Re: [discuss] Re: 32-bit dma allocations on 64-bit platforms

[William Lee Irwin III <wli@holomorphy.com>]

On Thu, Jun 24, 2004 at 04:09:45PM -0700, Andrew Morton wrote:
> A testcase would be, on a 2G box:
> a) free up as much memory as you can
> b) write a 1.2G file to fill highmem with pagecache
> c) malloc(800M), bzero(), sleep
> d) swapoff -a
> You now have a box which has almost all of lowmem pinned in anonymous
> memory.  It'll limp along and go oom fairly easily.
> Another testcase would be:
> a) free up as much memory as you can
> b) write a 1.2G file to fill highmem with pagecache
> c) malloc(800M), mlock it
> You now have most of lowmem mlocked.

These are approximately identical to the testcases I had in mind, except
neither of these is truly specific to 2GB and can have the various magic
numbers calculated from sysconf() and/or meminfo.


On Thu, Jun 24, 2004 at 04:09:45PM -0700, Andrew Morton wrote:
> In both situations the machine is really sick.  Probably the most risky
> scenario is a swapless machine in which lots of lowmem is allocated to
> anonymous memory.
> It should be the case that increasing lower_zone_peotection will fix all
> the above.  If not, it needs fixing.
> So we're down the question "what should we default to at bootup".  I find
> it hard to justify defaulting to a mode where we're super-defensive against
> this sort of thing, simply because nobody seems to be hitting the problems.
> Distributors can, if the must, bump lower_zone_protection in initscripts,
> and it's presumably pretty simple to write a boot script which parses
> /proc/meminfo's MemTotal and SwapTotal lines, producing an appropriate
> lower_zone_protection setting.

I'm going to beat on this in short order, but will be indisposed for an
hour or two before that begins.

Thanks.


-- wli