Re: [Xen-devel] [PATCH 1/2] xen/mm: fold PGC_broken into PGC_state bits

From: David Woodhouse <dwmw2@infradead.org>
To: Julien Grall <julien@xen.org>, Jan Beulich <jbeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Jeff Kubascik <jeff.kubascik@dornerworks.com>,
	Stewart Hildebrand <stewart.hildebrand@dornerworks.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH 1/2] xen/mm: fold PGC_broken into PGC_state bits
Date: Sun, 09 Feb 2020 13:22:32 +0000	[thread overview]
Message-ID: <d0072bd4be2151e1ad768f948f541694bb062d94.camel@infradead.org> (raw)
In-Reply-To: <28e1dfce-b4c5-7c73-0574-48fc4179443e@xen.org>

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On Fri, 2020-02-07 at 20:27 +0000, Julien Grall wrote:
> Could you please send the series in a separate thread? So we don't mix 
> the two discussions (i.e merge and free on boot allocated page) together.

Well, they're all part of the same mess, really.

There are cases where pages end up in free_heap_pages() which were
never vetted by init_heap_pages(). While that normally works just fine
— to the extent that we've never really cared — the hack excluding MFN0
is one of the things that gets 'missed' for such pages.

I was only looking at this because the early vmap support makes it a
tiny bit more likely that some pages will be freed that way after being
given out by the boot allocator, but there were plenty of reasons it
might happen already.

These patches basically legitimise that — we make it OK for
free_heap_pages() to be given a range which was never in the heap in
the first place. And in so doing, we fix the MFN0/zone merge trick even
when (for example) MFN0 was part of the dom0 initramfs and assigned
directly, but gets freed up later.

But sure, having failed to elicit any screams of "don't do it like
that", I'll fix up the things you pointed out and I'll repost it as
part of the series containing the early vmap() support, since that's
why I'm working on it.

Although at this point I'm half tempted to declare that there are *so*
many ways this can happen already, that the tiny little bit that it's
made more likely by the early vmap() support is basically in the noise.

In that case we can separate these patches out as an independent fix
rather than a dependency of early vmap.

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