Re: [Xen-devel] [PATCH 1/3] x86: calculate precisely the memory needed by init_memory_mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 17 Jun 2011, Ian Jackson wrote:
> stefano.stabellini@xxxxxxxxxxxxx writes ("[Xen-devel] [PATCH 1/3] x86: 
> calculate precisely the memory needed by init_memory_mapping"):
> > +   if (pgt_buf_end != pgt_buf_top)
> > +           printk(KERN_DEBUG "initial kernel pagetable allocation wasted 
> > %lx"
> > +                           " pages\n", pgt_buf_top - pgt_buf_end);
> 
> If (due to a bug) pgt_buf_end > pgt_buf_top, this will printk a
> message about wasting a negative number of pages, rather than
> crashing.  Is there something else that will catch this case ?

Thanks for reviewing this patch!

Yes, there is something else that catches this case: both the 32 bit
and the 64 bit versions of alloc_low_page contain this code:


unsigned long pfn = pgt_buf_end++;

if (pfn >= pgt_buf_top)
                panic("alloc_low_page: ran out of memory");


so we are safe from that point of view.

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