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

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jun 24, 2004 at 03:37:50PM -0700, William Lee Irwin III wrote:
> /*
> On Thu, Jun 24, 2004 at 02:54:41PM -0700, Andrew Morton wrote:
> >> First thing to do is to identify some workload which needs the patch. 
> 
> On Fri, Jun 25, 2004 at 12:21:50AM +0200, Andrea Arcangeli wrote:
> > that's quite trivial, boot a 2G box, malloc(1G), bzero(1GB), swapoff -a,
> > then the machine will lockup.
> > Depending on the architecture (more precisely depending if it starts
> > allocating ram from the end or from the start of the physical memory),
> > you may have to load 1G of data into pagecache first, like reading from
> > /dev/hda 1G (without closing the file) will work fine, then run the
> > above malloc + bzero + swapoff.
> > Most people will never report this because everybody has swap and they
> > simply run a lot slower than they could run if they didn't need to pass
> > through the swap device to relocate memory because memory would been allocated
> > in the right place in the first place. this plus the various oom killer
> > breakages that gets dominated by the nr_swap_pages > 0 check, are the
> > reasons 2.6 is unusable w/o swap. 
> 
> Have you tried with 2.6.7? The following program fails to trigger anything

I've definitely not tried 2.6.7 and I'm also reading a 2.6.5 codebase.
But you can sure trigger it if you run a big workload after the big
allocation.

> like what you've mentioned, though granted it was a 512MB allocation on
> a 1GB machine. swapoff(2) merely fails.

what you have to do is this:

1) swapoff -a (it must not fail!! it cannot fail if you run it first)
2) fill 130000K in pagecache, be very careful, not more than that, every
   mbyte matters
3) run your program and allocate 904000K!!! (not 512M!!!)

then keep using the machine until it lockups because it cannot reloate
the anonymous memory from the 900M of lowmem to the 130M of highmem.

But really I said you need >=2G to have a realistic chance of seeing it.

So don't be alarmed you cannot reproduce on a 1G box by allocating 512M
and with swap still enabled, you had none of the conditions that make it
reproducible.

I reproduced this dozen of times so I know how to reproduce it very
well (amittedly not in 2.6 because nobody crashed on this yet).