btrfs: zstd: enable negative compression levels mount option

Allow using the fast modes (negative compression levels) of zstd as a
mount option.

As per the results, the compression ratio is (expectedly) lower:

for level in {-15..-1} 1 2 3; \
do printf "level %3d\n" $level; \
  mount -o compress=zstd:$level /dev/sdb /mnt/test/; \
  grep sdb /proc/mounts; \
  cp -r /usr/bin       /mnt/test/; sync; compsize /mnt/test/bin; \
  cp -r /usr/share/doc /mnt/test/; sync; compsize /mnt/test/doc; \
  cp    enwik9         /mnt/test/; sync; compsize /mnt/test/enwik9; \
  cp    linux-6.13.tar /mnt/test/; sync; compsize /mnt/test/linux-6.13.tar; \
  rm -r /mnt/test/{bin,doc,enwik9,linux-6.13.tar}; \
  umount /mnt/test/; \
done |& tee results | \
awk '/^level/{print}/^TOTAL/{print$3"\t"$2"  |"}' | paste - - - - -

		266M	bin  |	45M	doc  |	953M	wiki |	1.4G	source
=============================+===============+===============+===============+
level -15	180M	67%  |	30M	68%  |	694M	72%  |	598M	40%  |
level -14	180M	67%  |	30M	67%  |	683M	71%  |	581M	39%  |
level -13	177M	66%  |	29M	66%  |	671M	70%  |	566M	38%  |
level -12	174M	65%  |	29M	65%  |	658M	69%  |	548M	37%  |
level -11	174M	65%  |	28M	64%  |	645M	67%  |	530M	35%  |
level -10	171M	64%  |	28M	62%  |	631M	66%  |	512M	34%  |
level  -9	165M	62%  |	27M	61%  |	615M	64%  |	493M	33%  |
level  -8	161M	60%  |	27M	59%  |	598M	62%  |	475M	32%  |
level  -7	155M	58%  |	26M	58%  |	582M	61%  |	457M	30%  |
level  -6	151M	56%  |	25M	56%  |	565M	59%  |	437M	29%  |
level  -5	145M	54%  |	24M	55%  |	545M	57%  |	417M	28%  |
level  -4	139M	52%  |	23M	52%  |	520M	54%  |	391M	26%  |
level  -3	135M	50%  |	22M	50%  |	495M	51%  |	369M	24%  |
level  -2	127M	47%  |	22M	48%  |	470M	49%  |	349M	23%  |
level  -1	120M	45%  |	21M	47%  |	452M	47%  |	332M	22%  |
level   1	110M	41%  |	17M	39%  |	362M	38%  |	290M	19%  |
level   2	106M	40%  |	17M	38%  |	349M	36%  |	288M	19%  |
level   3	104M	39%  |	16M	37%  |	340M	35%  |	276M	18%  |

The samples represent some data sets that can be commonly found and show
approximate compressibility. The fast levels trade off speed for ratio
and are best suitable for highly compressible data.

As can be seen above, comparing the results to the current default zstd
level 3, the negative levels are roughly 2x worse at -15 and the
ratio increases almost linearly with each level.

Signed-off-by: Daniel Vacek <neelx@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 10 insertions, 11 deletions

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 0c4d486c3048..1fe154e7cc02 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -740,7 +740,7 @@ static const struct btrfs_compress_op * const btrfs_compress_op[] = {
 	&btrfs_zstd_compress,
 };
 
-static struct list_head *alloc_workspace(int type, unsigned int level)
+static struct list_head *alloc_workspace(int type, int level)
 {
 	switch (type) {
 	case BTRFS_COMPRESS_NONE: return alloc_heuristic_ws();
@@ -818,7 +818,7 @@ static void btrfs_cleanup_workspace_manager(int type)
  * Preallocation makes a forward progress guarantees and we do not return
  * errors.
  */
-struct list_head *btrfs_get_workspace(int type, unsigned int level)
+struct list_head *btrfs_get_workspace(int type, int level)
 {
 	struct workspace_manager *wsm;
 	struct list_head *workspace;
@@ -968,14 +968,14 @@ static void put_workspace(int type, struct list_head *ws)
  * Adjust @level according to the limits of the compression algorithm or
  * fallback to default
  */
-static unsigned int btrfs_compress_set_level(int type, unsigned level)
+static int btrfs_compress_set_level(unsigned int type, int level)
 {
 	const struct btrfs_compress_op *ops = btrfs_compress_op[type];
 
 	if (level == 0)
 		level = ops->default_level;
 	else
-		level = min(level, ops->max_level);
+		level = min(max(level, ops->min_level), ops->max_level);
 
 	return level;
 }
@@ -1023,12 +1023,10 @@ int btrfs_compress_filemap_get_folio(struct address_space *mapping, u64 start,
  * @total_out is an in/out parameter, must be set to the input length and will
  * be also used to return the total number of compressed bytes
  */
-int btrfs_compress_folios(unsigned int type_level, struct address_space *mapping,
+int btrfs_compress_folios(unsigned int type, int level, struct address_space *mapping,
 			 u64 start, struct folio **folios, unsigned long *out_folios,
 			 unsigned long *total_in, unsigned long *total_out)
 {
-	int type = btrfs_compress_type(type_level);
-	int level = btrfs_compress_level(type_level);
 	const unsigned long orig_len = *total_out;
 	struct list_head *workspace;
 	int ret;
@@ -1590,18 +1588,19 @@ out:
 
 /*
  * Convert the compression suffix (eg. after "zlib" starting with ":") to
- * level, unrecognized string will set the default level
+ * level, unrecognized string will set the default level. Negative level
+ * numbers are allowed.
  */
-unsigned int btrfs_compress_str2level(unsigned int type, const char *str)
+int btrfs_compress_str2level(unsigned int type, const char *str)
 {
-	unsigned int level = 0;
+	int level = 0;
 	int ret;
 
 	if (!type)
 		return 0;
 
 	if (str[0] == ':') {
-		ret = kstrtouint(str + 1, 10, &level);
+		ret = kstrtoint(str + 1, 10, &level);
 		if (ret)
 			level = 0;
 	}