FAILED: patch "[PATCH] dm: prevent DAX mounts if not supported" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Ross Zwisler <[email protected]>
Date: Tue, 26 Jun 2018 16:30:41 -0600
Subject: [PATCH] dm: prevent DAX mounts if not supported

Currently device_supports_dax() just checks to see if the QUEUE_FLAG_DAX flag is set on the device's request queue to decide whether or not the device supports filesystem DAX. Really we should be using bdev_dax_supported() like filesystems do at mount time. This performs other tests like checking to make sure the dax_direct_access() path works.

We also explicitly clear QUEUE_FLAG_DAX on the DM device's request queue if any of the underlying devices do not support DAX. This makes the handling of QUEUE_FLAG_DAX consistent with the setting/clearing of most other flags in dm_table_set_restrictions().

Now that bdev_dax_supported() explicitly checks for QUEUE_FLAG_DAX, this will ensure that filesystems built upon DM devices will only be able to mount with DAX if all underlying devices also support DAX.

index 938766794c2e..3d0e2c198f06 100644
@@ -885,9 +885,7 @@ EXPORT_SYMBOL_GPL(dm_table_set_type);
 static int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
 			       sector_t start, sector_t len, void *data)
 {
-	struct request_queue *q = bdev_get_queue(dev->bdev);
-
-	return q && blk_queue_dax(q);
+	return bdev_dax_supported(dev->bdev, PAGE_SIZE);
 }
 
 static bool dm_table_supports_dax(struct dm_table *t)

@@ -1907,6 +1905,9 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 
 	if (dm_table_supports_dax(t))
 		blk_queue_flag_set(QUEUE_FLAG_DAX, q);
+	else
+		blk_queue_flag_clear(QUEUE_FLAG_DAX, q);
+
 	if (dm_table_supports_dax_write_cache(t))
 		dax_write_cache(t->md->dax_dev, true);
 

index a3b103e8e3ce..b0dd7027848b 100644
@@ -1056,8 +1056,7 @@ static long dm_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
 	if (len < 1)
 		goto out;
 	nr_pages = min(len, nr_pages);
-	if (ti->type->direct_access)
-		ret = ti->type->direct_access(ti, pgoff, nr_pages, kaddr, pfn);
+	ret = ti->type->direct_access(ti, pgoff, nr_pages, kaddr, pfn);
 
  out:
 	dm_put_live_table(md, srcu_idx);

Hey Greg,

It turns out that this series isn't needed for the v4.9-stable tree. The outcome we want with this series is to prevent DM devices made of one DAX-capable namespace and one DAX-incapable block device from using DAX.

We needed patches to prevent this from happening in v4.14-stable and v4.17-stable, but v4.9-stable already does the correct thing.

For those interested in the details, the DM device doesn't have QUEUE_FLAG_DAX set per the check in dm_setup_md_queue(), and we notice that in bdev_direct_access() via the blk_queue_dax() check. This causes the filesystem to see that DAX isn't supported.

So, it looks like the bug was introduced when that check was removed from bdev_direct_access(), not back when DAX support was initially introduced in DM like I thought previously.

Thanks,
- Ross