[PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support

[PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support

[David Disseldorp]

This patchset adds copy_file_range() support to gen_init_cpio. When
combined with data segment alignment, large-file archiving performance
is improved on Btrfs and XFS due to reflinks (see 7/7 benchmarks).

cpio data segment alignment is provided by "bending" the newc spec
to zero-pad the filename field. GNU cpio and Linux initramfs extractors
handle this fine as long as PATH_MAX isn't exceeded.

Changes since v1 RFC
- add alignment patches 6-7
- slightly rework commit and error messages
- rename l->len to avoid 1/i confusion

David Disseldorp (7):
      gen_init_cpio: write to fd instead of stdout stream
      gen_init_cpio: support -o <output_path> parameter
      gen_init_cpio: attempt copy_file_range for file data
      gen_init_cpio: avoid duplicate strlen calls
      gen_initramfs.sh: use gen_init_cpio -o parameter
      docs: initramfs: file data alignment via name padding
      gen_init_cpio: add -a <data_align> as reflink optimization

 .../driver-api/early-userspace/buffer-format.rst   |   5 +
 usr/gen_init_cpio.c                                | 234 ++++++++++++++-------
 usr/gen_initramfs.sh                               |   7 +-
 3 files changed, 166 insertions(+), 80 deletions(-)

[PATCH v2 1/7] gen_init_cpio: write to fd instead of stdout stream

[David Disseldorp]

In preparation for more efficient archiving using copy_file_range(),
switch from writing archive data to stdout to using STDOUT_FILENO and
I/O via write(), dprintf(), etc.
Basic I/O error handling is added to cover cases such as ENOSPC. Partial
writes are treated as errors.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_init_cpio.c | 139 ++++++++++++++++++++++++++------------------
 1 file changed, 81 insertions(+), 58 deletions(-)

diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
index edcdb8abfa31c..d8779fe4b8f1f 100644
--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -23,64 +23,71 @@
 #define xstr(s) #s
 #define str(s) xstr(s)
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
+#define CPIO_HDR_LEN 110
+#define padlen(_off, _align) (((_align) - ((_off) & ((_align) - 1))) % (_align))
 
+static char padding[512];
 static unsigned int offset;
 static unsigned int ino = 721;
 static time_t default_mtime;
 static bool do_file_mtime;
 static bool do_csum = false;
+static int outfd = STDOUT_FILENO;
 
 struct file_handler {
 	const char *type;
 	int (*handler)(const char *line);
 };
 
-static void push_string(const char *name)
+static int push_string(const char *name)
 {
 	unsigned int name_len = strlen(name) + 1;
+	ssize_t len;
+
+	len = write(outfd, name, name_len);
+	if (len != name_len)
+		return -1;
 
-	fputs(name, stdout);
-	putchar(0);
 	offset += name_len;
+	return 0;
 }
 
-static void push_pad (void)
+static int push_pad(size_t padlen)
 {
-	while (offset & 3) {
-		putchar(0);
-		offset++;
-	}
+	ssize_t len = 0;
+
+	if (!padlen)
+		return 0;
+
+	if (padlen < sizeof(padding))
+		len = write(outfd, padding, padlen);
+	if (len != padlen)
+		return -1;
+
+	offset += padlen;
+	return 0;
 }
 
-static void push_rest(const char *name)
+static int push_rest(const char *name)
 {
 	unsigned int name_len = strlen(name) + 1;
-	unsigned int tmp_ofs;
+	ssize_t len;
 
-	fputs(name, stdout);
-	putchar(0);
-	offset += name_len;
+	len = write(outfd, name, name_len);
+	if (len != name_len)
+		return -1;
 
-	tmp_ofs = name_len + 110;
-	while (tmp_ofs & 3) {
-		putchar(0);
-		offset++;
-		tmp_ofs++;
-	}
-}
+	offset += name_len;
 
-static void push_hdr(const char *s)
-{
-	fputs(s, stdout);
-	offset += 110;
+	return push_pad(padlen(name_len + CPIO_HDR_LEN, 4));
 }
 
-static void cpio_trailer(void)
+static int cpio_trailer(void)
 {
-	char s[256];
 	const char name[] = "TRAILER!!!";
+	int len;
 
-	sprintf(s, "%s%08X%08X%08lX%08lX%08X%08lX"
+	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
 		0,			/* ino */
@@ -96,23 +103,24 @@ static void cpio_trailer(void)
 		0,			/* rminor */
 		(unsigned)strlen(name)+1, /* namesize */
 		0);			/* chksum */
-	push_hdr(s);
-	push_rest(name);
+	offset += len;
 
