Re: Lockdep complain for zram

[Minchan Kim <minchan@kernel.org>]

On Fri, Nov 23, 2012 at 03:46:31PM +0100, Jerome Marchand wrote:
> On 11/23/2012 12:34 AM, Minchan Kim wrote:
> > On Thu, Nov 22, 2012 at 12:13:24PM +0100, Jerome Marchand wrote:
> >> On 11/21/2012 09:37 AM, Minchan Kim wrote:
> >>> Hi alls,
> >>>
> >>> Today, I saw below complain of lockdep.
> >>> As a matter of fact, I knew it long time ago but forgot that.
> >>> The reason lockdep complains is that now zram uses GFP_KERNEL
> >>> in reclaim path(ex, __zram_make_request) :(
> >>> I can fix it via replacing GFP_KERNEL with GFP_NOIO.
> >>> But more big problem is vzalloc in zram_init_device which calls GFP_KERNEL.
> >>> Of course, I can change it with __vmalloc which can receive gfp_t.
> >>> But still we have a problem. Althoug __vmalloc can handle gfp_t, it calls
> >>> allocation of GFP_KERNEL. That's why I sent the patch.
> >>> https://lkml.org/lkml/2012/4/23/77
> >>> Since then, I forgot it, saw the bug today and poped the question again.
> >>>
> >>> Yes. Fundamental problem is utter crap API vmalloc.
> >>> If we can fix it, everyone would be happy. But life isn't simple like seeing
> >>> my thread of the patch.
> >>>
> >>> So next option is to move zram_init_device into setting disksize time.
> >>> But it makes unnecessary metadata waste until zram is used really(That's why
> >>> Nitin move zram_init_device from disksize setting time to make_request) and
> >>> it makes user should set the disksize before using, which are behavior change.
> >>>
> >>> I would like to clean up this issue before promoting because it might change
> >>> usage behavior.
> >>>
> >>> Do you have any idea?
> >>
> >> This is a false positive due to the memory allocation in
> >> zram_init_device() called from zram_make_request(). It appears to
> >> lockdep that the allocation might trigger a request on the device that
> >> would try to take init_lock again, but in fact it doesn't. The device
> >> is not initialized yet, even less swapped on.
> > 
> > That's not a only swap case.
> > Let's think following usecase.
> > 
> > 1) Booting
> > 2) echo $((DISKSIZE)) > /sys/block/zram0/disksize
> > 3) dd if=/dev/zero of=/dev/zram0 bs=4K count=1
> > 4) Written 4K page(page-A) is still page cache and isn't submitted
> >    to zram block device.
> > 5) Memory pressure happen by some memory hogger.
> > 6) VM start to reclaim and write page-A to zram0.
> > 7) zram_init_device is called at last.
> > 8) allocate GFP_KERNEL in zram_init_device
> > 9) goto reclaim path again.
> > 10) deadlock.
> > 
> > So I think it's not false positive.
> 
> I guess you're right. That's a scenario I haven't imagined. At any rate, my
> patch fixes that.
> 
> > Even if it is, I think lock split isn't a good idea to just avoid
> > lockdep warn. It makes code unnecessary complicated and it would be more
> > error-prone. Let's not add another lock without performance trouble report
> > by the lock.
> > 
> > As I discussed with Nitin in this thread, lazy initialization don't have
> > much point and disksize setting option isn't consistent for user behavior.
> > And I expect Nitin will send patch "diet of table" soonish.
> > 
> > So just moving the initialzation part from reclaim context to process's one
> > is simple and clear solution, I believe.
> 
> Although that would avoid deadlocks (I guess, I'm not sure anymore...), it
> won't stop lockdep from complaining. It still makes an allocation while

Argh, I sent it by mistake anyway, It's false-positive by this patch now.
Anyway we need more patch to shut lockdep up. I just sent patchset.

> holding a lock that is also taken in a reclaim context.
> Anyway, I like the idea to removes the lazy initialization. It makes things
> more complicated without any actual advantage.

Thanks for the review, Jerome.

