Re: [PATCH v3 13/14] filesystem-dax: gate calls to dax_flush() on QUEUE_FLAG_WC

[Jan Kara]

On Fri 09-06-17 13:24:56, Dan Williams wrote:
> Some platforms arrange for cpu caches to be flushed on power-fail. On
> those platforms there is no requirement that the kernel track and flush
> potentially dirty cache lines. Given that we still insert entries into
> the radix for locking purposes this patch only disables the cache flush
> loop, not the dirty tracking.
> 
> Userspace can override the default cache setting via the block device
> queue "write_cache" attribute in sysfs.
> 
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

...

> -	dax_flush(dax_dev, pgoff, kaddr, size);
> +	if (test_bit(QUEUE_FLAG_WC, &bdev->bd_queue->queue_flags))
> +		dax_flush(dax_dev, pgoff, kaddr, size);

IMHO the check belongs into dax_flush() similarly as blkdev_issue_flush()
takes silently handles whether the flush is actually needed or not.

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR