[Virtio-fs] [PATCH] dax: dax_layout_busy_page() should not unmap cow pages

[Dan Williams]

On Fri, Aug 2, 2019 at 12:30 PM Vivek Goyal <vgoyal at redhat.com> wrote:
>
> As of now dax_layout_busy_page() calls unmap_mapping_range() with last
> argument as 1, which says even unmap cow pages. I am wondering who needs
> to get rid of cow pages as well.
>
> I noticed one interesting side affect of this. I mount xfs with -o dax and
> mmaped a file with MAP_PRIVATE and wrote some data to a page which created
> cow page. Then I called fallocate() on that file to zero a page of file.
> fallocate() called dax_layout_busy_page() which unmapped cow pages as well
> and then I tried to read back the data I wrote and what I get is old
> data from persistent memory. I lost the data I had written. This
> read basically resulted in new fault and read back the data from
> persistent memory.
>
> This sounds wrong. Are there any users which need to unmap cow pages
> as well? If not, I am proposing changing it to not unmap cow pages.
>
> I noticed this while while writing virtio_fs code where when I tried
> to reclaim a memory range and that corrupted the executable and I
> was running from virtio-fs and program got segment violation.
>
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---
>  fs/dax.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: rhvgoyal-linux/fs/dax.c
> ===================================================================
> --- rhvgoyal-linux.orig/fs/dax.c        2019-08-01 17:03:10.574675652 -0400
> +++ rhvgoyal-linux/fs/dax.c     2019-08-02 14:32:28.809639116 -0400
> @@ -600,7 +600,7 @@ struct page *dax_layout_busy_page(struct
>          * guaranteed to either see new references or prevent new
>          * references from being established.
>          */
> -       unmap_mapping_range(mapping, 0, 0, 1);
> +       unmap_mapping_range(mapping, 0, 0, 0);

Good find, yes, this looks correct to me and should also go to -stable.