Date:	Mon, 2 Feb 2015 10:30:28 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Ganesh Mahendran <opensource.ganesh@...il.com>
Subject: Re: [PATCH v1 2/2] zram: remove init_lock in zram_make_request

Hey Sergey,

On Sun, Feb 01, 2015 at 11:50:36PM +0900, Sergey Senozhatsky wrote:
> Hello Minchan,
> 
> the idea looks good and this is something I was trying to do, except
> that I used kref.
> 
> some review nitpicks are below. I also posted modified version of your
> patch so that will save some time.

Thanks a lot!

> 
> 
> >  static inline int init_done(struct zram *zram)
> >  {
> > -	return zram->meta != NULL;
> > +	return zram->disksize != 0;
> 
> we don't set ->disksize to 0 when create device. and I think
> it's better to use refcount here, but set it to 0 during device creation.
> (see the patch below)

There was a reason I didn't use refcount there.
I should have written down it.

We need something to prevent further I/O handling on other CPUs.
Otherwise, it's livelock. For example, new 'A' I/O rw path on CPU 1
can see non-zero refcount if another CPU is going on rw.
Then, another new 'B' I/O rw path on CPU 2 can see non-zero refcount
if A I/O is going on. Then, another new 'C' I/O rw path on CPU 3 can
see non-zero refcount if B I/O is going on. Finally, 'A' IO is done
on CPU 1 and next I/O 'D' on CPU 1 can see non-zero refcount because
'C' on CPU 3 is going on. Infinite loop.

> 
> > +static inline bool zram_meta_get(struct zram_meta *meta)
> > +{
> > +	if (!atomic_inc_not_zero(&meta->refcount))
> > +		return false;
> > +	return true;
> > +}
> 
> I've changed it to likely case first: `if recount return true'

Thanks!

> 
> >  static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >  {
> > +	struct zram_meta *meta;
> > +	u64 disksize;
> 
> not needed. (see the patch below).
> 
> > +
> >  	down_write(&zram->init_lock);
> >  
> >  	zram->limit_pages = 0;
> > @@ -728,14 +750,20 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >  		return;
> >  	}
> >  
> > +	meta = zram->meta;
> > +
> >  	zcomp_destroy(zram->comp);
> 
> we can't destoy zcomp before we see IO completion.

True.

> 
> >  	zram->max_comp_streams = 1;
> 
> we better keep original comp_streams number before we see IO completion.
> we don't know how many RW ops we have, so completion can happen earlier.

Yeb.

> 
> > -	zram_meta_free(zram->meta, zram->disksize);
> > -	zram->meta = NULL;
> > +	disksize = zram->disksize;
> > +	zram_meta_put(meta);
> > +	/* Read/write handler will not handle further I/O operation. */
> > +	zram->disksize = 0;
> 
> I keep it on its current position. (see below)
> 
> > +	wait_for_completion(&meta->complete);
> > +	/* I/O operation under all of CPU are done so let's free */
> > +	zram_meta_free(zram->meta, disksize);
> >  	/* Reset stats */
> >  	memset(&zram->stats, 0, sizeof(zram->stats));
> >  
> > -	zram->disksize = 0;
> >  	if (reset_capacity)
> >  		set_capacity(zram->disk, 0);
> >  
> > @@ -908,23 +936,25 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> >  {
> >  	struct zram *zram = queue->queuedata;
> >  
> > -	down_read(&zram->init_lock);
> > -	if (unlikely(!init_done(zram)))
> > +	if (unlikely(!zram_meta_get(zram->meta)))
> >  		goto error;
> >  
> > +	if (unlikely(!init_done(zram)))
> > +		goto put_meta;
> > +
> 
> here and later:
> we can't take zram_meta_get() first and then check for init_done(zram),
> because ->meta can be NULL, so it fill be ->NULL->refcount.

True.
Actually, it was totally RFC I forgot adding the tag in the night but I can't
escape from my shame with the escuse. Thanks!

> 
> let's keep ->completion and ->refcount in zram and rename zram_meta_[get|put]
> to zram_[get|put].

