[PATCH] btrfs: zlib: do not do unnecessary page copying for compression

[Qu Wenruo <wqu@suse.com>]

[BUG]
In function zlib_compress_folios(), we handle the input by:

- If there are multiple pages left
  We copy the page content into workspace->buf, and use workspace->buf
  as input for compression.

  But on x86_64 (which doesn't support dfltcc), that buffer size is just
  one page, so we're wasting our CPU time copying the page for no
  benefit.

- If there is only one page left
  We use the mapped page address as input for compression.

The problem is, this means we will copy the whole input range except the
last page (can be as large as 124K), without much obvious benefit.

Meanwhile the cost is pretty obvious.

[POSSIBLE REASON]
The possible reason may be related to the support of S390 hardware zlib
decompression acceleration.

As we allocate 4 pages (4 * 4K) as workspace input buffer just for s390.

[FIX]
I checked the dfltcc code, there seems to be no requirement on the
input buffer size.
The function dfltcc_can_deflate() only checks:

- If the compression settings are supported
  Only level/w_bits/strategy/level_mask is checked.

- If the hardware supports

No mention at all on the input buffer size, thus I believe there is no
need to waste time doing the page copying.

Maybe the hardware acceleration is so good for s390 that they can offset
the page copying cost, but it's definitely a penalty for non-s390
systems.

So fix the problem by:

- Use the same buffer size
  No matter if dfltcc support is enabled or not

- Always use page address as input

Cc: linux-s390@vger.kernel.org
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/zlib.c | 67 +++++++++++--------------------------------------
 1 file changed, 14 insertions(+), 53 deletions(-)

diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
index d9e5c88a0f85..9c88a841a060 100644
--- a/fs/btrfs/zlib.c
+++ b/fs/btrfs/zlib.c
@@ -65,21 +65,8 @@ struct list_head *zlib_alloc_workspace(unsigned int level)
 			zlib_inflate_workspacesize());
 	workspace->strm.workspace = kvzalloc(workspacesize, GFP_KERNEL | __GFP_NOWARN);
 	workspace->level = level;
-	workspace->buf = NULL;
-	/*
-	 * In case of s390 zlib hardware support, allocate lager workspace
-	 * buffer. If allocator fails, fall back to a single page buffer.
-	 */
-	if (zlib_deflate_dfltcc_enabled()) {
-		workspace->buf = kmalloc(ZLIB_DFLTCC_BUF_SIZE,
-					 __GFP_NOMEMALLOC | __GFP_NORETRY |
-					 __GFP_NOWARN | GFP_NOIO);
-		workspace->buf_size = ZLIB_DFLTCC_BUF_SIZE;
-	}
-	if (!workspace->buf) {
-		workspace->buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
-		workspace->buf_size = PAGE_SIZE;
-	}
+	workspace->buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	workspace->buf_size = PAGE_SIZE;
 	if (!workspace->strm.workspace || !workspace->buf)
 		goto fail;
 
@@ -103,7 +90,6 @@ int zlib_compress_folios(struct list_head *ws, struct address_space *mapping,
 	struct folio *in_folio = NULL;
 	struct folio *out_folio = NULL;
 	unsigned long bytes_left;
-	unsigned int in_buf_folios;
 	unsigned long len = *total_out;
 	unsigned long nr_dest_folios = *out_folios;
 	const unsigned long max_out = nr_dest_folios * PAGE_SIZE;
@@ -130,7 +116,6 @@ int zlib_compress_folios(struct list_head *ws, struct address_space *mapping,
 	folios[0] = out_folio;
 	nr_folios = 1;
 
-	workspace->strm.next_in = workspace->buf;
 	workspace->strm.avail_in = 0;
 	workspace->strm.next_out = cfolio_out;
 	workspace->strm.avail_out = PAGE_SIZE;
@@ -142,43 +127,19 @@ int zlib_compress_folios(struct list_head *ws, struct address_space *mapping,
 		 */
 		if (workspace->strm.avail_in == 0) {
 			bytes_left = len - workspace->strm.total_in;
-			in_buf_folios = min(DIV_ROUND_UP(bytes_left, PAGE_SIZE),
-					    workspace->buf_size / PAGE_SIZE);
-			if (in_buf_folios > 1) {
-				int i;
-
-				for (i = 0; i < in_buf_folios; i++) {
-					if (data_in) {
-						kunmap_local(data_in);
-						folio_put(in_folio);
-						data_in = NULL;
-					}
-					ret = btrfs_compress_filemap_get_folio(mapping,
-							start, &in_folio);
-					if (ret < 0)
-						goto out;
-					data_in = kmap_local_folio(in_folio, 0);
-					copy_page(workspace->buf + i * PAGE_SIZE,
-						  data_in);
-					start += PAGE_SIZE;
-				}
-				workspace->strm.next_in = workspace->buf;
-			} else {
-				if (data_in) {
-					kunmap_local(data_in);
-					folio_put(in_folio);
-					data_in = NULL;
-				}
-				ret = btrfs_compress_filemap_get_folio(mapping,
-						start, &in_folio);
-				if (ret < 0)
-					goto out;
-				data_in = kmap_local_folio(in_folio, 0);
-				start += PAGE_SIZE;
-				workspace->strm.next_in = data_in;
+			if (data_in) {
+				kunmap_local(data_in);
+				folio_put(in_folio);
+				data_in = NULL;
 			}
-			workspace->strm.avail_in = min(bytes_left,
-						       (unsigned long) workspace->buf_size);
+			ret = btrfs_compress_filemap_get_folio(mapping,
+					start, &in_folio);
+			if (ret < 0)
+				goto out;
+			data_in = kmap_local_folio(in_folio, 0);
+			start += PAGE_SIZE;
+			workspace->strm.next_in = data_in;
+			workspace->strm.avail_in = min(bytes_left, PAGE_SIZE);
 		}
 
 		ret = zlib_deflate(&workspace->strm, Z_SYNC_FLUSH);
-- 
2.45.1