> 
> Jerome
> 
> > 
> >>
> >> The following (quickly tested) patch should prevent lockdep complain.  
> >>
> >> Jerome
> >>
> >> ---
> >> >From ebb3514c4ee18276da7c5ca08025991b493ac204 Mon Sep 17 00:00:00 2001
> >> From: Jerome Marchand <jmarchan@redhat.com>
> >> Date: Thu, 22 Nov 2012 09:07:40 +0100
> >> Subject: [PATCH] staging: zram: Avoid lockdep warning
> >>
> >> zram triggers a lockdep warning. The cause of it is the call to
> >> zram_init_device() from zram_make_request(). The memory allocation in
> >> zram_init_device() could start a memory reclaim which in turn could
> >> cause swapout and (as it appears to lockdep) a call to
> >> zram_make_request(). However this is a false positive: an
> >> unititialized device can't be used as swap.
> >> A solution is to split init_lock in two lock. One mutex that protects
> >> init, reset and size setting and a rw_semaphore that protects requests
> >> and reset. Thus init and request would be protected by different locks
> >> and lockdep will be happy.
> >>
> >> Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
> >> ---
> >>  drivers/staging/zram/zram_drv.c   |   41 +++++++++++++++++++-----------------
> >>  drivers/staging/zram/zram_drv.h   |   16 ++++++++++---
> >>  drivers/staging/zram/zram_sysfs.c |   20 +++++++++---------
> >>  3 files changed, 44 insertions(+), 33 deletions(-)
> >>
> >> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> >> index fb4a7c9..b3bc3c4 100644
> >> --- a/drivers/staging/zram/zram_drv.c
> >> +++ b/drivers/staging/zram/zram_drv.c
> >> @@ -470,11 +470,11 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> >>  {
> >>  	struct zram *zram = queue->queuedata;
> >>  
> >> -	if (unlikely(!zram->init_done) && zram_init_device(zram))
> >> +	if (unlikely(!is_initialized(zram)) && zram_init_device(zram))
> >>  		goto error;
> >>  
> >> -	down_read(&zram->init_lock);
> >> -	if (unlikely(!zram->init_done))
> >> +	down_read(&zram->req_lock);
> >> +	if (unlikely(!is_initialized(zram)))
> >>  		goto error_unlock;
> >>  
> >>  	if (!valid_io_request(zram, bio)) {
> >> @@ -483,12 +483,12 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> >>  	}
> >>  
> >>  	__zram_make_request(zram, bio, bio_data_dir(bio));
> >> -	up_read(&zram->init_lock);
> >> +	up_read(&zram->req_lock);
> >>  
> >>  	return;
> >>  
> >>  error_unlock:
> >> -	up_read(&zram->init_lock);
> >> +	up_read(&zram->req_lock);
> >>  error:
> >>  	bio_io_error(bio);
> >>  }
> >> @@ -497,7 +497,7 @@ void __zram_reset_device(struct zram *zram)
> >>  {
> >>  	size_t index;
> >>  
> >> -	zram->init_done = 0;
> >> +	atomic_set(&zram->init_done, 0);
> >>  
> >>  	/* Free various per-device buffers */
> >>  	kfree(zram->compress_workmem);
> >> @@ -529,9 +529,12 @@ void __zram_reset_device(struct zram *zram)
> >>  
> >>  void zram_reset_device(struct zram *zram)
> >>  {
> >> -	down_write(&zram->init_lock);
> >> -	__zram_reset_device(zram);
> >> -	up_write(&zram->init_lock);
> >> +	mutex_lock(&zram->init_lock);
> >> +	down_write(&zram->req_lock);
> >> +	if (is_initialized(zram))
> >> +		__zram_reset_device(zram);
> >> +	up_write(&zram->req_lock);
> >> +	mutex_unlock(&zram->init_lock);
> >>  }
> >>  
> >>  int zram_init_device(struct zram *zram)
> >> @@ -539,10 +542,10 @@ int zram_init_device(struct zram *zram)
> >>  	int ret;
> >>  	size_t num_pages;
> >>  
> >> -	down_write(&zram->init_lock);
> >> +	mutex_lock(&zram->init_lock);
> >>  
> >> -	if (zram->init_done) {
> >> -		up_write(&zram->init_lock);
> >> +	if (is_initialized(zram)) {
> >> +		mutex_unlock(&zram->init_lock);
> >>  		return 0;
> >>  	}
> >>  
> >> @@ -583,8 +586,8 @@ int zram_init_device(struct zram *zram)
> >>  		goto fail;
> >>  	}
> >>  
> >> -	zram->init_done = 1;
> >> -	up_write(&zram->init_lock);
> >> +	atomic_set(&zram->init_done, 1);
> >> +	mutex_unlock(&zram->init_lock);
> >>  
> >>  	pr_debug("Initialization done!\n");
> >>  	return 0;
> >> @@ -594,7 +597,7 @@ fail_no_table:
> >>  	zram->disksize = 0;
> >>  fail:
> >>  	__zram_reset_device(zram);
> >> -	up_write(&zram->init_lock);
> >> +	mutex_unlock(&zram->init_lock);
> >>  	pr_err("Initialization failed: err=%d\n", ret);
> >>  	return ret;
> >>  }
> >> @@ -619,7 +622,8 @@ static int create_device(struct zram *zram, int device_id)
> >>  	int ret = 0;
> >>  
> >>  	init_rwsem(&zram->lock);
> >> -	init_rwsem(&zram->init_lock);
