Re: [RFC] Reproducible OOM with partial workaround

[Andrew Morton]

On Fri, 11 Jan 2013 22:51:35 +1100
paul.szabo@xxxxxxxxxxxxx wrote:

> Dear Andrew,
> 
> > Check /proc/slabinfo, see if all your lowmem got eaten up by buffer_heads.
> 
> Please see below: I do not know what any of that means. This machine has
> been running just fine, with all my users logging in here via XDMCP from
> X-terminals, dozens logged in simultaneously. (But, I think I could make
> it go OOM with more processes or logins.)

I'm counting 107MB in slab there.  Was this dump taken when the system
was at or near oom?

Please send a copy of the oom-killer kernel message dump, if you still
have one.

> > If so, you *may* be able to work around this by setting
> > /proc/sys/vm/dirty_ratio really low, so the system keeps a minimum
> > amount of dirty pagecache around.  Then, with luck, if we haven't
> > broken the buffer_heads_over_limit logic it in the past decade (we
> > probably have), the VM should be able to reclaim those buffer_heads.
> 
> I tried setting dirty_ratio to "funny" values, that did not seem to
> help.

Did you try setting it as low as possible?

> Did you notice my patch about bdi_position_ratio(), how it was
> plain wrong half the time (for negative x)? 

Nope, please resend.

> Anyway that did not help.
> 
> > Alternatively, use a filesystem which doesn't attach buffer_heads to
> > dirty pages.  xfs or btrfs, perhaps.
> 
> Seems there is also a problem not related to filesystem... or rather,
> the essence does not seem to be filesystem or caches. The filesystem
> thing now seems OK with my patch doing drop_caches.

hm, if doing a regular drop_caches fixes things then that implies the
problem is not with dirty pagecache.  Odd.

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