Message-ID: <20150126141709.GA985@swordfish>
Date:	Mon, 26 Jan 2015 23:17:09 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...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

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
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