[PATCH v6 09/17] zram: rework recompression loop

[Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>]

This reworks recompression loop handling:

- set a rule that stream-put NULLs the stream pointer
  If the loop returns with a non-NULL stream then it's a
  successfull recompression, otherwise the stream should
  always be NULL.

- do not count the number of recompressions
  Mark object as incompressible as soon as the algorithm
  with the highest priority failed to compress that object.

- count compression errors as resource usage
  Even if compression has failed, we still need to bump
  num_recomp_pages counter.

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 drivers/block/zram/zram_drv.c | 53 +++++++++++++----------------------
 1 file changed, 19 insertions(+), 34 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 5f8e963bd513..2e46ea542ecd 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1929,9 +1929,8 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	unsigned int comp_len_new;
 	unsigned int class_index_old;
 	unsigned int class_index_new;
-	u32 num_recomps = 0;
 	void *src, *dst;
-	int ret;
+	int ret = 0;
 
 	handle_old = zram_get_handle(zram, index);
 	if (!handle_old)
@@ -1974,7 +1973,6 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 		if (!zram->comps[prio])
 			continue;
 
-		num_recomps++;
 		zstrm = zcomp_stream_get(zram->comps[prio]);
 		src = kmap_local_page(page);
 		ret = zcomp_compress(zram->comps[prio], zstrm,
@@ -1983,7 +1981,8 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 
 		if (ret) {
 			zcomp_stream_put(zstrm);
-			return ret;
+			zstrm = NULL;
+			break;
 		}
 
 		class_index_new = zs_lookup_class_index(zram->mem_pool,
@@ -1993,6 +1992,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 		if (class_index_new >= class_index_old ||
 		    (threshold && comp_len_new >= threshold)) {
 			zcomp_stream_put(zstrm);
+			zstrm = NULL;
 			continue;
 		}
 
@@ -2000,14 +2000,6 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 		break;
 	}
 
-	/*
-	 * We did not try to recompress, e.g. when we have only one
-	 * secondary algorithm and the page is already recompressed
-	 * using that algorithm
-	 */
-	if (!zstrm)
-		return 0;
-
 	/*
 	 * Decrement the limit (if set) on pages we can recompress, even
 	 * when current recompression was unsuccessful or did not compress
@@ -2017,38 +2009,31 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	if (*num_recomp_pages)
 		*num_recomp_pages -= 1;
 
-	if (class_index_new >= class_index_old) {
+	/* Compression error */
+	if (ret)
+		return ret;
+
+	if (!zstrm) {
 		/*
 		 * Secondary algorithms failed to re-compress the page
-		 * in a way that would save memory, mark the object as
-		 * incompressible so that we will not try to compress
-		 * it again.
+		 * in a way that would save memory.
 		 *
-		 * We need to make sure that all secondary algorithms have
-		 * failed, so we test if the number of recompressions matches
-		 * the number of active secondary algorithms.
+		 * Mark the object incompressible if the max-priority
+		 * algorithm couldn't re-compress it.
 		 */
-		if (num_recomps == zram->num_active_comps - 1)
-			zram_set_flag(zram, index, ZRAM_INCOMPRESSIBLE);
+		if (prio < zram->num_active_comps)
+			return 0;
+		zram_set_flag(zram, index, ZRAM_INCOMPRESSIBLE);
 		return 0;
 	}
 
-	/* Successful recompression but above threshold */
-	if (threshold && comp_len_new >= threshold)
-		return 0;
-
 	/*
-	 * No direct reclaim (slow path) for handle allocation and no
-	 * re-compression attempt (unlike in zram_write_bvec()) since
-	 * we already have stored that object in zsmalloc. If we cannot
-	 * alloc memory for recompressed object then we bail out and
-	 * simply keep the old (existing) object in zsmalloc.
+	 * We are holding per-CPU stream mutex and entry lock so better
+	 * avoid direct reclaim.  Allocation error is not fatal since
+	 * we still have the old object in the mem_pool.
 	 */
 	handle_new = zs_malloc(zram->mem_pool, comp_len_new,
-			       __GFP_KSWAPD_RECLAIM |
-			       __GFP_NOWARN |
-			       __GFP_HIGHMEM |
-			       __GFP_MOVABLE);
+			       GFP_NOWAIT | __GFP_HIGHMEM | __GFP_MOVABLE);
 	if (IS_ERR_VALUE(handle_new)) {
 		zcomp_stream_put(zstrm);
 		return PTR_ERR((void *)handle_new);
-- 
2.48.1.601.g30ceb7b040-goog