[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(a)virtuozzo.com&gt; --- 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); } }