Good idea but still want to name it as zram_meta_get/put because zram_get naming
might confuse struct zram's refcount rather than zram_meta. :)

> 
> 
> 
> 
> please review a bit modified version of your patch.
> 
> /* the patch also reogranizes a bit order of struct zram members, to move
> member that we use more often together and to avoid paddings. nothing
> critical here. */
> 
> 
> next action items are:
> -- we actually can now switch from init_lock in some _show() fucntion to
> zram_get()/zram_put()
> -- address that theoretical and very unlikely in real live race condition
> of umount-reset vs. mount-rw.
> 
> 
> no concerns about performance of this version -- we probably will not get
> any faster than that.
> 
> 
> thanks a lot for your effort!
> 
> ---
> 
>  drivers/block/zram/zram_drv.c | 82 ++++++++++++++++++++++++++++++-------------
>  drivers/block/zram/zram_drv.h | 17 ++++-----
>  2 files changed, 66 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index aa5a4c5..6916790 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -44,7 +44,7 @@ static const char *default_compressor = "lzo";
>  static unsigned int num_devices = 1;
>  
>  #define ZRAM_ATTR_RO(name)						\
> -static ssize_t name##_show(struct device *d,		\
> +static ssize_t name##_show(struct device *d,				\
>  				struct device_attribute *attr, char *b)	\
>  {									\
>  	struct zram *zram = dev_to_zram(d);				\
> @@ -55,7 +55,7 @@ static DEVICE_ATTR_RO(name);
>  
>  static inline int init_done(struct zram *zram)
>  {
> -	return zram->meta != NULL;
> +	return atomic_read(&zram->refcount);
>  }
>  
>  static inline struct zram *dev_to_zram(struct device *dev)
> @@ -358,6 +358,23 @@ out_error:
>  	return NULL;
>  }
>  
> +static inline bool zram_get(struct zram *zram)
> +{
> +	if (atomic_inc_not_zero(&zram->refcount))
> +		return true;
> +	return false;
> +}
> +
> +/*
> + * We want to free zram_meta in process context to avoid
> + * deadlock between reclaim path and any other locks
> + */
> +static inline void zram_put(struct zram *zram)
> +{
> +	if (atomic_dec_and_test(&zram->refcount))
> +		complete(&zram->io_done);
> +}
> +
>  static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
>  {
>  	if (*offset + bvec->bv_len >= PAGE_SIZE)
> @@ -719,6 +736,9 @@ static void zram_bio_discard(struct zram *zram, u32 index,
>  
>  static void zram_reset_device(struct zram *zram, bool reset_capacity)
>  {
> +	struct zram_meta *meta;
> +	struct zcomp *comp;
> +
>  	down_write(&zram->init_lock);
>  
>  	zram->limit_pages = 0;
> @@ -728,14 +748,21 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
>  		return;
>  	}
>  
> -	zcomp_destroy(zram->comp);
> -	zram->max_comp_streams = 1;
> -	zram_meta_free(zram->meta, zram->disksize);
> -	zram->meta = NULL;
> +	meta = zram->meta;
> +	comp = zram->comp;
> +	/* ->refcount will go down to 0 eventually */
> +	zram_put(zram);
> +
> +	wait_for_completion(&zram->io_done);
> +	/* I/O operation under all of CPU are done so let's free */
> +	zram_meta_free(meta, disksize);
> +	zcomp_destroy(comp);
> +
>  	/* Reset stats */
>  	memset(&zram->stats, 0, sizeof(zram->stats));
> -
>  	zram->disksize = 0;
> +	zram->max_comp_streams = 1;
> +
>  	if (reset_capacity)
>  		set_capacity(zram->disk, 0);
>  
> @@ -783,6 +810,8 @@ static ssize_t disksize_store(struct device *dev,
>  		goto out_destroy_comp;
>  	}
>  
> +	init_completion(&zram->io_done);
> +	atomic_set(&zram->refcount, 1);
>  	zram->meta = meta;
>  	zram->comp = comp;
>  	zram->disksize = disksize;
