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

[Andrea Arcangeli]

On Thu, Dec 05, 2002 at 05:34:34PM -0800, Andrew Morton wrote:
> Andrea Arcangeli wrote:
> > 
> > ...
> > He may still suffer other known problems besides
> > the above two critical highmem fixes (for example if
> > lower_zone_reserve_ratio is not applied and there's no other fix around
> > it IMHO, that's generic OS problem not only for linux, and that was my
> > only sensible solution to fix it, the approch in mainline is way too
> > weak to make a real difference)
> 
> argh.  I hate that one ;)  Giving away 100 megabytes of memory
> hurts.

100M hurts on a 4G box? No-way ;)

it hurts when such 100M of normal zone are mlocked
by an highmem-capable users and you can't allocate one more inode but
you have still 3G free of highmem (google is doing this, they even drop
a check so they can mlock > half of the ram).

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.

> I've never been able to find the workload which makes this
> necessary.  Can you please describe an "exploit" against 

ask google...

> 2.4.20 which demonstrates the need for this?

even simpler, swapoff -a and malloc and have fun! ;) (again ask google,
they run w/o swap for obvious good reasons)

Or if you have enough time, wait those 100M to be filled by pagetables
on a 64G box.

Andrea