> >> +	mutex_init(&zram->init_lock);
> >> +	init_rwsem(&zram->req_lock);
> >>  	spin_lock_init(&zram->stat64_lock);
> >>  
> >>  	zram->queue = blk_alloc_queue(GFP_KERNEL);
> >> @@ -672,7 +676,7 @@ static int create_device(struct zram *zram, int device_id)
> >>  		goto out;
> >>  	}
> >>  
> >> -	zram->init_done = 0;
> >> +	atomic_set(&zram->init_done, 0);
> >>  
> >>  out:
> >>  	return ret;
> >> @@ -755,8 +759,7 @@ static void __exit zram_exit(void)
> >>  		zram = &zram_devices[i];
> >>  
> >>  		destroy_device(zram);
> >> -		if (zram->init_done)
> >> -			zram_reset_device(zram);
> >> +		zram_reset_device(zram);
> >>  	}
> >>  
> >>  	unregister_blkdev(zram_major, "zram");
> >> diff --git a/drivers/staging/zram/zram_drv.h b/drivers/staging/zram/zram_drv.h
> >> index df2eec4..f6bcead 100644
> >> --- a/drivers/staging/zram/zram_drv.h
> >> +++ b/drivers/staging/zram/zram_drv.h
> >> @@ -96,9 +96,12 @@ struct zram {
> >>  				   * against concurrent read and writes */
> >>  	struct request_queue *queue;
> >>  	struct gendisk *disk;
> >> -	int init_done;
> >> -	/* Prevent concurrent execution of device init, reset and R/W request */
> >> -	struct rw_semaphore init_lock;
> >> +	atomic_t init_done;
> >> +	/* Prevent concurrent execution of device init, reset and
> >> +	 * disksize_store */
> >> +	struct mutex init_lock;
> >> +	/* Prevent concurent execution device reset and R/W requests */
> >> +	struct rw_semaphore req_lock;
> >>  	/*
> >>  	 * This is the limit on amount of *uncompressed* worth of data
> >>  	 * we can store in a disk.
> >> @@ -108,6 +111,11 @@ struct zram {
> >>  	struct zram_stats stats;
> >>  };
> >>  
> >> +static inline int is_initialized(struct zram *zram)
> >> +{
> >> +	return atomic_read(&zram->init_done);
> >> +}
> >> +
> >>  extern struct zram *zram_devices;
> >>  unsigned int zram_get_num_devices(void);
> >>  #ifdef CONFIG_SYSFS
> >> @@ -115,6 +123,6 @@ extern struct attribute_group zram_disk_attr_group;
> >>  #endif
> >>  
> >>  extern int zram_init_device(struct zram *zram);
> >> -extern void __zram_reset_device(struct zram *zram);
> >> +extern void zram_reset_device(struct zram *zram);
> >>  
> >>  #endif
> >> diff --git a/drivers/staging/zram/zram_sysfs.c b/drivers/staging/zram/zram_sysfs.c
> >> index de1eacf..b300881 100644
> >> --- a/drivers/staging/zram/zram_sysfs.c
> >> +++ b/drivers/staging/zram/zram_sysfs.c
> >> @@ -62,16 +62,19 @@ static ssize_t disksize_store(struct device *dev,
> >>  	if (!disksize)
> >>  		return -EINVAL;
> >>  
> >> -	down_write(&zram->init_lock);
> >> -	if (zram->init_done) {
> >> -		up_write(&zram->init_lock);
> >> +	mutex_lock(&zram->init_lock);
> >> +	down_write(&zram->req_lock);
> >> +	if (is_initialized(zram)) {
> >> +		up_write(&zram->req_lock);
> >> +		mutex_unlock(&zram->init_lock);
> >>  		pr_info("Cannot change disksize for initialized device\n");
> >>  		return -EBUSY;
> >>  	}
> >>  
> >>  	zram->disksize = PAGE_ALIGN(disksize);
> >>  	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> >> -	up_write(&zram->init_lock);
> >> +	up_write(&zram->req_lock);
> >> +	mutex_unlock(&zram->init_lock);
> >>  
> >>  	return len;
> >>  }
> >> @@ -81,7 +84,7 @@ static ssize_t initstate_show(struct device *dev,
> >>  {
> >>  	struct zram *zram = dev_to_zram(dev);
> >>  
> >> -	return sprintf(buf, "%u\n", zram->init_done);
> >> +	return sprintf(buf, "%u\n", atomic_read(&zram->init_done));
> >>  }
> >>  
> >>  static ssize_t reset_store(struct device *dev,
> >> @@ -110,10 +113,7 @@ static ssize_t reset_store(struct device *dev,
> >>  	if (bdev)
> >>  		fsync_bdev(bdev);
> >>  
> >> -	down_write(&zram->init_lock);
> >> -	if (zram->init_done)
> >> -		__zram_reset_device(zram);
> >> -	up_write(&zram->init_lock);
> >> +	zram_reset_device(zram);
> >>  
> >>  	return len;
> >>  }
> >> @@ -186,7 +186,7 @@ static ssize_t mem_used_total_show(struct device *dev,
> >>  	u64 val = 0;
> >>  	struct zram *zram = dev_to_zram(dev);
> >>  
> >> -	if (zram->init_done)
> >> +	if (is_initialized(zram))
> >>  		val = zs_get_total_size_bytes(zram->mem_pool);
> >>  
> >>  	return sprintf(buf, "%llu\n", val);
> >> -- 
> >> 1.7.7.6
> >>
> >> --
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> > 
> 
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-- 
Kind regards,
Minchan Kim

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