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    SubjectRe: [PATCH] General purpose zeroed page slab
    On Thu, Oct 14, 2004 at 12:50:43PM -0400, Martin K. Petersen wrote:
    >
    > A while back Bill Irwin converted the page table code on ppc64 to use
    > a zeroed page slab. I recently did the same on ia64 and got a
    > significant performance improvement in terms of fault time (4 usec ->
    > 700 nsec).
    >
    > This cache needs to be initialized fairly early on and so far we've
    > called it from pgtable_cache_init() on both archs. However, Tony Luck
    > thought it might be useful to have a general purpose slab cache with
    > zeroed pages. And other architectures might decide to use it for
    > their page tables too.
    >
    > Consequently here's a patch that puts this functionality in slab.c.
    >
    > Signed-off-by: Martin K. Petersen <mkp@wildopensource.com>
    >
    > --
    > Martin K. Petersen Wild Open Source, Inc.
    > mkp@wildopensource.com http://www.wildopensource.com/
    >
    > diff -urN -X /usr/people/mkp/bin/dontdiff linux-pristine/include/linux/slab.h zero-slab/include/linux/slab.h
    > --- linux-pristine/include/linux/slab.h 2004-10-11 14:57:20.000000000 -0700
    > +++ zero-slab/include/linux/slab.h 2004-10-13 17:49:29.000000000 -0700
    > @@ -115,6 +115,7 @@
    > extern kmem_cache_t *signal_cachep;
    > extern kmem_cache_t *sighand_cachep;
    > extern kmem_cache_t *bio_cachep;
    > +extern kmem_cache_t *zero_page_cachep;
    >
    > extern atomic_t slab_reclaim_pages;
    >
    > diff -urN -X /usr/people/mkp/bin/dontdiff linux-pristine/mm/slab.c zero-slab/mm/slab.c
    > --- linux-pristine/mm/slab.c 2004-10-11 14:57:20.000000000 -0700
    > +++ zero-slab/mm/slab.c 2004-10-13 17:49:57.000000000 -0700
    > @@ -716,6 +716,13 @@
    >
    > static struct notifier_block cpucache_notifier = { &cpuup_callback, NULL, 0 };
    >
    > +kmem_cache_t *zero_page_cachep;
    > +
    > +static void zero_page_ctor(void *pte, kmem_cache_t *cache, unsigned long flags)
    > +{
    > + memset(pte, 0, PAGE_SIZE);
    > +}

    The means every user has to memset it to zero before free.
    Add a comment for that at least.

    Also that's pretty dumb. How about keeping track how much of the
    page got non zeroed (e.g. by using a few free words in struct page
    for a coarse grained dirty bitmap)

    Then you could memset on free only the parts that got actually
    changed, and never waste cache lines for anything else.

    -Andi

    -
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