> @@ -795,7 +824,6 @@ static ssize_t disksize_store(struct device *dev,
>  	 * so that revalidate_disk always sees up-to-date capacity.
>  	 */
>  	revalidate_disk(zram->disk);
> -
>  	return len;
>  
>  out_destroy_comp:
> @@ -908,23 +936,24 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
>  {
>  	struct zram *zram = queue->queuedata;
>  
> -	down_read(&zram->init_lock);
> -	if (unlikely(!init_done(zram)))
> +	if (unlikely(!zram_get(zram)))
>  		goto error;
>  
> +	if (unlikely(!init_done(zram)))
> +		goto put_zram;
> +
>  	if (!valid_io_request(zram, bio->bi_iter.bi_sector,
>  					bio->bi_iter.bi_size)) {
>  		atomic64_inc(&zram->stats.invalid_io);
> -		goto error;
> +		goto put_zram;
>  	}
>  
>  	__zram_make_request(zram, bio);
> -	up_read(&zram->init_lock);
> -
> +	zram_put(zram);
>  	return;
> -
> +put_zram:
> +	zram_put(zram);
>  error:
> -	up_read(&zram->init_lock);
>  	bio_io_error(bio);
>  }
>  
> @@ -946,21 +975,22 @@ static void zram_slot_free_notify(struct block_device *bdev,
>  static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  		       struct page *page, int rw)
>  {
> -	int offset, err;
> +	int offset, err = -EIO;
>  	u32 index;
>  	struct zram *zram;
>  	struct bio_vec bv;
>  
>  	zram = bdev->bd_disk->private_data;
> +	if (unlikely(!zram_get(zram)))
> +		goto out;
> +
> +	if (unlikely(!init_done(zram)))
> +		goto put_zram;
> +
>  	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
>  		atomic64_inc(&zram->stats.invalid_io);
> -		return -EINVAL;
> -	}
> -
> -	down_read(&zram->init_lock);
> -	if (unlikely(!init_done(zram))) {
> -		err = -EIO;
> -		goto out_unlock;
> +		err = -EINVAL;
> +		goto put_zram;
>  	}
>  
>  	index = sector >> SECTORS_PER_PAGE_SHIFT;
> @@ -971,8 +1001,9 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  	bv.bv_offset = 0;
>  
>  	err = zram_bvec_rw(zram, &bv, index, offset, rw);
> -out_unlock:
> -	up_read(&zram->init_lock);
> +put_zram:
> +	zram_put(zram);
> +out:
>  	/*
>  	 * If I/O fails, just return error(ie, non-zero) without
>  	 * calling page_endio.
> @@ -1041,6 +1072,7 @@ static int create_device(struct zram *zram, int device_id)
>  	int ret = -ENOMEM;
>  
>  	init_rwsem(&zram->init_lock);
> +	atomic_set(&zram->refcount, 0);
>  
>  	zram->queue = blk_alloc_queue(GFP_KERNEL);
>  	if (!zram->queue) {
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index b05a816..7138c82 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -100,24 +100,25 @@ struct zram_meta {
>  
>  struct zram {
>  	struct zram_meta *meta;
> +	struct zcomp *comp;
>  	struct request_queue *queue;
>  	struct gendisk *disk;
> -	struct zcomp *comp;
> -
>  	/* Prevent concurrent execution of device init, reset and R/W request */
>  	struct rw_semaphore init_lock;
>  	/*
> -	 * This is the limit on amount of *uncompressed* worth of data
> -	 * we can store in a disk.
> +	 * the number of pages zram can consume for storing compressed data
>  	 */
> -	u64 disksize;	/* bytes */
> +	unsigned long limit_pages;
> +	atomic_t refcount;
>  	int max_comp_streams;
> +
>  	struct zram_stats stats;
> +	struct completion io_done; /* notify IO under all of cpu are done */
>  	/*
> -	 * the number of pages zram can consume for storing compressed data
> +	 * This is the limit on amount of *uncompressed* worth of data
> +	 * we can store in a disk.
>  	 */
> -	unsigned long limit_pages;
> -
> +	u64 disksize;	/* bytes */
>  	char compressor[10];
>  };
>  #endif
> 

-- 
Kind regards,
Minchan Kim
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