Messages in this thread Patch in this message | | | Date | Fri, 16 Sep 2016 09:07:48 +1000 | From | Dave Chinner <> | Subject | Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs |
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On Thu, Sep 15, 2016 at 10:01:03AM -0700, Dan Williams wrote: > On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig <hch@lst.de> wrote: > > On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote: > >> The DAX property, page cache bypass, of a VMA is only detectable via the > >> vma_is_dax() helper to check the S_DAX inode flag. However, this is > >> only available internal to the kernel and is a property that userspace > >> applications would like to interrogate. > > > > They have absolutely no business knowing such an implementation detail. > > Hasn't that train already left the station with FS_XFLAG_DAX?
No, that's an admin flag, not a runtime hint for applications. Just because that flag is set on an inode, it does not mean that DAX is actually in use - it will be ignored if the backing dev is not dax capable.
> The other problem with hiding the DAX property is that it turns out to > not be a transparent acceleration feature. See xfs/086 xfs/088 > xfs/089 xfs/091 which fail with DAX and, as far as I understand, it is > due to the fact that DAX disallows delayed allocation behavior.
Which is not a bug, nor is it something that app developers should be surprised by.
i.e. Subtle differences in error reporting behaviour occur in filesystems /all the time/. Run the test on a non-dax filesystem with an extent size hint. It fails /exactly the same way as DAX/. Run it with direct IO - fails the same way as DAX. Run it with synchronous writes - it fails the same way as DAX.
IOWs, if an app can't handle the way DAX reports errors, then they are /broken/. Delayed allocation requires checking the return value of fsync() or close() to capture the allocation error - many more apps get that wrong than the ones that expect the immediate errors from write()...
Anyway: to domeonstrate that the nothign is actually broken, and you might sometimes need to fix tests and send patches to fstests@vger.kernel.org, this makes xfs/086 pass for me on DAX:
--- a/tests/xfs/086 +++ b/tests/xfs/086 @@ -96,7 +96,8 @@ _scratch_mount echo "+ modify files" for x in `seq 1 64`; do - $XFS_IO_PROG -f -c "pwrite -S 0x62 0 ${blksz}" "${TESTFILE}.${x}" >> $seqres.full + $XFS_IO_PROG -f -c "pwrite -S 0x62 0 ${blksz}" "${TESTFILE}.${x}" \ + >> $seqres.full 2>&1 done umount "${SCRATCH_MNT}" Cheers, Dave.
-- Dave Chinner david@fromorbit.com
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