[PATCH 5/5] zram: remove chained recompression

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Chained recompression has unpredictable behavior and is not useful
in practice.

First, systems usually configure just one alternative recompression
algorithm, which has slower compression/decompression but better
compression ratio.  A single alternative algorithm doesn't need
chaining.

Second, even with multiple recompression algorithms, chained
recompression is suboptimal.  If a lower priority algorithm
succeeds, the page is never attempted with a higher priority
algorithm, leading to worse memory savings.  If a lower priority
algorithm fails, the page is still attempted with a higher priority
algorithm, wasting resources on the failed lower priority attempt.

In either case, the system would be better off targeting a specific
priority directly.

Chained recompression also significantly complicates the code.
Remove it.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 Documentation/admin-guide/blockdev/zram.rst |  9 ---
 drivers/block/zram/zram_drv.c               | 83 ++++++---------------
 2 files changed, 24 insertions(+), 68 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 967b58c3aad2..60b07a7e30cd 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -533,15 +533,6 @@ unexpected results when the same algorithm is configured with different
 priorities (e.g. different parameters).  `priority` is the only way to
 guarantee that the expected algorithm will be used.
 
-During re-compression for every page, that matches re-compression criteria,
-ZRAM iterates the list of registered alternative compression algorithms in
-order of their priorities. ZRAM stops either when re-compression was
-successful (re-compressed object is smaller in size than the original one)
-and matches re-compression criteria (e.g. size threshold) or when there are
-no secondary algorithms left to try. If none of the secondary algorithms can
-successfully re-compressed the page such a page is marked as incompressible,
-so ZRAM will not attempt to re-compress it in the future.
-
 memory tracking
 ===============
 
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 118b0b277e37..ff1931e700c3 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2331,7 +2331,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
 #define RECOMPRESS_IDLE		(1 << 0)
 #define RECOMPRESS_HUGE		(1 << 1)
 
-static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio_max,
+static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio,
 				     struct zram_pp_ctl *ctl)
 {
 	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
@@ -2357,8 +2357,8 @@ static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio_max,
 		    test_slot_flag(zram, index, ZRAM_INCOMPRESSIBLE))
 			goto next;
 
-		/* Already compressed with same of higher priority */
-		if (get_slot_comp_priority(zram, index) + 1 >= prio_max)
+		/* Already compressed with same or higher priority */
+		if (get_slot_comp_priority(zram, index) >= prio)
 			goto next;
 
 		ok = place_pp_slot(zram, ctl, index);
@@ -2391,8 +2391,7 @@ static bool highest_priority_algorithm(struct zram *zram, u32 prio)
  * Corresponding ZRAM slot should be locked.
  */
 static int recompress_slot(struct zram *zram, u32 index, struct page *page,
-			   u64 *num_recomp_pages, u32 threshold, u32 prio,
-			   u32 prio_max)
+			   u64 *num_recomp_pages, u32 threshold, u32 prio)
 {
 	struct zcomp_strm *zstrm = NULL;
 	unsigned long handle_old;
@@ -2404,6 +2403,9 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	void *src;
 	int ret = 0;
 
+	if (!zram->comps[prio])
+		return -EINVAL;
+
 	handle_old = get_slot_handle(zram, index);
 	if (!handle_old)
 		return -EINVAL;
@@ -2426,51 +2428,10 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	 */
 	clear_slot_flag(zram, index, ZRAM_IDLE);
 
