Re: Strange dcache memory pressure when highmem enabled

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Oct 14, 2003 at 10:43:52PM -0700, Andrew Morton wrote:
> Neil Brown <neilb@cse.unsw.edu.au> wrote:
> >
> > I noticed that shrink_caches calls shrink_dcache_memory independant
> >   of the classzone that is being shrunk.  So if we are trying to
> >   shrink ZONE_HIGHMEM, the dentry_cache is shrunk, even though the
> >   dentry_cache doesn't live in highmem.  However I'm not sure if I have
> >   understood the classzones well enough for that observation even to
> >   make sense.
> 
> Makes heaps of sense.  Here's an instabackport of what we did in 2.6:
> 
>  mm/vmscan.c |   12 +++++++++---
>  1 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff -puN mm/vmscan.c~a mm/vmscan.c
> --- 24/mm/vmscan.c~a	2003-10-14 22:41:34.000000000 -0700
> +++ 24-akpm/mm/vmscan.c	2003-10-14 22:42:22.000000000 -0700
> @@ -640,11 +640,17 @@ int try_to_free_pages_zone(zone_t *class
>  			nr_pages = shrink_caches(classzone, gfp_mask, nr_pages, &failed_swapout);
>  			if (nr_pages <= 0)
>  				return 1;
> -			shrink_dcache_memory(vm_vfs_scan_ratio, gfp_mask);
> -			shrink_icache_memory(vm_vfs_scan_ratio, gfp_mask);
> +			if (classzone - classzone->zone_pgdat->node_zones <
> +						ZONE_HIGHMEM) {
> +				shrink_dcache_memory(vm_vfs_scan_ratio,
> +							gfp_mask);
> +				shrink_icache_memory(vm_vfs_scan_ratio,
> +							gfp_mask);
>  #ifdef CONFIG_QUOTA
> -			shrink_dqcache_memory(vm_vfs_scan_ratio, gfp_mask);
> +				shrink_dqcache_memory(vm_vfs_scan_ratio,
> +							gfp_mask);
>  #endif
> +			}
>  			if (!failed_swapout)
>  				failed_swapout = !swap_out(classzone);
>  		} while (--tries);

An highmem user can make use of lowmem too. Think a 1G box, it looks
wrong to disallow highmem to shrink the 800M of dcache in the normal/dma
zones.

I wonder if what he's suffering from is a reduced normal zone due the
mem_map_t being larger. The reduced normal zone will trigger the dcache
shrinking more frequently. But he may want to try again with 2.4.23pre7
with a classzone aware refill_inactive that will ensure the inactive
list has enough lowmem pages before shrink_caches claims failure.

Andrea - If you prefer relying on open source software, check these links:
	    rsync.kernel.org::pub/scm/linux/kernel/bkcvs/linux-2.[45]/
	    http://www.cobite.com/cvsps/