-	while (offset % 512) {
-		putchar(0);
-		offset++;
-	}
+	if (len != CPIO_HDR_LEN
+	 || push_rest(name) < 0
+	 || push_pad(padlen(offset, 512)) < 0)
+		return -1;
+
+	return 0;
 }
 
 static int cpio_mkslink(const char *name, const char *target,
 			 unsigned int mode, uid_t uid, gid_t gid)
 {
-	char s[256];
+	int len;
 
 	if (name[0] == '/')
 		name++;
-	sprintf(s,"%s%08X%08X%08lX%08lX%08X%08lX"
+	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
 		ino++,			/* ino */
@@ -128,12 +136,17 @@ static int cpio_mkslink(const char *name, const char *target,
 		0,			/* rminor */
 		(unsigned)strlen(name) + 1,/* namesize */
 		0);			/* chksum */
-	push_hdr(s);
-	push_string(name);
-	push_pad();
-	push_string(target);
-	push_pad();
+	offset += len;
+
+	if (len != CPIO_HDR_LEN
+	 || push_string(name) < 0
+	 || push_pad(padlen(offset, 4)) < 0
+	 || push_string(target) < 0
+	 || push_pad(padlen(offset, 4)) < 0)
+		return -1;
+
 	return 0;
+
 }
 
 static int cpio_mkslink_line(const char *line)
@@ -157,11 +170,11 @@ static int cpio_mkslink_line(const char *line)
 static int cpio_mkgeneric(const char *name, unsigned int mode,
 		       uid_t uid, gid_t gid)
 {
-	char s[256];
+	int len;
 
 	if (name[0] == '/')
 		name++;
-	sprintf(s,"%s%08X%08X%08lX%08lX%08X%08lX"
+	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
 		ino++,			/* ino */
@@ -177,8 +190,12 @@ static int cpio_mkgeneric(const char *name, unsigned int mode,
 		0,			/* rminor */
 		(unsigned)strlen(name) + 1,/* namesize */
 		0);			/* chksum */
-	push_hdr(s);
-	push_rest(name);
+	offset += len;
+
+	if (len != CPIO_HDR_LEN
+	 || push_rest(name) < 0)
+		return -1;
+
 	return 0;
 }
 
@@ -246,7 +263,7 @@ static int cpio_mknod(const char *name, unsigned int mode,
 		       uid_t uid, gid_t gid, char dev_type,
 		       unsigned int maj, unsigned int min)
 {
-	char s[256];
+	int len;
 
 	if (dev_type == 'b')
 		mode |= S_IFBLK;
@@ -255,7 +272,7 @@ static int cpio_mknod(const char *name, unsigned int mode,
 
 	if (name[0] == '/')
 		name++;
-	sprintf(s,"%s%08X%08X%08lX%08lX%08X%08lX"
+	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
 		ino++,			/* ino */
@@ -271,8 +288,12 @@ static int cpio_mknod(const char *name, unsigned int mode,
 		min,			/* rminor */
 		(unsigned)strlen(name) + 1,/* namesize */
 		0);			/* chksum */
-	push_hdr(s);
-	push_rest(name);
+	offset += len;
+
+	if (len != CPIO_HDR_LEN
+	 || push_rest(name) < 0)
+		return -1;
+
 	return 0;
 }
 
@@ -324,11 +345,9 @@ static int cpio_mkfile(const char *name, const char *location,
 			unsigned int mode, uid_t uid, gid_t gid,
 			unsigned int nlinks)
 {
-	char s[256];
 	struct stat buf;
 	unsigned long size;
-	int file;
-	int retval;
+	int file, retval, len;
 	int rc = -1;
 	time_t mtime;
 	int namesize;
@@ -386,7 +405,7 @@ static int cpio_mkfile(const char *name, const char *location,
 		if (name[0] == '/')
 			name++;
 		namesize = strlen(name) + 1;
-		sprintf(s,"%s%08X%08X%08lX%08lX%08X%08lX"
+		len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 		       "%08lX%08X%08X%08X%08X%08X%08X",
 			do_csum ? "070702" : "070701", /* magic */
 			ino,			/* ino */
@@ -402,9 +421,12 @@ static int cpio_mkfile(const char *name, const char *location,
 			0,			/* rminor */
 			namesize,		/* namesize */
 			size ? csum : 0);	/* chksum */
-		push_hdr(s);
-		push_string(name);
-		push_pad();
+		offset += len;
+
+		if (len != CPIO_HDR_LEN
+		 || push_string(name) < 0
+		 || push_pad(padlen(offset, 4)) < 0)
+			goto error;
 
 		while (size) {
 			unsigned char filebuf[65536];
@@ -417,14 +439,15 @@ static int cpio_mkfile(const char *name, const char *location,
 				goto error;
 			}
 
