Re: Memory pressure handling with iSCSI

[Andrew Morton <akpm@osdl.org>]

"Martin J. Bligh" <mbligh@mbligh.org> wrote:
>
> 
> > 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;
>                 }
>         }
> }

I don't think so.  We're getting the wrong answer out of
calculate_zone_totalpages() which is an init-time thing.

Maybe nr_free_zone_pages() is supposed to fix that up post-facto somehow,
but calculate_zone_totalpages() sure as heck shouldn't be putting 1568768
into my ZONE_NORMAL's ->node_present_pages.