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    SubjectRe: [PATCH 0/6] Support DAX for device-mapper dm-linear devices
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    On Mon, Jun 13, 2016 at 5:02 PM, Dan Williams <dan.j.williams@intel.com> wrote:
    > On Mon, Jun 13, 2016 at 4:59 PM, Kani, Toshimitsu <toshi.kani@hpe.com> wrote:
    >> On Mon, 2016-06-13 at 16:18 -0700, Dan Williams wrote:
    >>> Thanks Toshi!
    >>>
    >>> On Mon, Jun 13, 2016 at 3:21 PM, Toshi Kani <toshi.kani@hpe.com> wrote:
    >>> >
    >>> > This patch-set adds DAX support to device-mapper dm-linear devices
    >>> > used by LVM. It works with LVM commands as follows:
    >>> > - Creation of a logical volume with all DAX capable devices (such
    >>> > as pmem) sets the logical volume DAX capable as well.
    >>> > - Once a logical volume is set to DAX capable, the volume may not
    >>> > be extended with non-DAX capable devices.
    >>>
    >>> I don't mind this, but it seems a policy decision that the kernel does
    >>> not need to make. A sufficiently sophisticated user could cope with
    >>> DAX being available at varying LBAs. Would it be sufficient to move
    >>> this policy decision to userspace tooling?
    >>
    >> I think this is a kernel restriction. When a block device is declared as
    >> DAX capable, it should mean that the whole device is DAX capable. So, I
    >> think we need to assure the same to a mapped device.
    >
    > Hmm, but we already violate this with badblocks. The device is DAX
    > capable, but certain LBAs will return an error if direct_access is
    > attempted.

    Nevermind, for this to be useful we would need to fallback to regular
    mmap for a portion of the linear span. That's different than the
    badblocks case.

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