zram: add max_pages param to recompression

Introduce "max_pages" param to recompress device attribute which sets an
upper limit on the number of entries (pages) zram attempts to recompress
(in this particular recompression call).  S/W recompression can be quite
expensive so limiting the number of pages recompress touches can be quite
helpful.

Link: https://lkml.kernel.org/r/20240329094050.2815699-1-senozhatsky@chromium.orgSigned-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/blockdev/zram.rst

@@ -466,6 +466,11 @@ of equal or greater size:::
 	#recompress idle pages larger than 2000 bytes
 	echo "type=idle threshold=2000" > /sys/block/zramX/recompress
 
+It is also possible to limit the number of pages zram re-compression will
+attempt to recompress:::
+
+	echo "type=huge_idle max_pages=42" > /sys/block/zramX/recompress
+
 Recompression of idle pages requires memory tracking.
 
 During re-compression for every page, that matches re-compression criteria,

drivers/block/zram/zram_drv.c

@@ -1568,7 +1568,8 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
  * Corresponding ZRAM slot should be locked.
  */
 static int zram_recompress(struct zram *zram, u32 index, struct page *page,
-			   u32 threshold, u32 prio, u32 prio_max)
+			   u64 *num_recomp_pages, u32 threshold, u32 prio,
+			   u32 prio_max)
 {
 	struct zcomp_strm *zstrm = NULL;
 	unsigned long handle_old;
@@ -1645,6 +1646,15 @@ static int zram_recompress(struct zram *zram, u32 index, struct page *page,
 	if (!zstrm)
 		return 0;
 
+	/*
+	 * Decrement the limit (if set) on pages we can recompress, even
+	 * when current recompression was unsuccessful or did not compress
+	 * the page below the threshold, because we still spent resources
+	 * on it.
+	 */
+	if (*num_recomp_pages)
+		*num_recomp_pages -= 1;
+
 	if (class_index_new >= class_index_old) {
 		/*
 		 * Secondary algorithms failed to re-compress the page
@@ -1710,6 +1720,7 @@ static ssize_t recompress_store(struct device *dev,
 	struct zram *zram = dev_to_zram(dev);
 	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
 	char *args, *param, *val, *algo = NULL;
+	u64 num_recomp_pages = ULLONG_MAX;
 	u32 mode = 0, threshold = 0;
 	unsigned long index;
 	struct page *page;
@@ -1732,6 +1743,17 @@ static ssize_t recompress_store(struct device *dev,
 			continue;
 		}
 
+		if (!strcmp(param, "max_pages")) {
+			/*
+			 * Limit the number of entries (pages) we attempt to
+			 * recompress.
+			 */
+			ret = kstrtoull(val, 10, &num_recomp_pages);
+			if (ret)
+				return ret;
+			continue;
+		}
+
 		if (!strcmp(param, "threshold")) {
 			/*
 			 * We will re-compress only idle objects equal or
@@ -1788,6 +1810,9 @@ static ssize_t recompress_store(struct device *dev,
 	for (index = 0; index < nr_pages; index++) {
 		int err = 0;
 
+		if (!num_recomp_pages)
+			break;
+
 		zram_slot_lock(zram, index);
 
 		if (!zram_allocated(zram, index))
@@ -1807,8 +1832,8 @@ static ssize_t recompress_store(struct device *dev,
 		    zram_test_flag(zram, index, ZRAM_INCOMPRESSIBLE))
 			goto next;
 
-		err = zram_recompress(zram, index, page, threshold,
-				      prio, prio_max);
+		err = zram_recompress(zram, index, page, &num_recomp_pages,
+				      threshold, prio, prio_max);
 next:
 		zram_slot_unlock(zram, index);
 		if (err) {