Re: [PATCH] kvm: do not SetPageDirty from kvm_set_pfn_dirty for file mappings

[Maxim Patlasov]

On 02/12/2016 05:48 AM, Dmitry Monakhov wrote:
> Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> writes:
>
> > 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.
> Confirm. I've hit that BUGON
> [ 4442.219121] ------------[ cut here ]------------
> [ 4442.219188] kernel BUG at fs/ext4/inode.c:2285!
> <...>
>
> > 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.
> Please elaborate exact call-path which marks host-page.

wb_workfn ->
 wb_do_writeback ->
  wb_writeback ->
   __writeback_inodes_wb ->
    writeback_sb_inodes ->
     __writeback_single_inode ->
      do_writepages ->
       ext4_writepages ->
        mpage_prepare_extent_to_map ->
         mpage_process_page_bufs ->
          mpage_submit_page ->
           clear_page_dirty_for_io ->
             page_mkclean ->
              rmap_walk->
                rmap_walk_file ->
                 page_mkclean_one->
                  pte_wrprotect ->
                    pte_clear_flags(pte, _PAGE_RW)

Thanks,
Maxim

> > 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@xxxxxxxxxxxxx>
> > ---
> >   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);
> >   	}
> >   }
> >
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