Re: [PATCH 1/2] zram: free meta out of init_lock

[Minchan Kim]

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@xxxxxxxxxx>
> > To: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>, Minchan Kim
> >  <minchan@xxxxxxxxxx>
> > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>,
> >  linux-kernel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, Nitin Gupta
> >  <ngupta@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> > >> ---
> > >>  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. 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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