Date:	Tue, 27 Jan 2015 11:17:04 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nitin Gupta <ngupta@...are.org>,
	sergey.senozhatsky.work@...il.com
Subject: Re: [PATCH 1/2] zram: free meta out of init_lock

On (01/27/15 01:00), Minchan Kim wrote:
> On Mon, Jan 26, 2015 at 11:17:09PM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (01/26/15 10:33), Minchan Kim wrote:
> > > Hello,
> > > 
> > > On Sat, Jan 24, 2015 at 12:47:07AM +0900, Sergey Senozhatsky wrote:
> > > > On (01/23/15 15:48), Jerome Marchand wrote:
> > > > > On 01/23/2015 03:24 PM, Sergey Senozhatsky wrote:
> > > > > > On (01/23/15 14:58), Minchan Kim wrote:
> > > > > >> We don't need to call zram_meta_free, zcomp_destroy and zs_free
> > > > > >> under init_lock. What we need to prevent race with init_lock
> > > > > >> in reset is setting NULL into zram->meta (ie, init_done).
> > > > > >> This patch does it.
> > > > > >>
> > > > > >> Signed-off-by: Minchan Kim <minchan@...nel.org>
> > > > > >> ---
> > > > > >>  drivers/block/zram/zram_drv.c | 28 ++++++++++++++++------------
> > > > > >>  1 file changed, 16 insertions(+), 12 deletions(-)
> > > > > >>
> > > > > >> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > >> index 9250b3f54a8f..0299d82275e7 100644
> > > > > >> --- a/drivers/block/zram/zram_drv.c
> > > > > >> +++ b/drivers/block/zram/zram_drv.c
> > > > > >> @@ -708,6 +708,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> > > > > >>  {
> > > > > >>  	size_t index;
> > > > > >>  	struct zram_meta *meta;
> > > > > >> +	struct zcomp *comp;
> > > > > >>  
> > > > > >>  	down_write(&zram->init_lock);
> > > > > >>  
> > > > > >> @@ -719,20 +720,10 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> > > > > >>  	}
> > > > > >>  
> > > > > >>  	meta = zram->meta;
> > > > > >> -	/* Free all pages that are still in this zram device */
> > > > > >> -	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> > > > > >> -		unsigned long handle = meta->table[index].handle;
> > > > > >> -		if (!handle)
> > > > > >> -			continue;
> > > > > >> -
> > > > > >> -		zs_free(meta->mem_pool, handle);
> > > > > >> -	}
> > > > > >> -
> > > > > >> -	zcomp_destroy(zram->comp);
> > > > > > 
> > > > > > I'm not so sure about moving zcomp destruction. if we would have detached it
> > > > > > from zram, then yes. otherwise, think of zram ->destoy vs ->init race.
> > > > > > 
> > > > > > suppose,
> > > > > > CPU1 waits for down_write() init lock in disksize_store() with new comp already allocated;
> > > > > > CPU0 detaches ->meta and releases write init lock;
> > > > > > CPU1 grabs the lock and does zram->comp = comp;
> > > > > > CPU0 reaches the point of zcomp_destroy(zram->comp);
> > > > > 
> > > > > I don't see your point: this patch does not call
> > > > > zcomp_destroy(zram->comp) anymore, but zram_destroy(comp), where comp is
> > > > > the old zram->comp.
> > > > 
> > > > 
> > > > oh... yes. sorry! my bad.
> > > > 
> > > > 
> > > > 
> > > > anyway, on a second thought, do we even want to destoy meta out of init_lock?
> > > > 
> > > > I mean, it will let you init new device quicker. but... assume, you have
> > > > 30G zram (or any other bad-enough number). on CPU0 you reset device -- iterate
> > > > over 30G meta->table, etc. out of init_lock.
> > > > on CPU1 you concurrently re-init device and request again 30G.
> > > > 
> > > > how bad that can be?
> > > > 
> > > > 
> > > > 
> > > > diskstore called on already initialised device is also not so perfect.
> > > > we first will try to allocate ->meta (vmalloc pages for another 30G),
> > > > then allocate comp, then down_write() init lock to find out that device
