[Bug 1470] New: adjust Linux out-of-memory killer to stop sshd being killed

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https://bugzilla.mindrot.org/show_bug.cgi?id=1470

           Summary: adjust Linux out-of-memory killer to stop sshd being
                    killed
    Classification: Unclassified
           Product: Portable OpenSSH
           Version: 5.0p1
          Platform: All
               URL: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=34176
                    7
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: sshd
        AssignedTo: bitbucket at mindrot.org
        ReportedBy: cjwatson at debian.org


Created an attachment (id=1507)
 --> (http://bugzilla.mindrot.org/attachment.cgi?id=1507)
adjust Linux out-of-memory killer

In some out-of-memory situations, the Linux kernel will look for a
process to kill, employing some heuristics to try to guess what will
help. It doesn't always get this right and can occasionally end up
killing innocent bystanders (though as noted in the referenced bug log
it's possible to tweak this to be more accurate).

It is useful to instruct the OOM killer never to kill sshd, since
almost everyone wants it to keep on running so that they have a chance
of dealing with the problem remotely. Originally I implemented this in
an init script, by getting sshd's pid and writing to
/proc/$pid/oom_adj, but Vaclav Ovsik pointed out in
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=480020 that that ends
up immortalising child processes too.

The attached patch is based on Vaclav's, though I tidied it up and
moved chunks of it into openbsd-compat/port-linux. The use of an
environment variable for configuration is a bit odd. I didn't feel good
about introducing a port-specific configuration file key, and the
values you write into oom_adj have a pretty bizarre syntax (documented
in
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/filesystems/proc.txt;hb=HEAD)
which I think is unlikely to be portable to other systems. I'd
appreciate any better ideas here.

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