Re: [RFC 3/3] zram: add swap_get_free hint

[Minchan Kim]

Hi Heesub,

On Thu, Sep 04, 2014 at 03:26:14PM +0900, Heesub Shin wrote:
> Hello Minchan,
> 
> First of all, I agree with the overall purpose of your patch set.

Thank you.

> 
> On 09/04/2014 10:39 AM, Minchan Kim wrote:
> >This patch implement SWAP_GET_FREE handler in zram so that VM can
> >know how many zram has freeable space.
> >VM can use it to stop anonymous reclaiming once zram is full.
> >
> >Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> >---
> >  drivers/block/zram/zram_drv.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> >diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >index 88661d62e46a..8e22b20aa2db 100644
> >--- a/drivers/block/zram/zram_drv.c
> >+++ b/drivers/block/zram/zram_drv.c
> >@@ -951,6 +951,22 @@ static int zram_slot_free_notify(struct block_device *bdev,
> >  	return 0;
> >  }
> >
> >+static int zram_get_free_pages(struct block_device *bdev, long *free)
> >+{
> >+	struct zram *zram;
> >+	struct zram_meta *meta;
> >+
> >+	zram = bdev->bd_disk->private_data;
> >+	meta = zram->meta;
> >+
> >+	if (!zram->limit_pages)
> >+		return 1;
> >+
> >+	*free = zram->limit_pages - zs_get_total_pages(meta->mem_pool);
> 
> Even if 'free' is zero here, there may be free spaces available to
> store more compressed pages into the zs_pool. I mean calculation
> above is not quite accurate and wastes memory, but have no better
> idea for now.

Yeb, good point.

Actually, I thought about that but in this patchset, I wanted to
go with conservative approach which is a safe guard to prevent
system hang which is terrible than early OOM kill.

Whole point of this patchset is to add a facility to VM and VM
collaborates with zram via the interface to avoid worst case
(ie, system hang) and logic to throttle could be enhanced by
several approaches in future but I agree my logic was too simple
and conservative.

We could improve it with [anti|de]fragmentation in future but
at the moment, below simple heuristic is not too bad for first
step. :)


---
 drivers/block/zram/zram_drv.c | 15 ++++++++++-----
 drivers/block/zram/zram_drv.h |  1 +
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8e22b20aa2db..af9dfe6a7d2b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -410,6 +410,7 @@ static bool zram_free_page(struct zram *zram, size_t index)
 	atomic64_sub(zram_get_obj_size(meta, index),
 			&zram->stats.compr_data_size);
 	atomic64_dec(&zram->stats.pages_stored);
+	atomic_set(&zram->alloc_fail, 0);
 
 	meta->table[index].handle = 0;
 	zram_set_obj_size(meta, index, 0);
@@ -600,10 +601,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	alloced_pages = zs_get_total_pages(meta->mem_pool);
 	if (zram->limit_pages && alloced_pages > zram->limit_pages) {
 		zs_free(meta->mem_pool, handle);
+		atomic_inc(&zram->alloc_fail);
 		ret = -ENOMEM;
 		goto out;
 	}
 
+	atomic_set(&zram->alloc_fail, 0);
 	update_used_max(zram, alloced_pages);
 
 	cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
@@ -951,6 +954,7 @@ static int zram_slot_free_notify(struct block_device *bdev,
 	return 0;
 }
 
+#define FULL_THRESH_HOLD 32
 static int zram_get_free_pages(struct block_device *bdev, long *free)
 {
 	struct zram *zram;
@@ -959,12 +963,13 @@ static int zram_get_free_pages(struct block_device *bdev, long *free)
 	zram = bdev->bd_disk->private_data;
 	meta = zram->meta;
 
-	if (!zram->limit_pages)
-		return 1;
-
-	*free = zram->limit_pages - zs_get_total_pages(meta->mem_pool);
+	if (zram->limit_pages &&
+		(atomic_read(&zram->alloc_fail) > FULL_THRESH_HOLD)) {
+		*free = 0;
+		return 0;
+	}
 
-	return 0;
+	return 1;
 }
 
 static int zram_swap_hint(struct block_device *bdev,
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 779d03fa4360..182a2544751b 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -115,6 +115,7 @@ struct zram {
 	u64 disksize;	/* bytes */
 	int max_comp_streams;
 	struct zram_stats stats;
+	atomic_t alloc_fail;
 	/*
 	 * the number of pages zram can consume for storing compressed data
 	 */
-- 
2.0.0

> 
> heesub
> 
> >+
> >+	return 0;
> >+}
> >+
> >  static int zram_swap_hint(struct block_device *bdev,
> >  				unsigned int hint, void *arg)
> >  {
> >@@ -958,6 +974,8 @@ static int zram_swap_hint(struct block_device *bdev,
> >
> >  	if (hint == SWAP_SLOT_FREE)
> >  		ret = zram_slot_free_notify(bdev, (unsigned long)arg);
> >+	else if (hint == SWAP_GET_FREE)
> >+		ret = zram_get_free_pages(bdev, arg);
> >
> >  	return ret;
> >  }
> >
> 
> --
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-- 
Kind regards,
Minchan Kim
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