> > > > is initialised and we need to release allocated memory.
> > > > 
> > > > 
> > > > 
> > > > may be we better keep ->meta destruction under init_lock and additionally
> > > > move ->meta and ->comp allocation under init_lock in disksize_store()?
> > > > 
> > > > like the following one:
> > > > 
> > > > ---
> > > > 
> > > >  drivers/block/zram/zram_drv.c | 25 +++++++++++++------------
> > > >  1 file changed, 13 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > index 9250b3f..827ab21 100644
> > > > --- a/drivers/block/zram/zram_drv.c
> > > > +++ b/drivers/block/zram/zram_drv.c
> > > > @@ -765,9 +765,18 @@ static ssize_t disksize_store(struct device *dev,
> > > >  		return -EINVAL;
> > > >  
> > > >  	disksize = PAGE_ALIGN(disksize);
> > > > +	down_write(&zram->init_lock);
> > > > +	if (init_done(zram)) {
> > > > +		up_write(&zram->init_lock);
> > > > +		pr_info("Cannot change disksize for initialized device\n");
> > > > +		return -EBUSY;
> > > > +	}
> > > > +
> > > >  	meta = zram_meta_alloc(zram->disk->first_minor, disksize);
> > > > -	if (!meta)
> > > > -		return -ENOMEM;
> > > > +	if (!meta) {
> > > > +		err = -ENOMEM;
> > > > +		goto out_unlock;
> > > > +	}
> > > >  
> > > >  	comp = zcomp_create(zram->compressor, zram->max_comp_streams);
> > > >  	if (IS_ERR(comp)) {
> > > > @@ -777,13 +786,6 @@ static ssize_t disksize_store(struct device *dev,
> > > >  		goto out_free_meta;
> > > >  	}
> > > >  
> > > > -	down_write(&zram->init_lock);
> > > > -	if (init_done(zram)) {
> > > > -		pr_info("Cannot change disksize for initialized device\n");
> > > > -		err = -EBUSY;
> > > > -		goto out_destroy_comp;
> > > > -	}
> > > > -
> > > >  	zram->meta = meta;
> > > >  	zram->comp = comp;
> > > >  	zram->disksize = disksize;
> > > > @@ -799,11 +801,10 @@ static ssize_t disksize_store(struct device *dev,
> > > >  
> > > >  	return len;
> > > >  
> > > > -out_destroy_comp:
> > > > -	up_write(&zram->init_lock);
> > > > -	zcomp_destroy(comp);
> > > >  out_free_meta:
> > > >  	zram_meta_free(meta);
> > > > +out_unlock:
> > > > +	up_write(&zram->init_lock);
> > > >  	return err;
> > > >  }
> > > >  
> > > 
> > > The init_lock is really troublesome. We can't do call zram_meta_alloc
> > > under init_lock due to lockdep report. Please keep in mind.
> > >
> > 
> > ah... I do recall it, thanks for your reminder.
> > 
> > 
> > > The zram_rw_page is one of the function under reclaim path and hold it
> > > as read_lock while here holds it as write_lock.
> > > It's a false positive so that we might could make shut lockdep up
> > > by annotation but I don't want it but want to work with lockdep rather
> > > than disable. As well, there are other pathes to use init_lock to
> > > protect other data where would be victims of lockdep.
> > > 
> > > I didn't tell the motivation of this patch because it made you busy
> > > guys wasted. Let me tell it now. It was another lockdep report by
> > > kmem_cache_destroy for zsmalloc compaction about init_lock. That's why
> > > the patchset was one of the patch in compaction.
> > >
> > > Yes, the ideal is to remove horrible init_lock of zram in this phase and
> > > make code more simple and clear but I don't want to stuck zsmalloc
> > > compaction by the work.
> > 
> > 
> > > Having said that, I feel it's time to revisit
> > > to remove init_lock.
> > > At least, I will think over to find a solution to kill init_lock.
> > 
> > hm, can't think of anything quick...
> > 
> > 	-ss
> 
> Hello guys,
> 
> How about this?
> 
> It's based on Ganesh's patch.
> https://lkml.org/lkml/2015/1/24/50
(I see no similarities with Ganesh's patch)

