On Mon, Jan 26, 2015 at 11:17:09PM +0900, Sergey Senozhatsky wrote:

Hello,

On (01/26/15 10:33), Minchan Kim wrote:

quoted

Hello,

On Sat, Jan 24, 2015 at 12:47:07AM +0900, Sergey Senozhatsky wrote:

quoted

On (01/23/15 15:48), Jerome Marchand wrote:

quoted

On 01/23/2015 03:24 PM, Sergey Senozhatsky wrote:

quoted

On (01/23/15 14:58), Minchan Kim wrote:

quoted

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.

quoted

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.

quoted

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