[PATCH v3 6/8] x86: bump ZO_z_extra_bytes margin for zstd

Nick Terrell
2020-03-25T19:59:12Z

From: Nick Terrell <terrelln@fb.com>

Bump the ZO_z_extra_bytes margin for zstd.

Zstd needs 3 bytes per 128 KB, and has a 22 byte fixed overhead.
Zstd needs to maintain 128 KB of space at all times, since that is
the maximum block size. See the comments regarding in-place
decompression added in lib/decompress_unzstd.c for details.

Signed-off-by: Nick Terrell <terrelln@fb.com>
---
arch/x86/boot/header.S | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 97d9b6d6c1af..b820875c5c95 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -536,8 +536,14 @@ pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
# the size-dependent part now grows so fast.
#
# extra_bytes = (uncompressed_size >> 8) + 65536
+#
+# ZSTD compressed data grows by at most 3 bytes per 128K, and only has a 22
+# byte fixed overhead but has a maximum block size of 128K, so it needs a
+# larger margin.
+#
+# extra_bytes = (uncompressed_size >> 8) + 131072

-#define ZO_z_extra_bytes ((ZO_z_output_len >> 8) + 65536)
+#define ZO_z_extra_bytes ((ZO_z_output_len >> 8) + 131072)
#if ZO_z_output_len > ZO_z_input_len
# define ZO_z_extract_offset (ZO_z_output_len + ZO_z_extra_bytes - \
ZO_z_input_len)
--
2.25.1