On Sun, Aug 9, 2015 at 2:44 AM, Christoph Hellwig <h...@infradead.org> wrote: > On Fri, Aug 07, 2015 at 04:25:52AM -0400, Ming Lei wrote: >> On Fri, Aug 7, 2015 at 3:43 AM, Christoph Hellwig <h...@infradead.org> wrote: >> > I really disagree with the per-cmd use_dio tracking. >> >> Could you explain it in a bit? >> >> > >> > If we know at setup time that the loop device sector size is smaller >> > than the sector size of the underlying device we should never allow >> > dio, and othewise it should always work for data. >> >> Yes, that is just what I did in v7, and we can only do dio in case >> of 512 byte sector size of backing device(not considering the >> following patches from Hannes). >> >> When sector size of backing device isn't 512, most of transfer(buffered I/O >> and normal dio) is still 4k aligned, that is why I suggest to use per-cmd >> use_dio tracking. >> >> The patch avoids the race between buffered io and dio, doesn't it? >> The introduced cost is trivial and most of times it needn't to wait for >> completion of pending dio. > > All block filesystems can do direct I/O on a _sector size_, not > _block size_ boundary, e.g. for the typical setup of a 4k block size > xfs/btrfs/ext4 file system on a 512 byte sector device you can do 512 > byte aligned direct I/O.
It isn't the problem which lo_drain_pending_dio() in this patch is trying to avoid/fix. Now the logical block size of loop is 512byte, but the sector size of backing device may be 4K, so we can't do 512 byte algined direct I/O to the filesystem in this situation. With runtime switch to buffered I/O we can fix this problem, but most of times it won't fall in this case. > >> > is no need for draining or mode checking for an fsync - FLUSH is always >> > only guranteed to flush out I/O that has completed by the time it's >> > issued. >> >> Could you point it out in the patch? > > Basically your lo_drain_pending_dio() functionality is not needed. For non-512 byte sector size of backing device, it is needed as I described above. Thanks, Ming -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
