zram: mark incompressible page as ZRAM_HUGE

Mark incompressible pages so that we could investigate who is the owner
of the incompressible pages once the page is swapped out via using
upcoming zram memory tracker feature.

With it, we could prevent such pages to be swapped out by using mlock.
Otherwise we might remove them.

This patch exposes new stat for huge pages via mm_stat.

Link: http://lkml.kernel.org/r/20180416090946.63057-3-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/blockdev/zram.txt

@@ -218,6 +218,7 @@ line of text and contains the following stats separated by whitespace:
  same_pages       the number of same element filled pages written to this disk.
                   No memory is allocated for such pages.
  pages_compacted  the number of pages freed during compaction
+ huge_pages	  the number of incompressible pages
 
 9) Deactivate:
 	swapoff /dev/zram0

drivers/block/zram/zram_drv.c

@@ -729,14 +729,15 @@ static ssize_t mm_stat_show(struct device *dev,
 	max_used = atomic_long_read(&zram->stats.max_used_pages);
 
 	ret = scnprintf(buf, PAGE_SIZE,
-			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu\n",
+			"%8llu %8llu %8llu %8lu %8ld %8llu %8lu %8llu\n",
 			orig_size << PAGE_SHIFT,
 			(u64)atomic64_read(&zram->stats.compr_data_size),
 			mem_used << PAGE_SHIFT,
 			zram->limit_pages << PAGE_SHIFT,
 			max_used << PAGE_SHIFT,
 			(u64)atomic64_read(&zram->stats.same_pages),
-			pool_stats.pages_compacted);
+			pool_stats.pages_compacted,
+			(u64)atomic64_read(&zram->stats.huge_pages));
 	up_read(&zram->init_lock);
 
 	return ret;
@@ -805,6 +806,11 @@ static void zram_free_page(struct zram *zram, size_t index)
 {
 	unsigned long handle;
 
+	if (zram_test_flag(zram, index, ZRAM_HUGE)) {
+		zram_clear_flag(zram, index, ZRAM_HUGE);
+		atomic64_dec(&zram->stats.huge_pages);
+	}
+
 	if (zram_wb_enabled(zram) && zram_test_flag(zram, index, ZRAM_WB)) {
 		zram_wb_clear(zram, index);
 		atomic64_dec(&zram->stats.pages_stored);
@@ -973,6 +979,7 @@ compress_again:
 	}
 
 	if (unlikely(comp_len >= huge_class_size)) {
+		comp_len = PAGE_SIZE;
 		if (zram_wb_enabled(zram) && allow_wb) {
 			zcomp_stream_put(zram->comp);
 			ret = write_to_bdev(zram, bvec, index, bio, &element);
@@ -984,7 +991,6 @@ compress_again:
 			allow_wb = false;
 			goto compress_again;
 		}
-		comp_len = PAGE_SIZE;
 	}
 
 	/*
@@ -1046,6 +1052,11 @@ out:
 	zram_slot_lock(zram, index);
 	zram_free_page(zram, index);
 
+	if (comp_len == PAGE_SIZE) {
+		zram_set_flag(zram, index, ZRAM_HUGE);
+		atomic64_inc(&zram->stats.huge_pages);
+	}
+
 	if (flags) {
 		zram_set_flag(zram, index, flags);
 		zram_set_element(zram, index, element);

drivers/block/zram/zram_drv.h

@@ -47,6 +47,7 @@ enum zram_pageflags {
 	ZRAM_LOCK = ZRAM_FLAG_SHIFT,
 	ZRAM_SAME,	/* Page consists the same element */
 	ZRAM_WB,	/* page is stored on backing_device */
+	ZRAM_HUGE,	/* Incompressible page */
 
 	__NR_ZRAM_PAGEFLAGS,
 };
@@ -71,6 +72,7 @@ struct zram_stats {
 	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
 	atomic64_t notify_free;	/* no. of swap slot free notifications */
 	atomic64_t same_pages;		/* no. of same element filled pages */
+	atomic64_t huge_pages;		/* no. of huge pages */
 	atomic64_t pages_stored;	/* no. of pages currently stored */
 	atomic_long_t max_used_pages;	/* no. of maximum pages stored */
 	atomic64_t writestall;		/* no. of write slow paths */