Re: [PATCH 2.6.26-rc8-mm1] memrlimit: fix mmap_sem deadlock

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 04 Jul 2008 08:50:47 +0530 Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> > I was referring to the below (which is where the conversation ended).
> > 
> > It questions the basis of the whole feature.
> > 
> 
> In the email below, I referred to Hugh's comment on tracking total_vm as a more
> achievable target and it gives a rough approximation of something worth
> limiting. I agree with him on those points and mentioned my motivation for the
> memrlimit patchset. We also look forward to enhancing memrlimit to control
> mlock'ed pages (as it provides the generic infrastructure to control RLIMIT'ed
> resources). Given Hugh's comment, I looked at it from the more positive side
> rather the pessimistic angle. I've had discussions along these lines with Paul
> Menage and Kamezawa. In the past we've discussed and there are cases where
> memrlimit is not useful (large VM allocations with sparse usage), but there are
> cases as mentioned below in the motivation for memrlimits as to why and where
> they are useful.
> 
> If there are suggestions to help improve the feature or provide similar
> functionality without the noise; I am all ears

Well I've never reeeeeeealy understood what the whole feature is for.

+Advantages of providing this feature
+
+1. Control over virtual address space allows for a cgroup to fail gracefully
+   i.e., via a malloc or mmap failure as compared to OOM kill when no
+   pages can be reclaimed.
+2. It provides better control over how many pages can be swapped out when
+   the cgroup goes over its limit. A badly setup cgroup can cause excessive
+   swapping. Providing control over the address space allocations ensures
+   that the system administrator has control over the total swapping that
+   can take place.

umm, OK.  I'm not sure _why_ someone would want to do that.  Perhaps
some use-cases would help motivate us.  Perhaps desriptions of
real-world operational problems would would be improved or solved were
this feature available to the operator.

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