hm, you probably meant this one https://lkml.org/lkml/2015/1/23/406


at glance this makes things a bit more complicated, so I need to think more.

> From afda9fd2f6c40dd0745d8a6babe78c5cbdceddf5 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@...nel.org>
> Date: Mon, 26 Jan 2015 14:34:10 +0900
> Subject: [RFC] zram: remove init_lock in zram_make_request
> 
> Admin could reset zram during I/O operation going on so we have
> used zram->init_lock as read-side lock in I/O path to prevent
> sudden zram meta freeing.
> 
> However, the init_lock is really troublesome.
> We can't do call zram_meta_alloc under init_lock due to lockdep splat
> because zram_rw_page is one of the function under reclaim path and
> hold it as read_lock while other places in process context hold it
> as write_lock. So, we have used allocation out of the lock to avoid
> lockdep warn but it's not good for readability and fainally, I met
> another lockdep splat between init_lock and cpu_hotpulug from
> kmem_cache_destroy during wokring zsmalloc compaction. :(
> 
> Yes, the ideal is to remove horrible init_lock of zram in rw path.
> This patch removes it in rw path and instead, put init_done bool
> variable to check initialization done with smp_[wmb|rmb] and
> srcu_[un]read_lock to prevent sudden zram meta freeing
> during I/O operation.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  drivers/block/zram/zram_drv.c | 76 +++++++++++++++++++++++++++++--------------
>  drivers/block/zram/zram_drv.h |  5 +++
>  2 files changed, 57 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index a598ada817f0..e06ff975f997 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -32,6 +32,7 @@
>  #include <linux/string.h>
>  #include <linux/vmalloc.h>
>  #include <linux/err.h>
> +#include <linux/srcu.h>
>  
>  #include "zram_drv.h"
>  
> @@ -53,9 +54,16 @@ static ssize_t name##_show(struct device *d,		\
>  }									\
>  static DEVICE_ATTR_RO(name);
>  
> -static inline int init_done(struct zram *zram)
> +static inline bool init_done(struct zram *zram)
>  {
> -	return zram->meta != NULL;
> +	/*
> +	 * init_done can be used without holding zram->init_lock in
> +	 * read/write handler(ie, zram_make_request) but we should make sure
> +	 * that zram->init_done should set up after meta initialization is
> +	 * done. Look at disksize_store.
> +	 */
> +	smp_rmb();
> +	return zram->init_done;

->init_done returns back :)
can we rely on write ->meta; wmb; --- rmb; read ->meta?

how much performance do we lose on barriers?

>  }
>  
>  static inline struct zram *dev_to_zram(struct device *dev)
> @@ -326,6 +334,10 @@ static void zram_meta_free(struct zram_meta *meta)
>  	kfree(meta);
>  }
>  
> +static void rcu_zram_do_nothing(struct rcu_head *unused)
> +{
> +}
> +
>  static struct zram_meta *zram_meta_alloc(int device_id, u64 disksize)
>  {
>  	char pool_name[8];
> @@ -726,11 +738,8 @@ 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->meta = NULL;
>  	/* Reset stats */
>  	memset(&zram->stats, 0, sizeof(zram->stats));
>  
> @@ -738,8 +747,12 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
>  	if (reset_capacity)
>  		set_capacity(zram->disk, 0);
>  
> +	zram->init_done = false;

missing wmb?

I think we also better put comments after every wmb/rmb. like

	smp_wmb(); /* pairs with rmb() in foo() */


> +	call_srcu(&zram->srcu, &zram->rcu, rcu_zram_do_nothing);
> +	synchronize_srcu(&zram->srcu);
> +	zram_meta_free(zram->meta);
> +	zcomp_destroy(zram->comp);
>  	up_write(&zram->init_lock);
> -
>  	/*
>  	 * Revalidate disk out of the init_lock to avoid lockdep splat.
>  	 * It's okay because disk's capacity is protected by init_lock
> @@ -762,10 +775,19 @@ static ssize_t disksize_store(struct device *dev,
>  	if (!disksize)
>  		return -EINVAL;
>  
> +	down_write(&zram->init_lock);
> +	if (init_done(zram)) {
> +		pr_info("Cannot change disksize for initialized device\n");
> +		up_write(&zram->init_lock);
> +		return -EBUSY;
> +	}
> +
>  	disksize = PAGE_ALIGN(disksize);
>  	meta = zram_meta_alloc(zram->disk->first_minor, disksize);
> -	if (!meta)
> +	if (!meta) {
> +		up_write(&zram->init_lock);
>  		return -ENOMEM;
> +	}
>  
>  	comp = zcomp_create(zram->compressor, zram->max_comp_streams);
>  	if (IS_ERR(comp)) {
> @@ -775,17 +797,17 @@ static ssize_t disksize_store(struct device *dev,
>  		goto out_free_meta;
>  	}
>  
> -	down_write(&zram->init_lock);
> -	if (init_done(zram)) {
> -		pr_info("Cannot change disksize for initialized device\n");
> -		err = -EBUSY;
> -		goto out_destroy_comp;
> -	}
> -
>  	zram->meta = meta;
>  	zram->comp = comp;
>  	zram->disksize = disksize;
>  	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> +	/*
> +	 * Store operation of struct zram fields should complete
> +	 * before init_done set up because zram_bvec_rw doesn't
> +	 * hold an zram->init_lock.
> +	 */
> +	smp_wmb();
> +	zram->init_done = true;
>  	up_write(&zram->init_lock);
>  
>  	/*
> @@ -797,10 +819,8 @@ static ssize_t disksize_store(struct device *dev,
>  
>  	return len;
>  
> -out_destroy_comp:
> -	up_write(&zram->init_lock);
> -	zcomp_destroy(comp);
>  out_free_meta:
> +	up_write(&zram->init_lock);
>  	zram_meta_free(meta);

