Re: RFC for 2.6: avoid OOM at bounce buffer storm

[Martin Wilck <martin.wilck@fujitsu-siemens.com>]

Hi Andrew,

> Well.  As I said, I think what you're seeing here is recent changes to
> mm/page-writeback.c which reduce the amount of memory which we'll permit to
> be dirtied due to write() calls.  You'll probably find that the bounce
> buffer problem is also fixable by reducing /proc/sys/vm/dirty_ratio in
> 2.6.9, for the same reasons.
> 
> What concerns me is that there are other ways of dirtying lots of memory
> apart from write(): namely mmap(MAP_SHARED).  If someone dirties 90% of all
> memory via mmap() then we might again get into bounce buffer starvation.

I have tried the mmap(MAP_SHARED) method now extensively. I haven't been 
able to come anywhere near the catastrophic situations I saw with the 
2.6.9 kernel, even by dirtying the full 8GB in fractions of a second.

There was another strangeness there though: Even with the high memory 
pressure applied, The ZONE_NORMAL free memory would never go below 
~300MB. When the mem pressure got too high, the kernel would rather free 
almost slabs and start swapping than use those remaining 300M. It seems 
to me that the new logic is a bit too conservative with ZONE_NORMAL 
allocations.

Regards
Martin

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