-			if (fwrite(filebuf, this_read, 1, stdout) != 1) {
+			if (write(outfd, filebuf, this_read) != this_read) {
 				fprintf(stderr, "writing filebuf failed\n");
 				goto error;
 			}
 			offset += this_read;
 			size -= this_read;
 		}
-		push_pad();
+		if (push_pad(padlen(offset, 4)) < 0)
+			goto error;
 
 		name += namesize;
 	}
@@ -691,7 +714,7 @@ int main (int argc, char *argv[])
 		}
 	}
 	if (ec == 0)
-		cpio_trailer();
+		ec = cpio_trailer();
 
 	exit(ec);
 }
-- 
2.43.0

Re: [PATCH v2 1/7] gen_init_cpio: write to fd instead of stdout stream

[Nicolas Schier]

On Thu, Aug 14, 2025 at 03:17:59PM +1000, David Disseldorp wrote:
> In preparation for more efficient archiving using copy_file_range(),
> switch from writing archive data to stdout to using STDOUT_FILENO and
> I/O via write(), dprintf(), etc.
> Basic I/O error handling is added to cover cases such as ENOSPC. Partial
> writes are treated as errors.
> 
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
>  usr/gen_init_cpio.c | 139 ++++++++++++++++++++++++++------------------
>  1 file changed, 81 insertions(+), 58 deletions(-)
> 
> diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
> index edcdb8abfa31c..d8779fe4b8f1f 100644
> --- a/usr/gen_init_cpio.c
> +++ b/usr/gen_init_cpio.c
[...]
> @@ -96,23 +103,24 @@ static void cpio_trailer(void)
>  		0,			/* rminor */
>  		(unsigned)strlen(name)+1, /* namesize */
>  		0);			/* chksum */
> -	push_hdr(s);
> -	push_rest(name);
> +	offset += len;
>  
> -	while (offset % 512) {
> -		putchar(0);
> -		offset++;
> -	}
> +	if (len != CPIO_HDR_LEN
> +	 || push_rest(name) < 0
> +	 || push_pad(padlen(offset, 512)) < 0)


Thanks, patch looks good to me.

Just a minor coding style bike shedding: Starting continuation lines
with '||' seems to be rather unusual in Linux code:

    $ git grep -Ee '^\s*(\|\||&&)' **/*.c | wc -l
    64
    $ git grep -Ee '(\|\||&&)$' **/*.c | wc -l
    2553

Kind regards,
Nicolas

[PATCH v2 2/7] gen_init_cpio: support -o <output_path> parameter

[David Disseldorp]

This is another preparatory change to allow for reflink-optimized
cpio archives with file data written / cloned via copy_file_range().
The output file is truncated prior to write, so that it maps to
usr/gen_initramfs.sh usage. It may make sense to offer an append option
in future, for easier archive concatenation.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_init_cpio.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
index d8779fe4b8f1f..563594a0662a6 100644
--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -110,7 +110,7 @@ static int cpio_trailer(void)
 	 || push_pad(padlen(offset, 512)) < 0)
 		return -1;
 
-	return 0;
+	return fsync(outfd);
 }
 
 static int cpio_mkslink(const char *name, const char *target,
@@ -532,7 +532,7 @@ static int cpio_mkfile_line(const char *line)
 static void usage(const char *prog)
 {
 	fprintf(stderr, "Usage:\n"
-		"\t%s [-t <timestamp>] [-c] <cpio_list>\n"
+		"\t%s [-t <timestamp>] [-c] [-o <output_path>] <cpio_list>\n"
 		"\n"
 		"<cpio_list> is a file containing newline separated entries that\n"
 		"describe the files to be included in the initramfs archive:\n"
@@ -569,7 +569,8 @@ static void usage(const char *prog)
 		"as mtime for symlinks, directories, regular and special files.\n"
 		"The default is to use the current time for all files, but\n"
 		"preserve modification time for regular files.\n"
-		"-c: calculate and store 32-bit checksums for file data.\n",
+		"-c: calculate and store 32-bit checksums for file data.\n"
+		"<output_path>: write cpio to this file instead of stdout\n",
 		prog);
 }
 
@@ -611,7 +612,7 @@ int main (int argc, char *argv[])
 
 	default_mtime = time(NULL);
 	while (1) {
-		int opt = getopt(argc, argv, "t:ch");
+		int opt = getopt(argc, argv, "t:cho:");
 		char *invalid;
 
 		if (opt == -1)
@@ -630,6 +631,16 @@ int main (int argc, char *argv[])
 		case 'c':
 			do_csum = true;
 			break;
+		case 'o':
+			outfd = open(optarg,
+				     O_WRONLY | O_CREAT | O_LARGEFILE | O_TRUNC,
+				     0600);
+			if (outfd < 0) {
+				fprintf(stderr, "failed to open %s\n", optarg);
+				usage(argv[0]);
+				exit(1);
+			}
+			break;
 		case 'h':
 		case '?':
 			usage(argv[0]);
-- 
2.43.0

