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

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Date:   Wed, 14 Jun 2017 12:46:26 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, Jan Kara <jack@...e.cz>,
        dm-devel@...hat.com, Matthew Wilcox <mawilcox@...rosoft.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Jeff Moyer <jmoyer@...hat.com>, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>, hch@....de
Subject: Re: [PATCH v3 13/14] filesystem-dax: gate calls to dax_flush() on
 QUEUE_FLAG_WC

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@...e.cz>
> Cc: Jeff Moyer <jmoyer@...hat.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Matthew Wilcox <mawilcox@...rosoft.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

...

> -	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@...e.com>
SUSE Labs, CR