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    SubjectRe: [PATCH] [11/16] HWPOISON: check and isolate corrupted free pages v2
    On Tue, Jun 09, 2009 at 09:03:04PM +0800, Wu Fengguang wrote:
    > On Tue, Jun 09, 2009 at 06:02:29PM +0800, Nick Piggin wrote:
    > > On Wed, Jun 03, 2009 at 08:46:45PM +0200, Andi Kleen wrote:
    > > >
    > > > From: Wu Fengguang <fengguang.wu@intel.com>
    > > >
    > > > If memory corruption hits the free buddy pages, we can safely ignore them.
    > > > No one will access them until page allocation time, then prep_new_page()
    > > > will automatically check and isolate PG_hwpoison page for us (for 0-order
    > > > allocation).
    > >
    > > It would be kinda nice if this could be done in the handler
    > > directly (ie. take the page directly out of the allocator
    > > or pcp list). Completely avoiding fastpaths would be a
    > > wonderful goal.
    >
    > In fact Andi have code to do that. We prefer this one because
    > - it's simple
    > - it's good sanity check for possible software BUGs
    > - it mainly adds overhead to high order pages, which is acceptable

    Yeah it's not bad. But we don't have much other non-debug options
    that check for random memory corruption like this. Given that the
    struct page is a very tiny proportion of memory, then I'm not
    totally convinced that all this checking in the page allocator is
    worthwhile for everyone. It's a much bigger cost if checks and
    branches have to be there just for hwpoison.

    And I don't think removing a free page from the page allocator is
    too much more complex than removing a live page from the pagecache ;)


    >
    > Thanks,
    > Fengguang
    >
    > > >
    > > > + /* Don't complain about poisoned pages */
    > > > + if (PageHWPoison(page)) {
    > > > + __ClearPageBuddy(page);
    > > > + return;
    > > > + }
    > > > +
    >
    > I do think the above chunk is not absolutely necessary, though.
    >
    > Thanks,
    > Fengguang
    >
    >
    > > > /*
    > > > * Allow a burst of 60 reports, then keep quiet for that minute;
    > > > * or allow a steady drip of one report per second.
    > > > @@ -650,7 +656,7 @@
    > > > /*
    > > > * This page is about to be returned from the page allocator
    > > > */
    > > > -static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
    > > > +static inline int check_new_page(struct page *page)
    > > > {
    > > > if (unlikely(page_mapcount(page) |
    > > > (page->mapping != NULL) |
    > > > @@ -659,6 +665,18 @@
    > > > bad_page(page);
    > > > return 1;
    > > > }
    > > > + return 0;
    > > > +}
    > > > +
    > > > +static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
    > > > +{
    > > > + int i;
    > > > +
    > > > + for (i = 0; i < (1 << order); i++) {
    > > > + struct page *p = page + i;
    > > > + if (unlikely(check_new_page(p)))
    > > > + return 1;
    > > > + }
    > > >
    > > > set_page_private(page, 0);
    > > > set_page_refcounted(page);


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