Date:	Tue, 27 Jan 2015 00:52:01 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nitin Gupta <ngupta@...are.org>
Subject: Re: [PATCH 1/2] zram: free meta out of init_lock

Hello,

On Mon, Jan 26, 2015 at 03:34:06PM +0100, Jerome Marchand wrote:
> On 01/26/2015 02:33 AM, 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:
> >>> Date: Fri, 23 Jan 2015 15:48:05 +0100
> >>> From: Jerome Marchand <jmarchan@...hat.com>
> >>> To: Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Minchan Kim
> >>>  <minchan@...nel.org>
> >>> CC: Andrew Morton <akpm@...ux-foundation.org>,
> >>>  linux-kernel@...r.kernel.org, linux-mm@...ck.org, Nitin Gupta
> >>>  <ngupta@...are.org>
> >>> Subject: Re: [PATCH 1/2] zram: free meta out of init_lock
> >>> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101
> >>>  Thunderbird/31.3.0
> >>>
> >>> 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.
> > 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.
> 
> In my experience, reading a short explanation takes much less time that
> trying to figure out why something is done the way it is. Please add
> this explanation to the patch description. It might be very useful in
> the future to someone "git-blaming" this code.

This patch has two goals.

1. Avoid unnecessary lock
2. Prepare init_lock lockdep splot with upcoming zsmalloc compaction.

The compaction work doesn't come yet in mainline so I thought I don't
need to tell about 2 so if it become merging first by just 1's reason
before compaction work, everyone would happy without wasting the time
to look into lockdep splat.

Anyway, I will send an idea to remove init_lock in rw path.
Thanks!

> 
> Jerome
> 
> > 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.
> > 
> > Thanks!
> > 
> > 
> 
> 


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