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

On 17/02/16 14:59, Daniel Kiper wrote: > 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? The virtual p2m list's size is aligned to PMD_SIZE (2 MB). For 1 GB dom0 memory max_pfn will be a little bit above 0x40000 due to the BIOS area resulting in a 4 MB p2m list. And xen_max_p2m_pfn is reflecting this size. You could reduce it to max_pfn without any problem, as long as memory hotplug is disabled. At least I think so.