[PATCH v2 2/2] kbuild: pass --stream-size --no-content-size to zstd

[Alex Xu (Hello71)]

Otherwise, it allocates 2 GB of memory at once. Even though the majority
of this memory is never touched, the default heuristic overcommit
refuses this request if less than 2 GB of RAM+swap is currently
available. This results in "zstd: error 11 : Allocation error : not
enough memory" and the kernel failing to build.

When the size is specified, zstd will reduce the memory request
appropriately. For typical kernel sizes of ~32 MB, the largest mmap
request will be reduced to 512 MB, which will succeed on all but the
smallest devices.

For inputs around this size, --stream-size --no-content-size may
slightly decrease the compressed size, or slightly increase it:
https://github.com/facebook/zstd/issues/2848.

Signed-off-by: Alex Xu (Hello71) <alex_y_xu@xxxxxxxx>
---
 scripts/Makefile.lib | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index ca901814986a..c98a82ca38e6 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -466,12 +466,20 @@ quiet_cmd_xzmisc = XZMISC  $@
 # single pass, so zstd doesn't need to allocate a window buffer. When streaming
 # decompression is used, like initramfs decompression, zstd22 should likely not
 # be used because it would require zstd to allocate a 128 MB buffer.
+#
+# --stream-size to reduce zstd memory usage (otherwise zstd -22 --ultra
+# allocates, but does not use, 2 GB) and potentially improve compression.
+#
+# --no-content-size to save three bytes which we do not use (we use size_append).
+
+# zstd --stream-size is only supported since 1.4.4
+zstd_stream_size = $(shell $(ZSTD) -1c --stream-size=0 --no-content-size </dev/null >/dev/null 2>&1 && printf '%s' '--stream-size=$(total_size) --no-content-size')
 
 quiet_cmd_zstd = ZSTD    $@
-      cmd_zstd = { cat $(real-prereqs) | $(ZSTD) -19; $(size_append); } > $@
+      cmd_zstd = { cat $(real-prereqs) | $(ZSTD) $(zstd_stream_size) -19; $(size_append); } > $@
 
 quiet_cmd_zstd22 = ZSTD22  $@
-      cmd_zstd22 = { cat $(real-prereqs) | $(ZSTD) -22 --ultra; $(size_append); } > $@
+      cmd_zstd22 = { cat $(real-prereqs) | $(ZSTD) $(zstd_stream_size) -22 --ultra; $(size_append); } > $@
 
 # ASM offsets
 # ---------------------------------------------------------------------------
-- 
2.34.0