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. > The *only* way would be to have an interface to the hypervisor where we > "sense" if a memory location is safe to touch. I remember that xen or > hyper-v does that - they fake a zero page in that case, after querying > the hypervisor. But this does not sound like a clean approach to me, > especially es we need yet another hypervisor interface to sense for > memory provided via "some" device. > > If we can find a way to just tag pages as "don't touch", it would be the > easiest and cleanest solution in my opinion. If only we could have much more spare room in struct pages... -- Michal Hocko SUSE Labs
