Re: [RFC 2/9] Use NOMEMALLOC reclaim to allow reclaim if PF_MEMALLOC is set

[Nick Piggin]

On Mon, Aug 20, 2007 at 11:14:08PM +0200, Peter Zijlstra wrote:
> On Mon, 2007-08-20 at 13:27 -0700, Christoph Lameter wrote:
> > On Mon, 20 Aug 2007, Peter Zijlstra wrote:
> > 
> > > > Plus the same issue can happen today. Writes are usually not completed 
> > > > during reclaim. If the writes are sufficiently deferred then you have the 
> > > > same issue now.
> > > 
> > > Once we have initiated (disk) writeout we do not need more memory to
> > > complete it, all we need to do is wait for the completion interrupt.
> > 
> > We cannot reclaim the page as long as the I/O is not complete. If you 
> > have too many anonymous pages and the rest of memory is dirty then you can 
> > get into OOM scenarios even without this patch.
> 
> As long as the reserve is large enough to completely initialize writeout
> of a single page we can make progress. Once writeout is initialized the
> completion interrupt is guaranteed to happen (assuming working
> hardware).

Although interestingly, we are not guaranteed to have enough memory to
completely initialise writeout of a single page.

The buffer layer doesn't require disk blocks to be allocated at page
dirty-time. Allocating disk blocks can require complex filesystem operations
and readin of buffer cache pages. The buffer_head structures themselves may
not even be present and must be allocated :P

In _practice_, this isn't such a problem because we have dirty limits, and
we're almost guaranteed to have some clean pages to be reclaimed. In this
same way, networked filesystems are not a problem in practice. However
network swap, because there is no dirty limits on swap, can actually see
the deadlock problems.
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