-	class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old);
-
-	prio = max(prio, get_slot_comp_priority(zram, index) + 1);
-	/*
-	 * Recompression slots scan should not select slots that are
-	 * already compressed with a higher priority algorithm, but
-	 * just in case
-	 */
-	if (prio >= prio_max)
-		return 0;
-
-	/*
-	 * Iterate the secondary comp algorithms list (in order of priority)
-	 * and try to recompress the page.
-	 */
-	for (; prio < prio_max; prio++) {
-		if (!zram->comps[prio])
-			continue;
-
-		zstrm = zcomp_stream_get(zram->comps[prio]);
-		src = kmap_local_page(page);
-		ret = zcomp_compress(zram->comps[prio], zstrm,
-				     src, &comp_len_new);
-		kunmap_local(src);
-
-		if (ret) {
-			zcomp_stream_put(zstrm);
-			zstrm = NULL;
-			break;
-		}
-
-		class_index_new = zs_lookup_class_index(zram->mem_pool,
-							comp_len_new);
-
-		/* Continue until we make progress */
-		if (class_index_new >= class_index_old ||
-		    (threshold && comp_len_new >= threshold)) {
-			zcomp_stream_put(zstrm);
-			zstrm = NULL;
-			continue;
-		}
-
-		/* Recompression was successful so break out */
-		break;
-	}
+	zstrm = zcomp_stream_get(zram->comps[prio]);
+	src = kmap_local_page(page);
+	ret = zcomp_compress(zram->comps[prio], zstrm, src, &comp_len_new);
+	kunmap_local(src);
 
 	/*
 	 * Decrement the limit (if set) on pages we can recompress, even
@@ -2481,11 +2442,18 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	if (*num_recomp_pages)
 		*num_recomp_pages -= 1;
 
-	/* Compression error */
-	if (ret)
+	if (ret) {
+		zcomp_stream_put(zstrm);
 		return ret;
+	}
+
+	class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old);
+	class_index_new = zs_lookup_class_index(zram->mem_pool, comp_len_new);
+
+	if (class_index_new >= class_index_old ||
+	    (threshold && comp_len_new >= threshold)) {
+		zcomp_stream_put(zstrm);
 
-	if (!zstrm) {
 		/*
 		 * Secondary algorithms failed to re-compress the page
 		 * in a way that would save memory.
@@ -2535,11 +2503,11 @@ static ssize_t recompress_store(struct device *dev,
 				struct device_attribute *attr,
 				const char *buf, size_t len)
 {
-	u32 prio = ZRAM_SECONDARY_COMP, prio_max = ZRAM_MAX_COMPS;
 	struct zram *zram = dev_to_zram(dev);
 	char *args, *param, *val, *algo = NULL;
 	u64 num_recomp_pages = ULLONG_MAX;
 	struct zram_pp_ctl *ctl = NULL;
+	u32 prio = ZRAM_SECONDARY_COMP;
 	struct zram_pp_slot *pps;
 	u32 mode = 0, threshold = 0;
 	struct page *page = NULL;
@@ -2604,7 +2572,6 @@ static ssize_t recompress_store(struct device *dev,
 			 * "algorithm" name lookup is ambiguous.
 			 */
 			algo = NULL;
-			prio_max = min(prio + 1, ZRAM_MAX_COMPS);
 			continue;
 		}
 	}
@@ -2624,7 +2591,6 @@ static ssize_t recompress_store(struct device *dev,
 				continue;
 
 			if (!strcmp(zram->comp_algs[prio], algo)) {
-				prio_max = min(prio + 1, ZRAM_MAX_COMPS);
 				found = true;
 				break;
 			}
@@ -2653,7 +2619,7 @@ static ssize_t recompress_store(struct device *dev,
 		goto out;
 	}
 
-	scan_slots_for_recompress(zram, mode, prio_max, ctl);
+	scan_slots_for_recompress(zram, mode, prio, ctl);
 
 	ret = len;
 	while ((pps = select_pp_slot(ctl))) {
@@ -2667,8 +2633,7 @@ static ssize_t recompress_store(struct device *dev,
 			goto next;
 
 		err = recompress_slot(zram, pps->index, page,
-				      &num_recomp_pages, threshold,
-				      prio, prio_max);
+				      &num_recomp_pages, threshold, prio);
 next:
 		slot_unlock(zram, pps->index);
 		release_pp_slot(zram, pps);
-- 
2.53.0.473.g4a7958ca14-goog