > /proc/sys/kernel/shmmax
# cat /proc/sys/kernel/shmmax
0

That's why many open source software continue the silly game. But
again, I don't have knowledge about Fujitsu middleware. I'm waiting
kamezawa-san's answer.

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