Date:	Mon, 2 Feb 2015 13:28:47 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Ganesh Mahendran <opensource.ganesh@...il.com>
Subject: Re: [PATCH v1 2/2] zram: remove init_lock in zram_make_request

On Mon, Feb 02, 2015 at 01:01:24PM +0900, Sergey Senozhatsky wrote:
> On (02/02/15 11:44), Minchan Kim wrote:
> > > sure, I did think about this. and I actually didn't find any reason not
> > > to use ->refcount there. if user wants to reset the device, he first
> > > should umount it to make bdev->bd_holders check happy. and that's where
> > > IOs will be failed. so it makes sense to switch to ->refcount there, IMHO.
> > 
> > If we use zram as block device itself(not a fs or swap) and open the
> > block device as !FMODE_EXCL, bd_holders will be void.
> > 
> 
> hm.
> I don't mind to use ->disksize there, but personally I'd maybe prefer
> to use ->refcount, which just looks less hacky. zram's most common use
> cases are coming from ram swap device or ram device with fs. so it looks
> a bit like we care about some corner case here.

Maybe, but I always test zram with dd so it's not a corner case for me. :)

> 
> just my opinion, no objections against ->disksize != 0.

Thanks. It's a draft for v2. Please review.

BTW, you pointed out race between bdev_open/close and reset and
it's cleary bug although it's rare in real practice.
So, I want to fix it earlier than this patch and mark it as -stable
if we can fix it easily like Ganesh's work.
If it gets landing, we could make this patch rebased on it.

>From 699502b4e0c84b3d7b33f8754cf1c0109b16c012 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Mon, 2 Feb 2015 10:36:28 +0900
Subject: [PATCH v2] zram: remove init_lock in zram_make_request

Admin could reset zram during I/O operation going on so we have
used zram->init_lock as read-side lock in I/O path to prevent
sudden zram meta freeing.

However, the init_lock is really troublesome.
We can't do call zram_meta_alloc under init_lock due to lockdep splat
because zram_rw_page is one of the function under reclaim path and
hold it as read_lock while other places in process context hold it
as write_lock. So, we have used allocation out of the lock to avoid
lockdep warn but it's not good for readability and fainally, I met
another lockdep splat between init_lock and cpu_hotplug from
kmem_cache_destroy during working zsmalloc compaction. :(

Yes, the ideal is to remove horrible init_lock of zram in rw path.
This patch removes it in rw path and instead, add atomic refcount
for meta lifetime management and completion to free meta in process
context. It's important to free meta in process context because
some of resource destruction needs mutex lock, which could be held
if we releases the resource in reclaim context so it's deadlock,
again.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 drivers/block/zram/zram_drv.c | 85 ++++++++++++++++++++++++++++++-------------
 drivers/block/zram/zram_drv.h | 20 +++++-----
 2 files changed, 71 insertions(+), 34 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index aa5a4c5..c6d505c 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -55,7 +55,7 @@ static DEVICE_ATTR_RO(name);
 
 static inline int init_done(struct zram *zram)
 {
-	return zram->meta != NULL;
+	return zram->disksize != 0;
 }
 
 static inline struct zram *dev_to_zram(struct device *dev)
@@ -358,6 +358,18 @@ out_error:
 	return NULL;
 }
 
+static inline bool zram_meta_get(struct zram *zram)
+{
+	if (atomic_inc_not_zero(&zram->refcount))
+		return true;
+	return false;
+}
+
+static inline void zram_meta_put(struct zram *zram)
+{
+	atomic_dec(&zram->refcount);
+}
+
 static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
 {
 	if (*offset + bvec->bv_len >= PAGE_SIZE)
@@ -719,6 +731,10 @@ static void zram_bio_discard(struct zram *zram, u32 index,
 
 static void zram_reset_device(struct zram *zram, bool reset_capacity)
 {
+	struct zram_meta *meta;
+	struct zcomp *comp;
+	u64 disksize;
+
 	down_write(&zram->init_lock);
 
 	zram->limit_pages = 0;
@@ -728,19 +744,32 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
 		return;
 	}
 
-	zcomp_destroy(zram->comp);
-	zram->max_comp_streams = 1;
-	zram_meta_free(zram->meta, zram->disksize);
-	zram->meta = NULL;
+	meta = zram->meta;
+	comp = zram->comp;
+	disksize = zram->disksize;
+	zram->disksize = 0;
+	/*
+	 * ->refcount will go down to 0 eventually and rw handler cannot
+	 * handle further I/O by init_done checking.
+	 */
+	zram_meta_put(zram);
+	/*
+	 * We want to free zram_meta in process context to avoid
+	 * deadlock between reclaim path and any other locks
+	 */
+	wait_event(zram->io_done, atomic_read(&zram->refcount) == 0);
+
 	/* Reset stats */
 	memset(&zram->stats, 0, sizeof(zram->stats));
+	zram->max_comp_streams = 1;
 
