[PATCH v4] zram: support REQ_DISCARD

[Joonsoo Kim]

zram is ram based block device and can be used by backend of filesystem.
When filesystem deletes a file, it normally doesn't do anything on data
block of that file. It just marks on metadata of that file. This behavior
has no problem on disk based block device, but has problems on ram based
block device, since we can't free memory used for data block. To overcome
this disadvantage, there is REQ_DISCARD functionality. If block device
support REQ_DISCARD and filesystem is mounted with discard option,
filesystem sends REQ_DISCARD to block device whenever some data blocks are
discarded. All we have to do is to handle this request.

This patch implements to flag up QUEUE_FLAG_DISCARD and handle this
REQ_DISCARD request. With it, we can free memory used by zram if it isn't
used.

v2: handle unaligned case commented by Jerome
v3: conditionally set zero to discard_zeroes_data commented by Minchan
    reuse index, offset in __zram_make_request() commented by Sergey.
v4: replenish code comments suggested by Andrew.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 7631ef0..e3700cb 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -541,6 +541,48 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
 	return ret;
 }
 
+/*
+ * zram_bio_discard - handler on discard request
+ * @index: physical block index by PAGE_SIZE unit
+ * @offset: offset within physical block
+ */
+static void zram_bio_discard(struct zram *zram, u32 index,
+			     int offset, struct bio *bio)
+{
+	size_t n = bio->bi_iter.bi_size;
+
+	/*
+	 * zram manages data by physical block size unit. Because logical block
+	 * size isn't identical with physical block size on some arch, we
+	 * could get discard request pointing to specific offset within certain
+	 * physical block. Although we can handle this request by reading that
+	 * physiclal block and decompressing and partially zeroing and
+	 * re-compressing and then re-storing it, it isn't reasonable because
+	 * our intention of handling discard request is to save memory.
+	 * So skipping this logical block is approriate here.
+	 */
+	if (offset) {
+		if (n < offset)
+			return;
+
+		n -= offset;
+		index++;
+	}
+
+	while (n >= PAGE_SIZE) {
+		/*
+		 * discard request can be too large so that the zram can
+		 * be stucked for a long time if we handle the request
+		 * at once. So handle the request by PAGE_SIZE unit at a time.
+		 */
+		write_lock(&zram->meta->tb_lock);
+		zram_free_page(zram, index);
+		write_unlock(&zram->meta->tb_lock);
+		index++;
+		n -= PAGE_SIZE;
+	}
+}
+
 static void zram_reset_device(struct zram *zram, bool reset_capacity)
 {
 	size_t index;
@@ -676,6 +718,12 @@ static void __zram_make_request(struct zram *zram, struct bio *bio)
 	offset = (bio->bi_iter.bi_sector &
 		  (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
 
+	if (unlikely(bio->bi_rw & REQ_DISCARD)) {
+		zram_bio_discard(zram, index, offset, bio);
+		bio_endio(bio, 0);
+		return;
+	}
+
 	bio_for_each_segment(bvec, bio, iter) {
 		int max_transfer_size = PAGE_SIZE - offset;
 
@@ -845,6 +893,20 @@ static int create_device(struct zram *zram, int device_id)
 					ZRAM_LOGICAL_BLOCK_SIZE);
 	blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
 	blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
+	zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
+	zram->disk->queue->limits.max_discard_sectors = UINT_MAX;
+	/*
+	 * zram_bio_discard() will clear all logical blocks if logical block
+	 * size is identical with physical block size(PAGE_SIZE). But if it is
+	 * different, we will skip to discard some parts of logical blocks in
+	 * whole request range which isn't aligned to physical block size.
+	 * So we can't ensure that some discarded logical block is zeroed.
+	 */
+	if (ZRAM_LOGICAL_BLOCK_SIZE == PAGE_SIZE)
+		zram->disk->queue->limits.discard_zeroes_data = 1;
+	else
+		zram->disk->queue->limits.discard_zeroes_data = 0;
+	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, zram->disk->queue);
 
 	add_disk(zram->disk);
 
-- 
1.7.9.5

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