Ben-Ami Yassour wrote:
Anthony Liguori <[EMAIL PROTECTED]> wrote on 04/29/2008 05:32:09 PM:
Subject
[PATCH] Handle vma regions with no backing page
This patch allows VMA's that contain no backing page to be used for
guest
memory. This is a drop-in replacement for Ben-Ami's first page in
his direct
mmio series. Here, we continue to allow mmio pages to be represented
in the
rmap.
struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
{
- return pfn_to_page(gfn_to_pfn(kvm, gfn));
+ pfn_t pfn;
+
+ pfn = gfn_to_pfn(kvm, gfn);
+ if (pfn_valid(pfn))
+ return pfn_to_page(pfn);
+
+ return NULL;
}
We noticed that pfn_valid does not always works as expected by this
patch to indicate that a pfn has a backing page.
We have seen a case where CONFIG_NUMA was not set and then where
pfn_valid returned 1 for an mmio pfn.
We then changed the config file with CONFIG_NUMA set and it worked
fine as expected (since a different implementation of pfn_valid was
used).
How should we overcome this issue?
Looks like we need to reintroduce a refcount bit in the pte, and check
the page using the VMA.
Nick Piggin's lockless pagecache patches, which have the same issue,
also introduce a pte_special bit. We could follow a similar route.
http://www.mail-archive.com/[EMAIL PROTECTED]/msg04789.html
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
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