Re: [PATCH v2 2/7] gen_init_cpio: support -o <output_path> parameter

[Nicolas Schier]

On Thu, Aug 14, 2025 at 03:18:00PM +1000, David Disseldorp wrote:
> This is another preparatory change to allow for reflink-optimized
> cpio archives with file data written / cloned via copy_file_range().
> The output file is truncated prior to write, so that it maps to
> usr/gen_initramfs.sh usage. It may make sense to offer an append option
> in future, for easier archive concatenation.
> 
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
>  usr/gen_init_cpio.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
> index d8779fe4b8f1f..563594a0662a6 100644
> --- a/usr/gen_init_cpio.c
> +++ b/usr/gen_init_cpio.c
> @@ -110,7 +110,7 @@ static int cpio_trailer(void)
>  	 || push_pad(padlen(offset, 512)) < 0)
>  		return -1;
>  
> -	return 0;
> +	return fsync(outfd);
>  }
>  
>  static int cpio_mkslink(const char *name, const char *target,
> @@ -532,7 +532,7 @@ static int cpio_mkfile_line(const char *line)
>  static void usage(const char *prog)
>  {
>  	fprintf(stderr, "Usage:\n"
> -		"\t%s [-t <timestamp>] [-c] <cpio_list>\n"
> +		"\t%s [-t <timestamp>] [-c] [-o <output_path>] <cpio_list>\n"
>  		"\n"
>  		"<cpio_list> is a file containing newline separated entries that\n"
>  		"describe the files to be included in the initramfs archive:\n"
> @@ -569,7 +569,8 @@ static void usage(const char *prog)
>  		"as mtime for symlinks, directories, regular and special files.\n"
>  		"The default is to use the current time for all files, but\n"
>  		"preserve modification time for regular files.\n"
> -		"-c: calculate and store 32-bit checksums for file data.\n",
> +		"-c: calculate and store 32-bit checksums for file data.\n"
> +		"<output_path>: write cpio to this file instead of stdout\n",

gen_init_cpio writes only a single output file (instead of multiple
files to an output directory), I'd suggest to name the parameter just
'file' or 'output_file'.


I'd like to see the the '... -o ...' patch right after this one.



For compilability, I had to add

#define _LARGEFILE64_SOURCE

or

#define _GNU_SOURCE

as you do in the next patch, in order to get O_LARGEFILE defined.


Kind regards,
Nicolas

[PATCH v2 3/7] gen_init_cpio: attempt copy_file_range for file data

[David Disseldorp]

The copy_file_range syscall can improve copy performance by cloning
extents between cpio archive source and destination files.
Existing read / write based copy logic is retained for fallback in case
the copy_file_range syscall is unsupported or unavailable due to
cross-filesystem EXDEV, etc.

Clone or reflink, as opposed to copy, of source file extents into the
output cpio archive may (e.g. on Btrfs and XFS) require alignment of the
output to the filesystem block size. This could be achieved by inserting
padding entries into the cpio archive manifest.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_init_cpio.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
index 563594a0662a6..64421d410a88b 100644
--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -353,6 +354,7 @@ static int cpio_mkfile(const char *name, const char *location,
 	int namesize;
 	unsigned int i;
 	uint32_t csum = 0;
+	ssize_t this_read;
 
 	mode |= S_IFREG;
 
@@ -428,9 +430,19 @@ static int cpio_mkfile(const char *name, const char *location,
 		 || push_pad(padlen(offset, 4)) < 0)
 			goto error;
 
+		if (size) {
+			this_read = copy_file_range(file, NULL, outfd, NULL, size, 0);
+			if (this_read > 0) {
+				if (this_read > size)
+					goto error;
+				offset += this_read;
+				size -= this_read;
+			}
+			/* short or failed copy falls back to read/write... */
+		}
+
 		while (size) {
 			unsigned char filebuf[65536];
-			ssize_t this_read;
 			size_t this_size = MIN(size, sizeof(filebuf));
 
 			this_read = read(file, filebuf, this_size);
-- 
2.43.0

Re: [PATCH v2 3/7] gen_init_cpio: attempt copy_file_range for file data

[Nicolas Schier]

On Thu, Aug 14, 2025 at 03:18:01PM +1000, David Disseldorp wrote:
> The copy_file_range syscall can improve copy performance by cloning
> extents between cpio archive source and destination files.
> Existing read / write based copy logic is retained for fallback in case
> the copy_file_range syscall is unsupported or unavailable due to
> cross-filesystem EXDEV, etc.
> 
> Clone or reflink, as opposed to copy, of source file extents into the
> output cpio archive may (e.g. on Btrfs and XFS) require alignment of the
> output to the filesystem block size. This could be achieved by inserting
> padding entries into the cpio archive manifest.
> 
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
>  usr/gen_init_cpio.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)

Thanks.  I like introducing copy_file_range() here, it reduces the cpio
generation time on my system by a about 30% on a btrfs filesystem.
May cpio_mkfile_csum() now the slowest part?