 zram_meta_free(meta);
 up_write(&zram->init_lock);

 ?

>  	return err;
>  }
> @@ -905,9 +925,10 @@ out:
>   */
>  static void zram_make_request(struct request_queue *queue, struct bio *bio)
>  {
> +	int idx;
>  	struct zram *zram = queue->queuedata;
>  
> -	down_read(&zram->init_lock);
> +	idx = srcu_read_lock(&zram->srcu);
>  	if (unlikely(!init_done(zram)))
>  		goto error;
>  
> @@ -918,12 +939,12 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
>  	}
>  
>  	__zram_make_request(zram, bio);
> -	up_read(&zram->init_lock);
> +	srcu_read_unlock(&zram->srcu, idx);
>  
>  	return;
>  
>  error:
> -	up_read(&zram->init_lock);
> +	srcu_read_unlock(&zram->srcu, idx);
>  	bio_io_error(bio);
>  }
>  
> @@ -945,18 +966,20 @@ 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, idx;
>  	u32 index;
>  	struct zram *zram;
>  	struct bio_vec bv;
>  
>  	zram = bdev->bd_disk->private_data;
> +	idx = srcu_read_lock(&zram->srcu);
> +
>  	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
>  		atomic64_inc(&zram->stats.invalid_io);
> +		srcu_read_unlock(&zram->srcu, idx);
>  		return -EINVAL;
>  	}
>  
> -	down_read(&zram->init_lock);
>  	if (unlikely(!init_done(zram))) {
>  		err = -EIO;
>  		goto out_unlock;
> @@ -971,7 +994,7 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  
>  	err = zram_bvec_rw(zram, &bv, index, offset, rw);
>  out_unlock:
> -	up_read(&zram->init_lock);
> +	srcu_read_unlock(&zram->srcu, idx);
>  	/*
>  	 * If I/O fails, just return error(ie, non-zero) without
>  	 * calling page_endio.
> @@ -1041,6 +1064,11 @@ static int create_device(struct zram *zram, int device_id)
>  
>  	init_rwsem(&zram->init_lock);
>  
> +	if (init_srcu_struct(&zram->srcu)) {
> +		pr_err("Error initialize srcu for device %d\n", device_id);
> +		goto out;
> +	}
> +
>  	zram->queue = blk_alloc_queue(GFP_KERNEL);
>  	if (!zram->queue) {
>  		pr_err("Error allocating disk queue for device %d\n",
> @@ -1125,8 +1153,8 @@ static void destroy_device(struct zram *zram)
>  
>  	del_gendisk(zram->disk);
>  	put_disk(zram->disk);
> -
>  	blk_cleanup_queue(zram->queue);
> +	cleanup_srcu_struct(&zram->srcu);
>  }
>  
>  static int __init zram_init(void)
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index e492f6bf11f1..2042c310aea8 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -105,8 +105,13 @@ struct zram {
>  	struct gendisk *disk;
>  	struct zcomp *comp;
>  
> +	struct srcu_struct srcu;
> +	struct rcu_head rcu;
> +
>  	/* Prevent concurrent execution of device init, reset and R/W request */
>  	struct rw_semaphore init_lock;
> +	bool init_done;
> +
>  	/*
>  	 * This is the limit on amount of *uncompressed* worth of data
>  	 * we can store in a disk.
> -- 
> 1.9.1
> 
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