Re: [PATCH 10/10] mm: per device dirty threshold

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 Apr 2007 08:29:59 +0200 Miklos Szeredi <miklos@szeredi.hu> wrote:

> > > What about swapout?  That can increase the number of writeback pages,
> > > without decreasing the number of dirty pages, no?
> > 
> > Could we not solve that by enabling cap_account_writeback on
> > swapper_space, and thereby account swap writeback pages. Then the VM
> > knows it has outstanding IO and need not panic.
> 
> Hmm, I'm not sure that would be right, because then those writeback
> pages would be accounted twice: once for swapper_space, and once for
> the real device.
> 
> So there's a condition, when lots of anonymous pages are turned into
> swap-cache writeback pages, and we should somehow throttle this, because
> 
> >>>     This means that all memory is pinned and unreclaimable and the VM gets
> >>>     upset and goes oom.
> 
> although, it's not quite clear in my mind, how the VM gets upset about
> this.

I've been scratching my head on and off for a couple of days over this.

We've traditionally had reclaim problems when there's a huge amount of
dirty MAP_SHARED data, which the VM didn't know was dirty.  It's the old
"map a file which is the same size as physical memory and write to it all"
stresstest.

But we do not have such problems with anonymous memory, and I'm darned if I
can remember why :(