Kind regards,
Nicolas

Re: [PATCH v2 3/7] gen_init_cpio: attempt copy_file_range for file data

[David Disseldorp]

On Mon, 18 Aug 2025 20:40:39 +0200, Nicolas Schier wrote:
...
> Thanks.  I like introducing copy_file_range() here, it reduces the cpio
> generation time on my system by a about 30% on a btrfs filesystem.
> May cpio_mkfile_csum() now the slowest part?

cpio checksums are slow, but I don't think it's worth optimizing for at
this stage: they're optional (-c) and support for them was only recently
added (800c24dc34b93). Besides, there are better ways to guarantee
initramfs integrity.

Thanks, David

[PATCH v2 4/7] gen_init_cpio: avoid duplicate strlen calls

[David Disseldorp]

We determine the filename length for the cpio header, so shouldn't
recalculate it when writing out the filename.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_init_cpio.c | 40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
index 64421d410a88b..40f4cbd95844e 100644
--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -25,6 +25,7 @@
 #define str(s) xstr(s)
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
 #define CPIO_HDR_LEN 110
+#define CPIO_TRAILER "TRAILER!!!"
 #define padlen(_off, _align) (((_align) - ((_off) & ((_align) - 1))) % (_align))
 
 static char padding[512];
@@ -40,9 +41,8 @@ struct file_handler {
 	int (*handler)(const char *line);
 };
 
-static int push_string(const char *name)
+static int push_buf(const char *name, size_t name_len)
 {
-	unsigned int name_len = strlen(name) + 1;
 	ssize_t len;
 
 	len = write(outfd, name, name_len);
@@ -69,9 +69,8 @@ static int push_pad(size_t padlen)
 	return 0;
 }
 
-static int push_rest(const char *name)
+static int push_rest(const char *name, size_t name_len)
 {
-	unsigned int name_len = strlen(name) + 1;
 	ssize_t len;
 
 	len = write(outfd, name, name_len);
@@ -85,8 +84,8 @@ static int push_rest(const char *name)
 
 static int cpio_trailer(void)
 {
-	const char name[] = "TRAILER!!!";
 	int len;
+	unsigned int namesize = sizeof(CPIO_TRAILER);
 
 	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
@@ -102,12 +101,12 @@ static int cpio_trailer(void)
 		0,			/* minor */
 		0,			/* rmajor */
 		0,			/* rminor */
-		(unsigned)strlen(name)+1, /* namesize */
+		namesize,		/* namesize */
 		0);			/* chksum */
 	offset += len;
 
 	if (len != CPIO_HDR_LEN
-	 || push_rest(name) < 0
+	 || push_rest(CPIO_TRAILER, namesize) < 0
 	 || push_pad(padlen(offset, 512)) < 0)
 		return -1;
 
@@ -118,9 +117,12 @@ static int cpio_mkslink(const char *name, const char *target,
 			 unsigned int mode, uid_t uid, gid_t gid)
 {
 	int len;
+	unsigned int namesize, targetsize = strlen(target) + 1;
 
 	if (name[0] == '/')
 		name++;
+	namesize = strlen(name) + 1;
+
 	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
@@ -130,19 +132,19 @@ static int cpio_mkslink(const char *name, const char *target,
 		(long) gid,		/* gid */
 		1,			/* nlink */
 		(long) default_mtime,	/* mtime */
-		(unsigned)strlen(target)+1, /* filesize */
+		targetsize,		/* filesize */
 		3,			/* major */
 		1,			/* minor */
 		0,			/* rmajor */
 		0,			/* rminor */
-		(unsigned)strlen(name) + 1,/* namesize */
+		namesize,		/* namesize */
 		0);			/* chksum */
 	offset += len;
 
 	if (len != CPIO_HDR_LEN
-	 || push_string(name) < 0
+	 || push_buf(name, namesize) < 0
 	 || push_pad(padlen(offset, 4)) < 0
-	 || push_string(target) < 0
+	 || push_buf(target, targetsize) < 0
 	 || push_pad(padlen(offset, 4)) < 0)
 		return -1;
 
