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 -- Thanks, David / dhildenb
