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()). I wonder if we could convert the early allocated memory (PG_reserved) at some point (buddy initialized) into ordinary "simply allocated" memory. -- Thanks, David / dhildenb