@@ -172,9 +174,12 @@ static int cpio_mkgeneric(const char *name, unsigned int mode,
 		       uid_t uid, gid_t gid)
 {
 	int len;
+	unsigned int namesize;
 
 	if (name[0] == '/')
 		name++;
+	namesize = strlen(name) + 1;
+
 	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
@@ -189,12 +194,12 @@ static int cpio_mkgeneric(const char *name, unsigned int mode,
 		1,			/* minor */
 		0,			/* rmajor */
 		0,			/* rminor */
-		(unsigned)strlen(name) + 1,/* namesize */
+		namesize,		/* namesize */
 		0);			/* chksum */
 	offset += len;
 
 	if (len != CPIO_HDR_LEN
-	 || push_rest(name) < 0)
+	 || push_rest(name, namesize) < 0)
 		return -1;
 
 	return 0;
@@ -265,6 +270,7 @@ static int cpio_mknod(const char *name, unsigned int mode,
 		       unsigned int maj, unsigned int min)
 {
 	int len;
+	unsigned int namesize;
 
 	if (dev_type == 'b')
 		mode |= S_IFBLK;
@@ -273,6 +279,8 @@ static int cpio_mknod(const char *name, unsigned int mode,
 
 	if (name[0] == '/')
 		name++;
+	namesize = strlen(name) + 1;
+
 	len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 	       "%08X%08X%08X%08X%08X%08X%08X",
 		do_csum ? "070702" : "070701", /* magic */
@@ -287,12 +295,12 @@ static int cpio_mknod(const char *name, unsigned int mode,
 		1,			/* minor */
 		maj,			/* rmajor */
 		min,			/* rminor */
-		(unsigned)strlen(name) + 1,/* namesize */
+		namesize,		/* namesize */
 		0);			/* chksum */
 	offset += len;
 
 	if (len != CPIO_HDR_LEN
-	 || push_rest(name) < 0)
+	 || push_rest(name, namesize) < 0)
 		return -1;
 
 	return 0;
@@ -426,7 +434,7 @@ static int cpio_mkfile(const char *name, const char *location,
 		offset += len;
 
 		if (len != CPIO_HDR_LEN
-		 || push_string(name) < 0
+		 || push_buf(name, namesize) < 0
 		 || push_pad(padlen(offset, 4)) < 0)
 			goto error;
 
-- 
2.43.0

[PATCH v2 5/7] gen_initramfs.sh: use gen_init_cpio -o parameter

[David Disseldorp]

gen_init_cpio can now write to a file directly, so use it when
gen_initramfs.sh is called with -o (e.g. usr/Makefile invocation).

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_initramfs.sh | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/usr/gen_initramfs.sh b/usr/gen_initramfs.sh
index 14b5782f961a8..7eba2fddf0ef2 100755
--- a/usr/gen_initramfs.sh
+++ b/usr/gen_initramfs.sh
@@ -193,7 +193,8 @@ root_gid=0
 dep_list=
 timestamp=
 cpio_list=$(mktemp ${TMPDIR:-/tmp}/cpiolist.XXXXXX)
-output="/dev/stdout"
+# gen_init_cpio writes to stdout by default
+output=""
 
 trap "rm -f $cpio_list" EXIT
 
@@ -207,7 +208,7 @@ while [ $# -gt 0 ]; do
 			shift
 			;;
 		"-o")	# generate cpio image named $1
-			output="$1"
+			output="-o $1"
 			shift
 			;;
 		"-u")	# map $1 to uid=0 (root)
@@ -246,4 +247,4 @@ done
 
 # If output_file is set we will generate cpio archive
 # we are careful to delete tmp files
-usr/gen_init_cpio $timestamp $cpio_list > $output
+usr/gen_init_cpio $output $timestamp $cpio_list
-- 
2.43.0

Re: [PATCH v2 5/7] gen_initramfs.sh: use gen_init_cpio -o parameter

[Nicolas Schier]

On Thu, Aug 14, 2025 at 03:18:03PM +1000, David Disseldorp wrote:
> gen_init_cpio can now write to a file directly, so use it when
> gen_initramfs.sh is called with -o (e.g. usr/Makefile invocation).
> 
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
>  usr/gen_initramfs.sh | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/usr/gen_initramfs.sh b/usr/gen_initramfs.sh
> index 14b5782f961a8..7eba2fddf0ef2 100755
> --- a/usr/gen_initramfs.sh
> +++ b/usr/gen_initramfs.sh
> @@ -193,7 +193,8 @@ root_gid=0
>  dep_list=
>  timestamp=
>  cpio_list=$(mktemp ${TMPDIR:-/tmp}/cpiolist.XXXXXX)
> -output="/dev/stdout"
> +# gen_init_cpio writes to stdout by default
> +output=""
>  
>  trap "rm -f $cpio_list" EXIT
>  
> @@ -207,7 +208,7 @@ while [ $# -gt 0 ]; do
>  			shift
>  			;;
>  		"-o")	# generate cpio image named $1
> -			output="$1"
> +			output="-o $1"
>  			shift
>  			;;
>  		"-u")	# map $1 to uid=0 (root)
> @@ -246,4 +247,4 @@ done
>  
>  # If output_file is set we will generate cpio archive
>  # we are careful to delete tmp files
> -usr/gen_init_cpio $timestamp $cpio_list > $output
> +usr/gen_init_cpio $output $timestamp $cpio_list

I think it would have been sufficient to replace '> $output' by
'-o $output'.

