On Tue 24-07-18 15:27:51, David Hildenbrand wrote: > On 24.07.2018 15:13, Michal Hocko wrote: > > On Tue 24-07-18 14:17:12, David Hildenbrand wrote: > >> On 24.07.2018 09:25, Michal Hocko wrote: > >>> On Mon 23-07-18 19:20:43, David Hildenbrand wrote: > >>>> On 23.07.2018 14:30, Michal Hocko wrote: > >>>>> On Mon 23-07-18 13:45:18, Vlastimil Babka wrote: > >>>>>> On 07/20/2018 02:34 PM, David Hildenbrand wrote: > >>>>>>> Dumping tools (like makedumpfile) right now don't exclude reserved pages. > >>>>>>> So reserved pages might be access by dump tools although nobody except > >>>>>>> the owner should touch them. > >>>>>> > >>>>>> Are you sure about that? Or maybe I understand wrong. Maybe it changed > >>>>>> recently, but IIRC pages that are backing memmap (struct pages) are also > >>>>>> PG_reserved. And you definitely do want those in the dump. > >>>>> > >>>>> You are right. reserve_bootmem_region will make all early bootmem > >>>>> allocations (including those backing memmaps) PageReserved. I have asked > >>>>> several times but I haven't seen a satisfactory answer yet. Why do we > >>>>> even care for kdump about those. If they are reserved the nobody should > >>>>> really look at those specific struct pages and manipulate them. Kdump > >>>>> tools are using a kernel interface to read the content. If the specific > >>>>> content is backed by a non-existing memory then they should simply not > >>>>> return anything. > >>>>> > >>>> > >>>> "new kernel" provides an interface to read memory from "old kernel". > >>>> > >>>> The new kernel has no idea about > >>>> - which memory was added/online in the old kernel > >>>> - where struct pages of the old kernel are and what their content is > >>>> - which memory is save to touch and which not > >>>> > >>>> Dump tools figure all that out by interpreting the VMCORE. They e.g. > >>>> identify "struct pages" and see if they should be dumped. The "new > >>>> kernel" only allows to read that memory. It cannot hinder to crash the > >>>> system (e.g. if a dump tool would try to read a hwpoison page). > >>>> > >>>> So how should the "new kernel" know if a page can be touched or not? > >>> > >>> I am sorry I am not familiar with kdump much. But from what I remember > >>> it reads from /proc/vmcore and implementation of this interface should > >>> simply return EINVAL or alike when you try to dump inaccessible memory > >>> range. > >> > >> Oh, and BTW, while something like -EINVAL could work, we usually don't > >> want to try to read certain pages at all (e.g. ballooned pages - > >> accessing the page might work but involves quite some overhead in the > >> hypervisor). > >> > >> So we should either handle this in dump tools (reserved + ...?) or while > >> doing the read similar to XEN (is_ram_page()). > > > > Yes, I think this is the proper way. Just test for PageOnline > > in read_from_oldmem/copy_oldmem_page. Btw. we already page > > pfn_to_online_page which performs the per-section online/offline > > status. This should be extendable to consider your new PageOffline > > state. > > That is the important bit: > > What the new kernel sees is not what the old kernel saw. > > Checking for pfn_to_online_page() from > read_from_oldmem/copy_oldmem_page() is plain wrong. > > E.g. ACPI hotplug memory is not even added in the new kernel - see > "acpi_no_memhotplug" which is used in kdump environments. > > The only thing we can do is > - query the hypervisor > - try to access and get an exception But we do preserve struct page's (aka memmap) from the crash kernel, don't we? So you have the whole state there. Or am I missing something? -- Michal Hocko SUSE Labs
