On 05.12.18 18:32, Matthew Wilcox wrote: > On Wed, Dec 05, 2018 at 04:05:12PM +0100, David Hildenbrand wrote: >> On 05.12.18 15:35, Matthew Wilcox wrote: >>> On Wed, Dec 05, 2018 at 01:28:51PM +0100, David Hildenbrand wrote: >>>> I don't see a reason why we have to document "Some of them might not even >>>> exist". If there is a user, we should document it. E.g. for balloon >>>> drivers we now use PG_offline to indicate that a page might currently >>>> not be backed by memory in the hypervisor. And that is independent from >>>> PG_reserved. >>> >>> I think you're confused by the meaning of "some of them might not even >>> exist". What this means is that there might not be memory there; maybe >>> writes to that memory will be discarded, or maybe they'll cause a machine >>> check. Maybe reads will return ~0, or 0, or cause a machine check. >>> We just don't know what's there, and we shouldn't try touching the memory. >> >> If there are users, let's document it. And I need more details for that :) >> >> 1. machine check: if there is a HW error, we set PG_hwpoison (except >> ia64 MCA, see the list) >> >> 2. Writes to that memory will be discarded >> >> Who is the user of that? When will we have such pages right now? >> >> 3. Reads will return ~0, / 0? >> >> I think this is a special case of e.g. x86? But where do we have that, >> are there any user? > > When there are gaps in the physical memory. As in, if you put that > physical address on the bus (or in a packet), no device will respond > to it. Look: > > 00000000-00000fff : Reserved > 00001000-00057fff : System RAM > 00058000-00058fff : Reserved > 00059000-0009dfff : System RAM > 0009e000-000fffff : Reserved > > Those examples I gave are examples of how various different architectures > respond to "no device responded to this memory access". > Okay, so for this memory we will have a) vmmaps b) Memory block devices c) A sections that is online So essentially "Gaps in physical memory" which is part of a online section. This might be a candidate for PG_offline as well. Thanks for the info, I'll try to find out how such things are handled. In general I assume this memory has to be readable, because otherwise kdump and friends would crash already when trying to dump? -- Thanks, David / dhildenb
