Re: Maybe a VM bug in 2.4.18-18 from RH 8.0?

[Andrea Arcangeli]

On Thu, Dec 05, 2002 at 06:15:59PM -0800, William Lee Irwin III wrote:
> On Fri, Dec 06, 2002 at 02:44:29AM +0100, Andrea Arcangeli wrote:
> > Or it hurts when you can't allocate an inode because such 100M are in
> > pagetables on a 64G box and you still have 60G free of highmem.
> 
> This is the zone vs. zone watermark stuff that penalizes/fails
> allocations made with a given GFP mask from being satisfied by
> fallback. This is largely old news wrt. various kinds of inability
> to pressure those ZONE_NORMAL (maybe also ZONE_DMA) consumers.
> 
> Admission control for fallback is valuable, sure. I suspect the
> question akpm raised is about memory utilization. My own issues are
> centered around allocations targeted directly at ZONE_NORMAL,
> which fallback prevention does not address, so the watermark patch
> is not something I'm personally very concerned about.

you must be very concerned about it too.

If you don't have the fallback prevention all your efforts around the
allocations targeted directoy zone normal will be completely worthless.

Either that or you want to drop ZONE_NORMAL enterely because it means
nothing uses zone-normal dynamically anymore (ZONE_NORMAL seen as a
place that is directly mapped, not necessairly always 32bit dma
capable).

> 64GB isn't getting any testing that I know of; I'd hold off until
> someone's actually stood up and confessed to attempting to boot
> Linux on such a beast. Or until I get some more RAM. =)

64GB is an example, a good example for this thing, but a 16G machine or
a 4G machine can run in the very same issues. As said just swapoff -a
and malloc(1G) and such 1G is all ZONE_NORMAL before you could allocate
enough inodes for your workload. Or alloc 1G of pagetables by setting
everything protnone, and sugh 1G of pagetables goes in zone-normal
because the highmem is filled by cache. Choose whatever is your
preferred example of real life bug fixed by the lowmem-reservation patch
that is absolutely necessary to run stable on a big box with normal zone
and highmem (not only a 64G box).

The only place where you must not be concerned about these fixes are the
64bit archs.

Andrea