Re: [PATCH 1/3] bdi: Track users that require stable page writes

[Boaz Harrosh <bharrosh@panasas.com>]

On 11/01/2012 12:58 AM, Darrick J. Wong wrote:
> This creates a per-backing-device counter that tracks the number of users which
> require pages to be held immutable during writeout.  Eventually it will be used
> to waive wait_for_page_writeback() if nobody requires stable pages.
> 

There is two things I do not like:
1. Please remind me why we need the users counter?
   If the device needs it, it will always be needed. The below 
   queue_unrequire_stable_pages call at blk_integrity_unregister
   only happens at device destruction time, no?

   It was also said that maybe individual filesystems would need
   stable pages where other FSs using the same BDI do not. But
   since the FS is at the driving seat in any case, it can just
   do wait_for_writeback() regardless and can care less about
   the bdi flag and the other FSs. Actually all those FSs already
   do this this, and do not need any help. So this reason is
   mute.

2. I hate the atomic_read for every mkwrite. I think we can do
   better, since as you noted it can never be turned off, only
   on, at init time. And because of 1. above it is not dynamic.
   I think I like your previous simple bool better.

But I do like a lot the new request_queue API you added here.
Thanks

Boaz
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  Documentation/ABI/testing/sysfs-class-bdi |    7 +++++
>  block/blk-integrity.c                     |    4 +++
>  include/linux/backing-dev.h               |   16 ++++++++++++
>  include/linux/blkdev.h                    |   10 ++++++++
>  mm/backing-dev.c                          |   38 +++++++++++++++++++++++++++++
>  5 files changed, 75 insertions(+)
> 
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-bdi b/Documentation/ABI/testing/sysfs-class-bdi
> index 5f50097..218a618 100644
> --- a/Documentation/ABI/testing/sysfs-class-bdi
> +++ b/Documentation/ABI/testing/sysfs-class-bdi
> @@ -48,3 +48,10 @@ max_ratio (read-write)
>  	most of the write-back cache.  For example in case of an NFS
>  	mount that is prone to get stuck, or a FUSE mount which cannot
>  	be trusted to play fair.
> +
> +stable_pages_required (read-write)
> +
> +	If set, the backing device requires that all pages comprising a write
> +	request must not be changed until writeout is complete.  The system
> +	administrator can turn this on if the hardware does not do so already.
> +	However, once enabled, this flag cannot be disabled.
> diff --git a/block/blk-integrity.c b/block/blk-integrity.c
> index da2a818..cf2dd95 100644
> --- a/block/blk-integrity.c
> +++ b/block/blk-integrity.c
> @@ -420,6 +420,8 @@ int blk_integrity_register(struct gendisk *disk, struct blk_integrity *template)
>  	} else
>  		bi->name = bi_unsupported_name;
>  
> +	queue_require_stable_pages(disk->queue);
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL(blk_integrity_register);
> @@ -438,6 +440,8 @@ void blk_integrity_unregister(struct gendisk *disk)
>  	if (!disk || !disk->integrity)
>  		return;
>  
> +	queue_unrequire_stable_pages(disk->queue);
> +
>  	bi = disk->integrity;
>  
>  	kobject_uevent(&bi->kobj, KOBJ_REMOVE);
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 2a9a9ab..0554f5d 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -109,6 +109,7 @@ struct backing_dev_info {
>  	struct dentry *debug_dir;
>  	struct dentry *debug_stats;
>  #endif
> +	atomic_t stable_page_users;
>  };
>  
>  int bdi_init(struct backing_dev_info *bdi);
> @@ -307,6 +308,21 @@ long wait_iff_congested(struct zone *zone, int sync, long timeout);
>  int pdflush_proc_obsolete(struct ctl_table *table, int write,
>  		void __user *buffer, size_t *lenp, loff_t *ppos);
>  
> +static inline void bdi_require_stable_pages(struct backing_dev_info *bdi)
> +{
> +	atomic_inc(&bdi->stable_page_users);
> +}
> +
> +static inline void bdi_unrequire_stable_pages(struct backing_dev_info *bdi)
> +{
> +	atomic_dec(&bdi->stable_page_users);
> +}
> +
> +static inline bool bdi_cap_stable_pages_required(struct backing_dev_info *bdi)
> +{
> +	return atomic_read(&bdi->stable_page_users) > 0;
> +}
> +
>  static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
>  {
>  	return !(bdi->capabilities & BDI_CAP_NO_WRITEBACK);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 1756001..bf927c0 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -458,6 +458,16 @@ struct request_queue {
>  				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
>  				 (1 << QUEUE_FLAG_ADD_RANDOM))
>  
> +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 void queue_lockdep_assert_held(struct request_queue *q)
>  {
>  	if (q->queue_lock)
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index d3ca2b3..dd9f5ed 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -221,12 +221,48 @@ static ssize_t max_ratio_store(struct device *dev,
>  }
>  BDI_SHOW(max_ratio, bdi->max_ratio)
>  
> +static ssize_t stable_pages_required_store(struct device *dev,
> +					   struct device_attribute *attr,
> +					   const char *buf, size_t count)
> +{
> +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> +	unsigned int spw;
> +	ssize_t ret;
> +
> +	ret = kstrtouint(buf, 10, &spw);
> +	if (ret < 0)
> +		return ret;
> +
> +	/*
> +	 * SPW could be enabled due to hw requirement, so don't
> +	 * let users disable it.
> +	 */
> +	if (bdi_cap_stable_pages_required(bdi) && spw == 0)
> +		return -EINVAL;
> +
> +	if (spw != 0)
> +		atomic_inc(&bdi->stable_page_users);
> +
> +	return count;
> +}
> +
> +static ssize_t stable_pages_required_show(struct device *dev,
> +					  struct device_attribute *attr,
> +					  char *page)
> +{
> +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> +
> +	return snprintf(page, PAGE_SIZE-1, "%d\n",
> +			bdi_cap_stable_pages_required(bdi) ? 1 : 0);
> +}
> +
>  #define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
>  
>  static struct device_attribute bdi_dev_attrs[] = {
>  	__ATTR_RW(read_ahead_kb),
>  	__ATTR_RW(min_ratio),
>  	__ATTR_RW(max_ratio),
> +	__ATTR_RW(stable_pages_required),
>  	__ATTR_NULL,
>  };
>  
> @@ -650,6 +686,8 @@ int bdi_init(struct backing_dev_info *bdi)
>  	bdi->write_bandwidth = INIT_BW;
>  	bdi->avg_write_bandwidth = INIT_BW;
>  
> +	atomic_set(&bdi->stable_page_users, 0);
> +
>  	err = fprop_local_init_percpu(&bdi->completions);
>  
>  	if (err) {
> 


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