Re: [PATCH] [11/16] HWPOISON: check and isolate corrupted free pages v2

Nick Piggin
2009-06-09T13:28:54Z

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);