From d75996dd022b6d83bd14af59b2775b1aa639e4b9 Mon Sep 17 00:00:00 2001
From: Vivek Goyal <vgoyal@redhat.com>
Date: Fri, 2 Aug 2019 15:29:56 -0400
Subject: [PATCH] dax: dax_layout_busy_page() should not unmap cow pages
Git-commit: d75996dd022b6d83bd14af59b2775b1aa639e4b9
Patch-mainline: v5.3-rc4
References: bsc#1148698

Vivek: 

    "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."

Dan: 

    "In fact the unmap_mapping_range() in this path is only to synchronize
     against get_user_pages_fast() and force it to call back into the
     filesystem to re-establish the mapping. COW pages should be left
     untouched by dax_layout_busy_page()."

Cc: <stable@vger.kernel.org>
Fixes: 5fac7408d828 ("mm, fs, dax: handle layout changes to pinned dax mappings")
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Link: https://lore.kernel.org/r/20190802192956.GA3032@redhat.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Jan Kara <jack@suse.cz>

---
 fs/dax.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -676,7 +676,7 @@ 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);
 
 	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
 				min(end - index, (pgoff_t)PAGEVEC_SIZE),