5:12 a.m.
On 02/12/2016 05:48 AM, Dmitry Monakhov wrote:
Maxim Patlasov <mpatlasov(a)virtuozzo.com> 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(a)virtuozzo.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);
> }
> }
>
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