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

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

On Thu, Sep 17, 2020 at 10:49 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Thu, Sep 17, 2020 at 7:58 AM Huang Adrian <adrianhuang0701@gmail.com> wrote:
> >
> > On Thu, Sep 17, 2020 at 6:42 PM Jan Kara <jack@suse.cz> wrote:
> > >
> > > On Thu 17-09-20 02:28:57, Dan Williams wrote:
> > > > 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;
> > > >  }
> > >
> > > Yeah, thanks for this! I was actually looking into this when writing the
> > > patch and somehow convinced myself we will always be called through
> > > bdev_dax_supported() which does dax_read_lock() for us. But apparently I
> > > was wrong...
> >
> > Hold on. This patch hit another regression when I ran the full test of
> > the lvm2-testsuite tool today.
>
> Are you sure it's this patch?
>
> The dax_read_lock() should have zero interaction with the
> synchronize_srcu() that __dm_suspend() performs. The too srcu domains
> should not conflict... I don't even see a dax_read_lock() in this
> path.

Confirmed. I hit the hang in the lvm2-testsuite on vanilla 5.9-rc5.
So, I'm going to proceed with Jan's patch plus my fixup.
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