[PATCH v2 6/7] docs: initramfs: file data alignment via name padding

[David Disseldorp]

The existing cpio extraction logic reads (maximum PATH_MAX) name_len
bytes from the archive into the collected name buffer and ensures that
the trailing byte is a null-terminator. This allows the actual file name
to be shorter than name_len, with the name string simply zero-terminated
prior to the last byte.

Initramfs generators, such as dracut-cpio[1], can take advantage of name
zero-padding to align file data segments within the archive to
filesystem block boundaries. Block boundary alignment may allow the
copy_file_range syscall to reflink archive source and destination
extents.

Link: https://github.com/dracutdevs/dracut/commit/300e4b116c624bca1b9e7251708b1ae656fe9157 [1]
Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 Documentation/driver-api/early-userspace/buffer-format.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/driver-api/early-userspace/buffer-format.rst b/Documentation/driver-api/early-userspace/buffer-format.rst
index 726bfa2fe70da..4597a91100b7b 100644
--- a/Documentation/driver-api/early-userspace/buffer-format.rst
+++ b/Documentation/driver-api/early-userspace/buffer-format.rst
@@ -86,6 +86,11 @@ c_mtime is ignored unless CONFIG_INITRAMFS_PRESERVE_MTIME=y is set.
 The c_filesize should be zero for any file which is not a regular file
 or symlink.
 
+c_namesize may account for more than one trailing '\0', as long as the
+value doesn't exceed PATH_MAX.  This can be useful for ensuring that a
+subsequent file data segment is aligned, e.g. to a filesystem block
+boundary.
+
 The c_chksum field contains a simple 32-bit unsigned sum of all the
 bytes in the data field.  cpio(1) refers to this as "crc", which is
 clearly incorrect (a cyclic redundancy check is a different and
-- 
2.43.0

[PATCH v2 7/7] gen_init_cpio: add -a <data_align> as reflink optimization

[David Disseldorp]

As described in buffer-format.rst, the existing initramfs.c extraction
logic works fine if the cpio filename field is padded out with trailing
zeros, with a caveat that the padded namesize can't exceed PATH_MAX.

Add filename zero-padding logic to gen_init_cpio, which can be triggered
via the new -a <data_align> parameter. Performance and storage
utilization is improved for Btrfs and XFS workloads, as copy_file_range
can reflink the entire source file into a filesystem block-size aligned
destination offset within the cpio archive.

Btrfs benchmarks run on 6.15.8-1-default (Tumbleweed) x86_64 host:
  > truncate --size=2G /tmp/backing.img
  > /sbin/mkfs.btrfs /tmp/backing.img
  ...
  Sector size:        4096        (CPU page size: 4096)
  ...
  > sudo mount /tmp/backing.img mnt
  > sudo chown $USER mnt
  > cd mnt
  mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
  ...
  mnt> echo "file /foo foo 0755 0 0" > list
  mnt> perf stat -r 10 gen_init_cpio -o unaligned_btrfs list
  ...
            0.023496 +- 0.000472 seconds time elapsed  ( +-  2.01% )

  mnt> perf stat -r 10 gen_init_cpio -o aligned_btrfs -a 4096 list
  ...
           0.0010010 +- 0.0000565 seconds time elapsed  ( +-  5.65% )

  mnt> /sbin/xfs_io -c "fiemap -v" unaligned_btrfs
  unaligned_btrfs:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..40967]:      695040..736007   40968   0x1
  mnt> /sbin/xfs_io -c "fiemap -v" aligned_btrfs
  aligned_btrfs:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..7]:          26768..26775         8   0x0
     1: [8..40967]:      269056..310015   40960 0x2000
     2: [40968..40975]:  26776..26783         8   0x1
  mnt> /sbin/btrfs fi du unaligned_btrfs aligned_btrfs
       Total   Exclusive  Set shared  Filename
    20.00MiB    20.00MiB       0.00B  unaligned_btrfs
    20.01MiB     8.00KiB    20.00MiB  aligned_btrfs

