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

From: Matthew Wilcox <willy@infradead.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-s390@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
	yi.z.zhang@linux.intel.com,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-kernel@vger.kernel.org,
	Pavel Tatashin <pasha.tatashin@oracle.com>,
	linux-mm@kvack.org, linux-m68k@lists.linux-m68k.org,
	linux-mediatek@lists.infradead.org,
	Miles Chen <miles.chen@mediatek.com>,
	Anthony Yznaga <anthony.yznaga@oracle.com>,
	linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC 7/7] mm: better document PG_reserved
Date: Wed, 5 Dec 2018 09:32:01 -0800	[thread overview]
Message-ID: <20181205173201.GA11646@bombadil.infradead.org> (raw)
In-Reply-To: <46d0e90f-f0bb-815e-7a5b-4429de1c502a@redhat.com>

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".

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