linux-kernel - Re: [PATCH 1/4] bdi: Track users that require stable page writes

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Date:	Wed, 21 Nov 2012 05:56:24 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	axboe@...nel.dk, lucho@...kov.net, jack@...e.cz, ericvh@...il.com,
	tytso@....edu, rminnich@...dia.gov, viro@...iv.linux.org.uk,
	martin.petersen@...cle.com, neilb@...e.de, david@...morbit.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
	v9fs-developer@...ts.sourceforge.net, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/4] bdi: Track users that require stable page writes

> +static inline void bdi_require_stable_pages(struct backing_dev_info *bdi)
> +{
> +	bdi->capabilities |= BDI_CAP_STABLE_WRITES;
> +}
> +
> +static inline void bdi_unrequire_stable_pages(struct backing_dev_info *bdi)
> +{
> +	bdi->capabilities &= ~BDI_CAP_STABLE_WRITES;
> +}

Any reason to provide these wrappers while other BDI_CAP_ values don't
have it/

Also what protects bdi->capabilities against concurrent updates now that
it gets modified at runtime?

> +static inline void queue_require_stable_pages(struct request_queue *q)
> +{
> +	bdi_require_stable_pages(&q->backing_dev_info);
> +}
> +
> +static inline void queue_unrequire_stable_pages(struct request_queue *q)
> +{
> +	bdi_unrequire_stable_pages(&q->backing_dev_info);
> +}
> +
> +static inline int queue_requires_stable_pages(struct request_queue *q)
> +{
> +	return bdi_cap_stable_pages_required(&q->backing_dev_info);
> +}

Independent of the above I see no point in these wrappers that just
provide a single dereference.

> +static ssize_t stable_pages_required_store(struct device *dev,
> +					   struct device_attribute *attr,
> +					   const char *buf, size_t count)

Can you add a rationale on why we'd want to allow users to change the
value?  I can't really think of any.

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