On Thu, Feb 14, 2019 at 04:04:48PM -0500, Theodore Y. Ts'o wrote:
> On Mon, Jan 14, 2019 at 03:37:08PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> >
> > Revert bcca9876a3428c10417c660b78933e6e70e8a5f5, because
> > fallocate(PUNCH_HOLE) on block devices was changed to use zeroout
> > instead of discard shortly after block device fallocate was merged.
> > zeroout isn't necessarily a "drop storage" operation like discard is,
> > so we prefer to use that on block devices.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> Thanks, applied.
>
> - Ted
I just noticed this patch, sorry. I think we can still use fallocate,
but we need to set the right flags to make sure it uses discard instead
of zeroout. See fs/block_dev.c
switch (mode) {
case FALLOC_FL_ZERO_RANGE:
case FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE:
error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
GFP_KERNEL, BLKDEV_ZERO_NOUNMAP);
break;
case FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE:
error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
GFP_KERNEL, BLKDEV_ZERO_NOFALLBACK);
break;
case FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE | FALLOC_FL_NO_HIDE_STALE:
error = blkdev_issue_discard(bdev, start >> 9, len >> 9,
GFP_KERNEL, 0);
break;
default:
return -EOPNOTSUPP;
}
So if we want a discard (meaning we want to unallocate the blocks
without necessarily making sure we can't read stale data from it) we
have to use FALLOC_FL_NO_HIDE_STALE.
So the flags would be FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE | FALLOC_FL_NO_HIDE_STALE
Ted, Darrick what do you think ? Can we keep the
bcca9876a3428c10417c660b78933e6e70e8a5f5 commit and just change the
flags ?
-Lukas
