Re: [PATCH -mm 13/25] Noreclaim LRU Infrastructure

[Rik van Riel <riel@redhat.com>]

On Sun, 8 Jun 2008 13:57:04 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> > > > From: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
> > 
> > > > The noreclaim infrastructure is enabled by a new mm Kconfig option
> > > > [CONFIG_]NORECLAIM_LRU.
> > > 
> > > Having a config option for this really sucks, and needs extra-special
> > > justification, rather than none.
> > 
> > I believe the justification is that it uses a page flag.
> > 
> > PG_noreclaim would be the 20th page flag used, meaning there are
> > 4 more free if 8 bits are used for zone and node info, which would
> > give 6 bits for NODE_SHIFT or 64 NUMA nodes - probably overkill
> > for 32 bit x86.
> > 
> > If you want I'll get rid of CONFIG_NORECLAIM_LRU and make everything
> > just compile in always.
> 
> Seems unlikely to be useful?  The only way in which this would be an
> advantage if if we hae some other feature which also needs a page flag
> but which will never be concurrently enabled with this one.
> 
> > Please let me know what your preference is.
> 
> Don't use another page flag?

I don't see how that would work.  We need a way to identify
the status of the page.

> > > > +#ifdef CONFIG_NORECLAIM_LRU
> > > > +	PG_noreclaim,		/* Page is "non-reclaimable"  */
> > > > +#endif
> > > 
> > > I fear that we're messing up the terminology here.
> > > 
> > > Go into your 2.6.25 tree and do `grep -i reclaimable */*.c'.  The term
> > > already means a few different things, but in the vmscan context,
> > > "reclaimable" means that the page is unreferenced, clean and can be
> > > stolen.  "reclaimable" also means a lot of other things, and we just
> > > made that worse.
> > > 
> > > Can we think of a new term which uniquely describes this new concept
> > > and use that, rather than flogging the old horse?
> > 
> > Want to reuse the BSD term "pinned" instead?
> 
> mm, "pinned" in Linuxland means "someone took a ref on it to prevent it
> from being reclaimed".
> 
> As a starting point: what, in your english-language-paragraph-length
> words, does this flag mean?

"Cannot be reclaimed because someone has it locked in memory
through mlock, or the page belongs to something that cannot
be evicted like ramfs."

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