Re: Posix Shared Mem patch

On Tue, Jun 26, 2012 at 2:53 PM, Alvaro Herrera
<alvherre(at)commandprompt(dot)com> wrote:
>
> Excerpts from Daniel Farina's message of mar jun 26 17:40:16 -0400 2012:
>
>> On that, I used to be of the opinion that this is a good compromise (a
>> small amount of interlock space, plus mostly posix shmem), but I've
>> heard since then (I think via AgentM indirectly, but I'm not sure)
>> that there are cases where even the small SysV segment can cause
>> problems -- notably when other software tweaks shared memory settings
>> on behalf of a user, but only leaves just-enough for the software
>> being installed.
>
> This argument is what killed the original patch. If you want to get
> anything done *at all* I think it needs to be dropped. Changing shmem
> implementation is already difficult enough --- you don't need to add the
> requirement that the interlocking mechanism be changed simultaneously.
> You (or whoever else) can always work on that as a followup patch.

True, but then again, I did very intentionally write:

> Excerpts from Daniel Farina's message of mar jun 26 17:40:16 -0400 2012:
>> *I wouldn't let perfect be the enemy of good* to make progress
>> here, but it appears this was a witnessed real problem, so it may
>> be worth reconsidering if there is a way we can safely remove all
>> SysV by finding an alternative to the nattach mechanic.

(Emphasis mine).

I don't think that -hackers at the time gave the zero-shmem rationale
much weight (I also was not that happy about the safety mechanism of
that patch), but upon more reflection (and taking into account *other*
software that may mangle shmem settings) I think it's something at
least worth thinking about again one more time. What killed the patch
was an attachment to the deemed-less-safe stategy for avoiding bogus
shmem attachments already in it, but I don't seem to recall anyone
putting a whole lot of thought at the time into the zero-shmem case
from what I could read on the list, because a small interlock with
nattach seemed good-enough.

I'm simply suggesting that for additional benefits it may be worth
thinking about getting around nattach and thus SysV shmem, especially
with regard to safety, in an open-ended way. Maybe there's a solution
(like Robert's FIFO suggestion?) that is not too onerous and can
satisfy everyone.

--
fdr