Re: [Bugme-new] [Bug 38032] New: default values of /proc/sys/net/ipv4/udp_mem does not consider huge page allocation

For anyone who may not have read the bugzilla, a
possibly larger concern subsequently discovered is
that actual kernel memory consumption is double the
total of the values reported by 'netstat -nau', at
least when mostly small packets are received and
a RHEL 5 kernel is in use.  The tunable enforces based
on the 'netstat' value rather than the actual value
in the RH kernel.  Maybe not an issue in the
mainline, but it took a few additional system
hangs in the lab before we figured this out
and divided the 'udm_mem' maximum value in half.


At 04:03 PM 7/6/2011 -0700, Andrew Morton wrote:

(switched to email.  Please respond via emailed reply-to-all, 
not via the bugzilla web interface).

(cc's added)

On Tue, 21 Jun 2011 00:35:22 GMT
bugzilla-daemon@bugzilla.kernel.org wrote:

quoted

https://bugzilla.kernel.org/show_bug.cgi?id=38032

           Summary: default values of 

/proc/sys/net/ipv4/udp_mem does not

quoted

                    consider huge page allocatio
           Product: Memory Management
           Version: 2.5
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
        AssignedTo: akpm@linux-foundation.org
        ReportedBy: starlight@binnacle.cx
        Regression: No


In the RHEL 5.5 back-port of this tunable we ran into trouble locking up
systems because the boot-time default is set based on physical memory does not
account for the hugepages= in the boot parameters.  So the UDP socket buffer
limit can exceed phyisical memory.  Don't know if this is an issue in mainline
kernels but it seems likely so reporting this as a courtesy.  Seems like it
would be easy to fix the default to account for the memory reserved by
hugepages which is not available for slab allocations.

https://bugzilla.redhat.com/show_bug.cgi?id=714833

Yes, we've made similar mistakes in other places.

I don't think we really have an official formula for what callers
should be doing here.  net/ipv4/udp.c:udp_init() does

       nr_pages = totalram_pages - totalhigh_pages;             
              

which assumes that totalram_pages does not include the pages which were
lost to hugepage allocations.

I *think* that this is now the case, but it wasn't always the case - we
made relatively recent fixes to the totalram_pages maintenance.

Perhaps UDP should be using the misnamed nr_free_buffer_pages() 
here.

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