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Re: [PATCH RFC 7/7] mm: better document PG_reserved

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".