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

[Minchan Kim <minchan@kernel.org>]

On Mon, Jan 26, 2015 at 11:17:09PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (01/26/15 10:33), 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:
> > > > 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@kernel.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.
> >
> 
> ah... I do recall it, thanks for your reminder.
> 
> 
> > 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.
> 
> hm, can't think of anything quick...
> 
> 	-ss

Hello guys,

How about this?

It's based on Ganesh's patch.
https://lkml.org/lkml/2015/1/24/50

>From afda9fd2f6c40dd0745d8a6babe78c5cbdceddf5 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Mon, 26 Jan 2015 14:34:10 +0900
Subject: [RFC] 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_hotpulug from
kmem_cache_destroy during wokring 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, put init_done bool
variable to check initialization done with smp_[wmb|rmb] and
srcu_[un]read_lock to prevent sudden zram meta freeing
during I/O operation.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 drivers/block/zram/zram_drv.c | 76 +++++++++++++++++++++++++++++--------------
 drivers/block/zram/zram_drv.h |  5 +++
 2 files changed, 57 insertions(+), 24 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a598ada817f0..e06ff975f997 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -32,6 +32,7 @@
 #include <linux/string.h>
 #include <linux/vmalloc.h>
 #include <linux/err.h>
+#include <linux/srcu.h>
 
 #include "zram_drv.h"
 
@@ -53,9 +54,16 @@ static ssize_t name##_show(struct device *d,		\
 }									\
 static DEVICE_ATTR_RO(name);
 
-static inline int init_done(struct zram *zram)
+static inline bool init_done(struct zram *zram)
 {
-	return zram->meta != NULL;
+	/*
+	 * init_done can be used without holding zram->init_lock in
+	 * read/write handler(ie, zram_make_request) but we should make sure
+	 * that zram->init_done should set up after meta initialization is
+	 * done. Look at disksize_store.
+	 */
+	smp_rmb();
+	return zram->init_done;
 }
 
 static inline struct zram *dev_to_zram(struct device *dev)
@@ -326,6 +334,10 @@ static void zram_meta_free(struct zram_meta *meta)
 	kfree(meta);
 }
 
+static void rcu_zram_do_nothing(struct rcu_head *unused)
+{
+}
+
 static struct zram_meta *zram_meta_alloc(int device_id, u64 disksize)
 {
 	char pool_name[8];
@@ -726,11 +738,8 @@ 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->meta = NULL;
 	/* Reset stats */
 	memset(&zram->stats, 0, sizeof(zram->stats));
 
@@ -738,8 +747,12 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
 	if (reset_capacity)
 		set_capacity(zram->disk, 0);
 
+	zram->init_done = false;
+	call_srcu(&zram->srcu, &zram->rcu, rcu_zram_do_nothing);
+	synchronize_srcu(&zram->srcu);
+	zram_meta_free(zram->meta);
+	zcomp_destroy(zram->comp);
 	up_write(&zram->init_lock);
-
 	/*
 	 * Revalidate disk out of the init_lock to avoid lockdep splat.
 	 * It's okay because disk's capacity is protected by init_lock
@@ -762,10 +775,19 @@ static ssize_t disksize_store(struct device *dev,
 	if (!disksize)
 		return -EINVAL;
 
+	down_write(&zram->init_lock);
+	if (init_done(zram)) {
+		pr_info("Cannot change disksize for initialized device\n");
+		up_write(&zram->init_lock);
+		return -EBUSY;
+	}
+
 	disksize = PAGE_ALIGN(disksize);
 	meta = zram_meta_alloc(zram->disk->first_minor, disksize);
-	if (!meta)
+	if (!meta) {
+		up_write(&zram->init_lock);
 		return -ENOMEM;
+	}
 
 	comp = zcomp_create(zram->compressor, zram->max_comp_streams);
 	if (IS_ERR(comp)) {
@@ -775,17 +797,17 @@ 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;
 	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
+	/*
+	 * Store operation of struct zram fields should complete
+	 * before init_done set up because zram_bvec_rw doesn't
+	 * hold an zram->init_lock.
+	 */
+	smp_wmb();
+	zram->init_done = true;
 	up_write(&zram->init_lock);
 
 	/*
@@ -797,10 +819,8 @@ static ssize_t disksize_store(struct device *dev,
 
 	return len;
 
-out_destroy_comp:
-	up_write(&zram->init_lock);
-	zcomp_destroy(comp);
 out_free_meta:
+	up_write(&zram->init_lock);
 	zram_meta_free(meta);
 	return err;
 }
@@ -905,9 +925,10 @@ out:
  */
 static void zram_make_request(struct request_queue *queue, struct bio *bio)
 {
+	int idx;
 	struct zram *zram = queue->queuedata;
 
-	down_read(&zram->init_lock);
+	idx = srcu_read_lock(&zram->srcu);
 	if (unlikely(!init_done(zram)))
 		goto error;
 
@@ -918,12 +939,12 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
 	}
 
 	__zram_make_request(zram, bio);
-	up_read(&zram->init_lock);
+	srcu_read_unlock(&zram->srcu, idx);
 
 	return;
 
 error:
-	up_read(&zram->init_lock);
+	srcu_read_unlock(&zram->srcu, idx);
 	bio_io_error(bio);
 }
 
@@ -945,18 +966,20 @@ 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, idx;
 	u32 index;
 	struct zram *zram;
 	struct bio_vec bv;
 
 	zram = bdev->bd_disk->private_data;
+	idx = srcu_read_lock(&zram->srcu);
+
 	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
 		atomic64_inc(&zram->stats.invalid_io);
+		srcu_read_unlock(&zram->srcu, idx);
 		return -EINVAL;
 	}
 
-	down_read(&zram->init_lock);
 	if (unlikely(!init_done(zram))) {
 		err = -EIO;
 		goto out_unlock;
@@ -971,7 +994,7 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
 
 	err = zram_bvec_rw(zram, &bv, index, offset, rw);
 out_unlock:
-	up_read(&zram->init_lock);
+	srcu_read_unlock(&zram->srcu, idx);
 	/*
 	 * If I/O fails, just return error(ie, non-zero) without
 	 * calling page_endio.
@@ -1041,6 +1064,11 @@ static int create_device(struct zram *zram, int device_id)
 
 	init_rwsem(&zram->init_lock);
 
+	if (init_srcu_struct(&zram->srcu)) {
+		pr_err("Error initialize srcu for device %d\n", device_id);
+		goto out;
+	}
+
 	zram->queue = blk_alloc_queue(GFP_KERNEL);
 	if (!zram->queue) {
 		pr_err("Error allocating disk queue for device %d\n",
@@ -1125,8 +1153,8 @@ static void destroy_device(struct zram *zram)
 
 	del_gendisk(zram->disk);
 	put_disk(zram->disk);
-
 	blk_cleanup_queue(zram->queue);
+	cleanup_srcu_struct(&zram->srcu);
 }
 
 static int __init zram_init(void)
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index e492f6bf11f1..2042c310aea8 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -105,8 +105,13 @@ struct zram {
 	struct gendisk *disk;
 	struct zcomp *comp;
 
+	struct srcu_struct srcu;
+	struct rcu_head rcu;
+
 	/* Prevent concurrent execution of device init, reset and R/W request */
 	struct rw_semaphore init_lock;
+	bool init_done;
+
 	/*
 	 * This is the limit on amount of *uncompressed* worth of data
 	 * we can store in a disk.
-- 
1.9.1