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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Vanilla Linux and has_foreign_mapping
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 24 Apr 2008 17:18:13 -0700
Cc: Mark McLoughlin <markmc@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Andrea Arcangeli <andrea@xxxxxxxxxxxx>, Eduardo Habkost <ehabkost@xxxxxxxxxx>, Michael Abd-El-Malek <mabdelmalek@xxxxxxx>, Christoph Lameter <clameter@xxxxxxx>
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Keir Fraser wrote:
I'm not really familiar with the pv_ops code I'm afraid. But thinking about
this some more I've realised there's no way really to avoid making the
early-unpin logic aware of gntdev mappings. This is because if we do pin pte
pages, and require them to remain pinned across early-unpin, then
pgd_unpin() must not attempt to make those pte pages writable. That will
fail, because the pages are still pinned! You'd either need to handle the
failure to make the page writable, or have a per-page flag to indicate which
pte pages contain gntdev mappings. Frankly you may as well stick with the
per-mm-context has_foreign_mappings flag.

So the issue is that a pte page containing a _PAGE_IO pte must remain pinned while it contains that mapping? Would shooting down the mapping allow it to be unpinned, or does that need to be deferred until some later point (if so, when?)?

I guess the downside is that we'd need to scan the pte looking for _PAGE_IO mappings, which is a bit of a pain. Skipping that would mean hiding a flag somewhere...

Is it a pain to add a pv_ops-subtype-specific flag to mm_context? If so you
could maintain a set datastructure instead, indicating which mm_contexts
contain foreign mappings.

So, in 2.6.18-xen mm->has_foreign_mapping makes it skip early-unpin, but puts it off until pgd_free(). Presumably that works because all the vma's all been unmapped by then...

I wonder if Christoph/Andrea's mmu notifier patch is useful here...  Maybe?

Christoph/Andrea: would the mmu notifier mechanism allow us to efficiently generate the set of ptes mapping granted pages (pages from other domains), so we can shoot them down during process exit?


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