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    SubjectRe: [PATCH -mm 13/25] Noreclaim LRU Infrastructure
    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."

    --
    All rights reversed.


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