[Xen-devel] Re: [PATCH] Balloon driver crash

[M A Young <m.a.young@xxxxxxxxxxxx>]

On Tue, 8 Jun 2010, Jeremy Fitzhardinge wrote:

> On 06/08/2010 02:25 AM, M A Young wrote:
> > This patch fixes an error in commit
> > f4685d0ed041523d1bd5124c5872459681aca876
> > "Add hugepage support to balloon driver" that was causing the balloon
> > driver to crash when shrinking the memory used by Domain-0.
> Thanks.  But why haven't I seen this?
Good question, but I think you only see the crash in a limited memory 
situation. I don't claim to be an expert in what is going on, but a few 
lines earlier you have the the loop
        for (i = 0; i < nr_pages; i++) {
                if ((page = alloc_pages(GFP_BALLOON, balloon_order)) == 
NULL) {
                        nr_pages = i;
                        need_sleep = 1;
                        break;
                }

                pfn = page_to_pfn(page);
                frame_list[i] = pfn_to_mfn(pfn);

                scrub_page(page);
        }

If alloc_pages(GFP_BALLOON, balloon_order) is never NULL during the loop 
then page is left set to be one of the pages you are freeing. From then on 
PageHighMem(pfn_to_page(lpfn)) and PageHighMem(page) could easily the same 
and you will only have a problem if some of the pages you are freeing are 
PageHighMem and some aren't.
If alloc_pages(GFP_BALLOON, balloon_order) is null somewhere in the loop 
(presumably because it can't find enough pages to free) then the 
process will crash.
        Michael Young

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