Re: [Crash-utility] p2m stuff and crash tool

On Tue, Feb 16, 2016 at 01:55:33PM +0100, Juergen Gross wrote: > Hi Daniel, > > On 16/02/16 12:35, Daniel Kiper wrote: >> Hey Juergen, [...] >> After that I decided to take a look at Linux kernel upstream. I saw >> that xen_max_p2m_pfn in xen_build_mfn_list_list() is equal to "the >> end of last usable machine memory region available for a given >> dom0_mem argument + something", e.g. >> >> For dom0_mem=1g,max:1g: >> >> (XEN) Xen-e820 RAM map: >> (XEN) 0000000000000000 - 000000000009fc00 (usable) >> (XEN) 000000000009fc00 - 00000000000a0000 (reserved) >> (XEN) 00000000000f0000 - 0000000000100000 (reserved) >> (XEN) 0000000000100000 - 000000007ffdf000 (usable) <--- HERE >> (XEN) 000000007ffdf000 - 0000000080000000 (reserved) >> (XEN) 00000000b0000000 - 00000000c0000000 (reserved) >> (XEN) 00000000feffc000 - 00000000ff000000 (reserved) >> (XEN) 00000000fffc0000 - 0000000100000000 (reserved) >> (XEN) 0000000100000000 - 0000000180000000 (usable) >> >> Hence xen_max_p2m_pfn == 0x80000 >> >> Later I reviewed most of your p2m related commits and I realized >> that you played whack-a-mole game with p2m bugs. Sadly, I was not >> able to identify exactly one (or more) commit which would fix the >> same issue (well, there are some which fixes similar stuff but not >> the same one described above). So, if you explain to me why >> xen_max_p2m_pfn is set to that value and does not e.g. max_pfn then >> it will be much easier for me to write proper fix and maybe fix >> the same issue in upstream kernel if it is needed (well, crash >> tool does not work with new p2m layout so first of all I must fix it; >> I hope that you will help me to that sooner or later). > > The reason for setting xen_max_p2m_pfn to nr_pages initially is it's > usage in __pfn_to_mfn(): this must work with the initial p2m list > supplied by the hypervisor which just has only nr_pages entries. That make sense. > Later it is updated to the number of entries the linear p2m list is > able to hold. This size has to include possible hotplugged memory > in prder to be able to make use of that memory later (remember: the > p2m list's size is limited by the virtual space allocated for it via > xen_vmalloc_p2m_tree()). However, I have memory hotplug disabled and kernel set xen_max_p2m_pfn to 0x80000 (2 Gib) even if dom0 memory was set to 1 GiB. Hmmm... Why? I suppose that if xen_max_p2m_pfn == max_pfn then everything should work. Am I missing something?