On 24.07.2018 15:35, Michal Hocko wrote: > 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? >
Yes, they are preserved but we don't interpret them, that is up to dump tools. We only provide access to the vmcore, which includes read/writing the memory indicated in it. The struct pages are simply part of the vmcore. Completely hidden from the new kernel. Finding/interpreting the struct pages is not (and most probably should never) be done in the kernel. E.g. The old kernel could be a different kernel version, different memory configuration (!SPARSE, SPARSE ...), page flags could be different ... it's not a straight forward access. That's why dump tools interpret struct pages instead. And also why I want a simple identifier in them so user space dump tools can figure out "this page is better not to be touched, the content is stale or not accessible". So I see right now: - Pg_reserved + e.g. new page type (or some other unique identifier in combination with Pg_reserved) -> Avoid reads of pages we know are offline - extend is_ram_page() -> Fake zero memory for pages we know are offline Or even both (avoid reading and don't crash the kernel if it is being done). I am not a friend of the "try to access and get an exception" approach. -- Thanks, David / dhildenb