[PATCH 02/12] KVM: guest_memfd: delay folio_mark_uptodate() until after successful preparation

Paolo Bonzini
2024-07-11T22:28:03Z

The up-to-date flag as is now is not too useful; it tells guest_memfd not
to overwrite the contents of a folio, but it doesn't say that the page
is ready to be mapped into the guest. For encrypted guests, mapping
a private page requires that the "preparation" phase has succeeded,
and at the same time the same page cannot be prepared twice.

So, ensure that folio_mark_uptodate() is only called on a prepared page. If
kvm_gmem_prepare_folio() or the post_populate callback fail, the folio
will not be marked up-to-date; it's not a problem to call clear_highpage()
again on such a page prior to the next preparation attempt.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
virt/kvm/guest_memfd.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 522e1b28e7ae..1ea632dbae57 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -73,8 +73,6 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool

for (i = 0; i < nr_pages; i++)
clear_highpage(folio_page(folio, i));
-
- folio_mark_uptodate(folio);
}

if (prepare) {
@@ -84,6 +82,8 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool
folio_put(folio);
return ERR_PTR(r);
}
+
+ folio_mark_uptodate(folio);
}

/*
@@ -646,6 +646,8 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long

p = src ? src + i * PAGE_SIZE : NULL;
ret = post_populate(kvm, gfn, pfn, p, max_order, opaque);
+ if (!ret)
+ folio_mark_uptodate(folio);

folio_put(folio);
if (ret)
--
2.43.0