-	zram->disksize = 0;
 	if (reset_capacity)
 		set_capacity(zram->disk, 0);
 
 	up_write(&zram->init_lock);
-
+	/* I/O operation under all of CPU are done so let's free */
+	zram_meta_free(meta, disksize);
+	zcomp_destroy(comp);
 	/*
 	 * Revalidate disk out of the init_lock to avoid lockdep splat.
 	 * It's okay because disk's capacity is protected by init_lock
@@ -783,6 +812,8 @@ static ssize_t disksize_store(struct device *dev,
 		goto out_destroy_comp;
 	}
 
+	init_waitqueue_head(&zram->io_done);
+	zram_meta_get(zram);
 	zram->meta = meta;
 	zram->comp = comp;
 	zram->disksize = disksize;
@@ -838,8 +869,8 @@ static ssize_t reset_store(struct device *dev,
 	/* Make sure all pending I/O is finished */
 	fsync_bdev(bdev);
 	bdput(bdev);
-
 	zram_reset_device(zram, true);
+
 	return len;
 
 out:
@@ -908,23 +939,24 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
 {
 	struct zram *zram = queue->queuedata;
 
-	down_read(&zram->init_lock);
-	if (unlikely(!init_done(zram)))
+	if (unlikely(!zram_meta_get(zram)))
 		goto error;
 
+	if (unlikely(!init_done(zram)))
+		goto put_zram;
+
 	if (!valid_io_request(zram, bio->bi_iter.bi_sector,
 					bio->bi_iter.bi_size)) {
 		atomic64_inc(&zram->stats.invalid_io);
-		goto error;
+		goto put_zram;
 	}
 
 	__zram_make_request(zram, bio);
-	up_read(&zram->init_lock);
-
+	zram_meta_put(zram);
 	return;
-
+put_zram:
+	zram_meta_put(zram);
 error:
-	up_read(&zram->init_lock);
 	bio_io_error(bio);
 }
 
@@ -946,21 +978,22 @@ static void zram_slot_free_notify(struct block_device *bdev,
 static int zram_rw_page(struct block_device *bdev, sector_t sector,
 		       struct page *page, int rw)
 {
-	int offset, err;
+	int offset, err = -EIO;
 	u32 index;
 	struct zram *zram;
 	struct bio_vec bv;
 
 	zram = bdev->bd_disk->private_data;
+	if (unlikely(!zram_meta_get(zram)))
+		goto out;
+
+	if (unlikely(!init_done(zram)))
+		goto put_zram;
+
 	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
 		atomic64_inc(&zram->stats.invalid_io);
-		return -EINVAL;
-	}
-
-	down_read(&zram->init_lock);
-	if (unlikely(!init_done(zram))) {
-		err = -EIO;
-		goto out_unlock;
+		err = -EINVAL;
+		goto put_zram;
 	}
 
 	index = sector >> SECTORS_PER_PAGE_SHIFT;
@@ -971,8 +1004,9 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
 	bv.bv_offset = 0;
 
 	err = zram_bvec_rw(zram, &bv, index, offset, rw);
-out_unlock:
-	up_read(&zram->init_lock);
+put_zram:
+	zram_meta_put(zram);
+out:
 	/*
 	 * If I/O fails, just return error(ie, non-zero) without
 	 * calling page_endio.
@@ -1041,6 +1075,7 @@ static int create_device(struct zram *zram, int device_id)
 	int ret = -ENOMEM;
 
 	init_rwsem(&zram->init_lock);
+	atomic_set(&zram->refcount, 0);
 
 	zram->queue = blk_alloc_queue(GFP_KERNEL);
 	if (!zram->queue) {
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index b05a816..6085335 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -100,24 +100,26 @@ struct zram_meta {
 
 struct zram {
 	struct zram_meta *meta;
+	struct zcomp *comp;
 	struct request_queue *queue;
 	struct gendisk *disk;
-	struct zcomp *comp;
-
-	/* Prevent concurrent execution of device init, reset and R/W request */
+	/* Prevent concurrent execution of device init */
 	struct rw_semaphore init_lock;
 	/*
-	 * This is the limit on amount of *uncompressed* worth of data
-	 * we can store in a disk.
+	 * the number of pages zram can consume for storing compressed data
 	 */
-	u64 disksize;	/* bytes */
+	unsigned long limit_pages;
+	atomic_t refcount; /* refcount for zram_meta */
 	int max_comp_streams;
+
 	struct zram_stats stats;
+	/* wait all IO under all of cpu are done */
+	wait_queue_head_t io_done;
 	/*
-	 * the number of pages zram can consume for storing compressed data
+	 * This is the limit on amount of *uncompressed* worth of data
+	 * we can store in a disk.
 	 */
-	unsigned long limit_pages;
-
+	u64 disksize;	/* bytes */
 	char compressor[10];
 };
 #endif
-- 
1.9.3



> 
> I need to check fs/block_dev. can we switch away from ->bd_holders?
> 
> > Another topic: As I didn't see enough fs/block_dev.c bd_holders in zram
> > would be mess. I guess we need to study hotplug of device and implement
> > it for zram reset rather than strange own konb. It should go TODO. :(
> 
> ok, need to investigate this later.
> let's land current activities first.
> 
> 	-ss

-- 
Kind regards,
Minchan Kim
--
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