Re: [PATCH] only irq-safe atomic ops

[Andrew Morton <akpm@zip.com.au>]

Andi Kleen wrote:
> 
> Andrew Morton <akpm@zip.com.au> writes:
> > > >
> > > I can tell you that Irix has just such a global counter for the amount of
> > > delayed allocate pages - and it gets to be a major point of cache contention
> > > once you get to larger cpu counts. So avoiding that from the start would
> > > be good.
> >
> > Ah, good info.  Thanks.  I'll fix it with a big "FIXME" comment for now,
> > fix it for real when Rusty's per-CPU infrastructure appears.
> 
> Just curious -- how do you want to fix it for real?
> As far as I can see a delalloc counter needs to be exact to avoid OOM
> deadlocks, but making it per CPU would require doing the accounting inexact.

The counter is used only for making writeback decisions.  It is
completely analogous to nr_buffers_type[BUF_DIRTY], except it counts
pages.   So it does not need to be exact for readers.

If we needed exact reader-accounting for the number of dirty pages in the
machine then we'd need a ton of new locking in fun places like __free_pte(),
and that still doesn't account for pages which are only pte-dirty, and it's
not obvious what we'd do with reader-exact dirty page info anyway?


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