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    SubjectRe: [PATCH v3 05/12] fs: remove unneeded IS_DAX() check
    On Tue, Feb 11, 2020 at 08:38:31AM -0800, Ira Weiny wrote:
    > On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote:
    > > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote:
    > > > From: Ira Weiny <ira.weiny@intel.com>
    > > >
    > > > The IS_DAX() check in io_is_direct() causes a race between changing the
    > > > DAX state and creating the iocb flags.
    > > >
    > > > Remove the check because DAX now emulates the page cache API and
    > > > therefore it does not matter if the file state is DAX or not when the
    > > > iocb flags are created.
    > >
    > > This statement is ... weird.
    > >
    > > DAX doesn't "emulate" the page cache API at all
    >
    > ah... yea emulate is a bad word here.
    >
    > > - it has it's own
    > > read/write methods that filesystems call based on the iomap
    > > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths
    > > through the filesystems: the DAX IO path, the direct IO path, and
    > > the buffered IO path.
    > >
    > > Indeed, it seems like this works a bit by luck: Ext4 and XFS always
    > > check IS_DAX(inode) in the read/write_iter methods before checking
    > > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the
    > > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we
    > > forgot to clean up io_is_direct() and it's only due to the ordering
    > > of checks that we went down the DAX path correctly....
    > >
    > > That said, the code change is good, but the commit message needs a
    > > rewrite.
    >
    > How about?
    >
    > <commit msg>
    > fs: Remove unneeded IS_DAX() check
    >
    > The IS_DAX() check in io_is_direct() causes a race between changing the
    > DAX state and creating the iocb flags.

    This is irrelevant - the check is simply wrong and has been since
    ~2016 when we moved DAX to use the iomap infrastructure...

    Cheers,

    Dave.

    --
    Dave Chinner
    david@fromorbit.com

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