Re: [PATCH] dm: Call proper helper to determine dax support

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Sep 16, 2020 at 8:15 AM Jan Kara <jack@suse.cz> wrote:
>
> DM was calling generic_fsdax_supported() to determine whether a device
> referenced in the DM table supports DAX. However this is a helper for "leaf" device drivers so that
> they don't have to duplicate common generic checks. High level code
> should call dax_supported() helper which that calls into appropriate
> helper for the particular device. This problem manifested itself as
> kernel messages:
>
> dm-3: error: dax access failed (-95)
>
> when lvm2-testsuite run in cases where a DM device was stacked on top of
> another DM device.
>
> Fixes: 7bf7eac8d648 ("dax: Arrange for dax_supported check to span multiple devices")
> Tested-by: Adrian Huang <ahuang12@lenovo.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  drivers/dax/super.c   |  4 ++++
>  drivers/md/dm-table.c |  3 +--
>  include/linux/dax.h   | 11 +++++++++--
>  3 files changed, 14 insertions(+), 4 deletions(-)
>
> This patch should go in together with Adrian's
> https://lore.kernel.org/linux-nvdimm/20200916133923.31-1-adrianhuang0701@gmail.com
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e5767c83ea23..b6284c5cae0a 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -325,11 +325,15 @@ EXPORT_SYMBOL_GPL(dax_direct_access);
>  bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
>                 int blocksize, sector_t start, sector_t len)
>  {
> +       if (!dax_dev)
> +               return false;
> +

Hi Jan, Thanks for this.

>         if (!dax_alive(dax_dev))
>                 return false;

One small fixup to quiet lockdep because dax_supported() calls
dax_alive() it expects that dax_read_lock() is held. So I'm testing
with this incremental change:

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index bed1ff0744ec..229f461e7def 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -860,9 +860,14 @@ EXPORT_SYMBOL_GPL(dm_table_set_type);
 int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
                        sector_t start, sector_t len, void *data)
 {
-       int blocksize = *(int *) data;
+       int blocksize = *(int *) data, id;
+       bool rc;

-       return dax_supported(dev->dax_dev, dev->bdev, blocksize, start, len);
+       id = dax_read_lock();
+       rc = dax_supported(dev->dax_dev, dev->bdev, blocksize, start, len);
+       dax_read_unlock(id);
+
+       return rc;
 }
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