Re: Memory pressure handling with iSCSI

["Martin J. Bligh" <mbligh@mbligh.org>]

> It happens here, a bit.  My machine goes up to 60% dirty when it should be
> clamping at 40%.
> 
> The variable `total_pages' in page-writeback.c (from
> nr_free_pagecache_pages()) is too high.  I trace it back to here:
> 
> On node 0 totalpages: 1572864
>   DMA zone: 4096 pages, LIFO batch:1
>   Normal zone: 1568768 pages, LIFO batch:31
>   HighMem zone: 0 pages, LIFO batch:1
> 
> This machine only has 4G of memory, so the platform code is overestimating
> the number of pages by 50%.  Can you please check your dmesg, see if your
> system is also getting this wrong?

I think we're repeatedly iterating over the same zones by walking the 
zonelists:

static unsigned int nr_free_zone_pages(int offset)
{
        pg_data_t *pgdat;
        unsigned int sum = 0;
        int i;

        for_each_pgdat(pgdat) {
                struct zone *zone;

                for (i = 0; i < MAX_NR_ZONES; i++) {
                        unsigned long size, high;

                        zone = pgdat->node_zones[i];
                        size = zone->present_pages;
                        high = zone->pages_high;

                        if (size > high)
                                sum += size - high;
                }
        }
}

Does that look more sensible? I'd send you a real patch, except the
box just crashed ;-)

M.