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    SubjectRe: [PATCH -mm 13/25] Noreclaim LRU Infrastructure
    On Sun, 8 Jun 2008 16:34:13 -0400 Rik van Riel <riel@redhat.com> wrote:

    > On Fri, 6 Jun 2008 18:05:06 -0700
    > Andrew Morton <akpm@linux-foundation.org> wrote:
    > > On Fri, 06 Jun 2008 16:28:51 -0400
    > > Rik van Riel <riel@redhat.com> 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?

    > > > --- linux-2.6.26-rc2-mm1.orig/include/linux/page-flags.h 2008-05-29 16:21:04.000000000 -0400
    > > > +++ linux-2.6.26-rc2-mm1/include/linux/page-flags.h 2008-06-06 16:05:15.000000000 -0400
    > > > @@ -94,6 +94,9 @@ enum pageflags {
    > > > PG_reclaim, /* To be reclaimed asap */
    > > > PG_buddy, /* Page is free, on buddy lists */
    > > > PG_swapbacked, /* Page is backed by RAM/swap */
    > > > +#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?

    > > > +/**
    > > > + * add_page_to_noreclaim_list
    > > > + * @page: the page to be added to the noreclaim list
    > > > + *
    > > > + * Add page directly to its zone's noreclaim list. To avoid races with
    > > > + * tasks that might be making the page reclaimble while it's not on the
    > > > + * lru, we want to add the page while it's locked or otherwise "invisible"
    > > > + * to other tasks. This is difficult to do when using the pagevec cache,
    > > > + * so bypass that.
    > > > + */
    > >
    > > How does a task "make a page reclaimable"? munlock()? fsync()?
    > > exit()?
    > >
    > > Choice of terminology matters...
    >
    > Lee? Kosaki-san?



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