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

[Sergey Senozhatsky]

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;
 }
 
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