RE: [External] Re: [PATCH 1/1] dax: Fix stack overflow when mounting fsdax pmem device

[Adrian Huang12 <ahuang12@lenovo.com>]

> -----Original Message-----
> From: Jan Kara <jack@suse.cz>
> 
> I'm not sure how you can get __generic_fsdax_supported() called for dm-0?
> Possibly because there's another dm device stacked on top of it and
> dm_table_supports_dax() calls generic_fsdax_supported()? That actually seems
> to be a bug in dm_table_supports_dax() (device_supports_dax() in particular).
> I'd think it should be calling dax_supported() instead of
> generic_fsdax_supported() so that proper device callback gets called when
> determining whether a device supports DAX or not.
> 

Yes, you're right. There's another dm device stacked on top of it. 

When applying the following patch and running 'lvm2-testsuite --only activate-minor.sh', the following error messages are observed.

dm-3: error: dax access failed (-95)
dm-3: error: dax access failed (-95)
dm-3: error: dax access failed (-95)

The commands 'lvchange $vg/foo -My --major=255 --minor=123' and 'lvchange $vg/foo -a y' in activate-minor.sh (https://fossies.org/linux/LVM2/test/shell/activate-minor.sh) create another dm device (dm-123) on top of dm-3. Please see the following command output.

# ls -l /dev/mapper
total 0
lrwxrwxrwx. 1 root root       7 Sep 16 02:12 LVMTEST14781pv1 -> ../dm-3
lrwxrwxrwx. 1 root root       9 Sep 16 02:12 LVMTEST14781vg-foo -> ../dm-123
crw-------.      1 root root      10, 236 Sep 16 01:41 control
lrwxrwxrwx. 1 root root       7 Sep 16 01:41 rhel-home -> ../dm-2
lrwxrwxrwx. 1 root root       7 Sep 16 01:41 rhel-root -> ../dm-0
lrwxrwxrwx. 1 root root       7 Sep 16 01:41 rhel-swap -> ../dm-1

# ls -l /dev/dm*
brw-rw----. 1 root disk 253,   0 Sep 16 01:41 /dev/dm-0
brw-rw----. 1 root disk 253,   1 Sep 16 01:41 /dev/dm-1
brw-rw----. 1 root disk 253, 123 Sep 16 02:12 /dev/dm-123
brw-rw----. 1 root disk 253,   2 Sep 16 01:41 /dev/dm-2
brw-rw----. 1 root disk 253,   3 Sep 16 02:12 /dev/dm-3

# dmsetup table
rhel-home: 0 344326144 linear 8:19 16345088
LVMTEST14781vg-foo: 0 1024 linear 253:3 2048
rhel-swap: 0 16343040 linear 8:19 2048
rhel-root: 0 104857600 linear 8:19 360671232
LVMTEST14781pv1: 0 69632 linear 1:0 0

I also use trace-cmd tool (command: trace-cmd record -p function -l '*dax*'  -l '*dm_*' -l 'linear_*')  to record the whole call path:
   dm_get_md_type
   dm_table_supports_dax
      linear_iterate_devices
      device_supports_dax
         __generic_fsdax_supported (dax_dev is valid for dm-3)
            dm_dax_direct_access
               dax_get_private
               dm_dax_get_live_target
               dm_table_find_target
               linear_dax_direct_access
                  bdev_dax_pgoff
                  dax_direct_access (dax_dev is NULL for physical device. Return -EOPNOTSUPP)
            dm_dax_direct_access
               dax_get_private
               dm_dax_get_live_target
               dm_table_find_target
               linear_dax_direct_access
                  bdev_dax_pgoff
                  dax_direct_access (dax_dev is NULL for physical device. Return -EOPNOTSUPP)

Please find the attachment for the full log. You can see three dm_table_supports_dax() calls in the attachment, which aligns with the dmesg output (three dax error messages). 

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e5767c83ea23..11d0541e6f8f 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -85,6 +85,12 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
                return false;
        }

+       if (!dax_dev) {
+               pr_debug("%s: error: dax unsupported by block device\n",
+                               bdevname(bdev, buf));
+               return false;
+       }
+
        err = bdev_dax_pgoff(bdev, start, PAGE_SIZE, &pgoff);
        if (err) {
                pr_info("%s: error: unaligned partition for dax\n",
@@ -100,12 +106,6 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
                return false;
        }

-       if (!dax_dev || !bdev_dax_supported(bdev, blocksize)) {
-               pr_debug("%s: error: dax unsupported by block device\n",
-                               bdevname(bdev, buf));
-               return false;
-       }
-
        id = dax_read_lock();
        len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
        len2 = dax_direct_access(dax_dev, pgoff_end, 1, &end_kaddr, &end_pfn);

-- Adrian
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