linux-kernel - [PATCH] kvm: do not SetPageDirty from kvm_set_pfn

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Date:	Thu, 11 Feb 2016 10:13:29 -0800
From:	Maxim Patlasov <mpatlasov@...tuozzo.com>
To:	pbonzini@...hat.com
Cc:	kvm@...r.kernel.org, linux-nvdimm@...ts.01.org, gleb@...nel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, devel@...nvz.org
Subject: [PATCH] kvm: do not SetPageDirty from kvm_set_pfn_dirty for file
 mappings

The patch solves the following problem: file system specific routines
involved in ordinary routine writeback process BUG_ON page_buffers()
because a page goes to writeback without buffer-heads attached.

The way how kvm_set_pfn_dirty calls SetPageDirty works only for anon
mappings. For file mappings it is obviously incorrect - there page_mkwrite
must be called. It's not easy to add page_mkwrite call to kvm_set_pfn_dirty
because there is no universal way to find vma by pfn. But actually
SetPageDirty may be simply skipped in those cases. Below is a
justification.

When guest modifies the content of a page with file mapping, kernel kvm
makes the page dirty by the following call-path:

vmx_handle_exit ->
 handle_ept_violation ->
  __get_user_pages ->
   page_mkwrite ->
    SetPageDirty

Since then, the page is dirty from both guest and host point of view. Then
the host makes writeback and marks the page as write-protected. So any
further write from the guest triggers call-path above again.

So, for file mappings, it's not possible to have new data written to a page
inside the guest w/o corresponding SetPageDirty on the host.

This makes explicit SetPageDirty from kvm_set_pfn_dirty redundant.

Signed-off-by: Maxim Patlasov <mpatlasov@...tuozzo.com>
---
 virt/kvm/kvm_main.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a11cfd2..5a7d3fa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1582,7 +1582,8 @@ void kvm_set_pfn_dirty(kvm_pfn_t pfn)
 	if (!kvm_is_reserved_pfn(pfn)) {
 		struct page *page = pfn_to_page(pfn);

-		if (!PageReserved(page))
+		if (!PageReserved(page) &&
+		    (!page->mapping || PageAnon(page)))
 			SetPageDirty(page);
 	}
 }