[PATCH] fix page->count discrepancy for zero page

From: Dave Hansen (haveblue_at_us.ibm.com)
Date: 06/29/04

  • Next message: Pat Gefre: "Re: [PATCH 2.6] Altix serial driver" 
    To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:	Mon, 28 Jun 2004 15:05:53 -0700

    While writing some analysis tools for memory hot-remove, we came across
    a single page which had a ->count that always increased, without bound.
    It ended up always being the zero page, and it was caused by a leaked
    reference in some do_wp_page() code that ends up avoiding PG_reserved
    pages.

    Basically what happens is that page_cache_release()/put_page() ignore
    PG_reserved pages, while page_cache_get()/get_page() go ahead and take
    the reference. So, each time there's a COW fault on the zero-page, you
    get a leaked page->count increment.

    It's pretty rare to have a COW fault on anything that's PG_reserved, in
    fact, I can't think of anything else that this applies to other than the
    zero page.

    In any case, it the bug doesn't cause any real problems, but it is a bit
    of an annoyance and is obviously incorrect. We've been running with
    this patch for about 3 months now, and haven't run into any problems
    with it.

    Attached patch is against 2.6.7 and applies to -mm3 properly.

    -- Dave

    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

  • Next message: Pat Gefre: "Re: [PATCH 2.6] Altix serial driver"

    Relevant Pages