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Date:   Thu, 16 May 2019 11:22:49 -0700
From:   Dan Williams <>
To:     Vaibhav Jain <>
Cc:     Andrew Morton <>,
        Vishal Verma <>,
        Keith Busch <>,
        Dave Jiang <>,
        "Aneesh Kumar K.V" <>,
        linux-nvdimm <>,
        Linux Kernel Mailing List <>,
        Chandan Rajendra <>,
        Mike Snitzer <>
Subject: Re: [PATCH] dax: Fix last_page check in __bdev_dax_supported()

On Wed, May 15, 2019 at 10:55 PM Vaibhav Jain <> wrote:
> Presently __bdev_dax_supported() checks if first sector of last
> page ( last_page ) on the block device is aligned to page
> boundary. However the code to compute 'last_page' assumes that there
> are 8 sectors/page assuming a 4K page-size.
> This assumption breaks on architectures which use a different page
> size specifically PPC64 where page-size == 64K. Hence a warning is
> seen while trying to mount a xfs/ext4 file-system with dax enabled:
> $ sudo mount -o dax /dev/pmem0 /mnt/pmem
> XFS (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
> XFS (pmem0): DAX unsupported by block device. Turning off DAX.
> The patch fixes this issue by updating calculation of 'last_var' to
> take into account number-of-sectors/page instead of assuming it to be
> '8'.

Yes, I noticed this too and fixed it up in a wider change that also
allows device-mapper to validate each component device. Does this
patch work for you?

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