XFS benchmarks run on same host:
  > sudo umount mnt && rm /tmp/backing.img
  > truncate --size=2G /tmp/backing.img
  > /sbin/mkfs.xfs /tmp/backing.img
  ...
           =                       reflink=1    ...
  data     =                       bsize=4096   blocks=524288, imaxpct=25
  ...
  > sudo mount /tmp/backing.img mnt
  > sudo chown $USER mnt
  > cd mnt
  mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
  ...
  mnt> echo "file /foo foo 0755 0 0" > list
  mnt> perf stat -r 10 gen_init_cpio -o unaligned_xfs list
  ...
            0.011069 +- 0.000469 seconds time elapsed  ( +-  4.24% )

  mnt> perf stat -r 10 gen_init_cpio -o aligned_xfs -a 4096 list
  ...
            0.001273 +- 0.000288 seconds time elapsed  ( +- 22.60% )

  mnt> /sbin/xfs_io -c "fiemap -v" unaligned_xfs
   unaligned_xfs:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..40967]:      106176..147143   40968   0x0
     1: [40968..65023]:  147144..171199   24056 0x801
  mnt> /sbin/xfs_io -c "fiemap -v" aligned_xfs
   aligned_xfs:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..7]:          120..127             8   0x0
     1: [8..40967]:      192..41151       40960 0x2000
     2: [40968..40975]:  236728..236735       8   0x0
     3: [40976..106495]: 236736..302255   65520 0x801

The alignment is best-effort; a stderr message is printed if alignment
can't be achieved due to PATH_MAX overrun, with fallback to non-padded
filename. This allows it to still be useful for opportunistic alignment,
e.g. on aarch64 Btrfs with 64K block-size. Alignment failure messages
provide an indicator that reordering of the cpio-manifest may be
beneficial.

Archive read performance for reflinked initramfs images may suffer due
to the effects of fragmentation, particularly on spinning disks. To
mitigate excessive fragmentation, files with lengths less than
data_align aren't padded.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 usr/gen_init_cpio.c | 50 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 12 deletions(-)

diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
index 40f4cbd95844e..75bf95d327171 100644
--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -28,13 +28,15 @@
 #define CPIO_TRAILER "TRAILER!!!"
 #define padlen(_off, _align) (((_align) - ((_off) & ((_align) - 1))) % (_align))
 
-static char padding[512];
+/* zero-padding the filename field for data alignment is limited by PATH_MAX */
+static char padding[PATH_MAX];
 static unsigned int offset;
 static unsigned int ino = 721;
 static time_t default_mtime;
 static bool do_file_mtime;
 static bool do_csum = false;
 static int outfd = STDOUT_FILENO;
+static unsigned int dalign;
 
 struct file_handler {
 	const char *type;
@@ -359,7 +361,7 @@ static int cpio_mkfile(const char *name, const char *location,
 	int file, retval, len;
 	int rc = -1;
 	time_t mtime;
-	int namesize;
+	int namesize, namepadlen;
 	unsigned int i;
 	uint32_t csum = 0;
 	ssize_t this_read;
@@ -407,14 +409,27 @@ static int cpio_mkfile(const char *name, const char *location,
 	}
 
 	size = 0;
+	namepadlen = 0;
 	for (i = 1; i <= nlinks; i++) {
-		/* data goes on last link */
-		if (i == nlinks)
-			size = buf.st_size;
-
 		if (name[0] == '/')
 			name++;
 		namesize = strlen(name) + 1;
+
+		/* data goes on last link, after any alignment padding */
+		if (i == nlinks)
+			size = buf.st_size;
+
+		if (dalign && size > dalign) {
+			namepadlen = padlen(offset + CPIO_HDR_LEN + namesize,
+					    dalign);
+			if (namesize + namepadlen > PATH_MAX) {
+				fprintf(stderr,
+					"%s: best-effort alignment %u missed\n",
+					name, dalign);
+				namepadlen = 0;
+			}
+		}
+
 		len = dprintf(outfd, "%s%08X%08X%08lX%08lX%08X%08lX"
 		       "%08lX%08X%08X%08X%08X%08X%08X",
 			do_csum ? "070702" : "070701", /* magic */
@@ -429,13 +444,13 @@ static int cpio_mkfile(const char *name, const char *location,
 			1,			/* minor */
 			0,			/* rmajor */
 			0,			/* rminor */
-			namesize,		/* namesize */
+			namesize + namepadlen,	/* namesize */
 			size ? csum : 0);	/* chksum */
 		offset += len;
 
 		if (len != CPIO_HDR_LEN
 		 || push_buf(name, namesize) < 0
-		 || push_pad(padlen(offset, 4)) < 0)
+		 || push_pad(namepadlen ? namepadlen : padlen(offset, 4)) < 0)
 			goto error;
 
 		if (size) {
@@ -552,8 +567,7 @@ static int cpio_mkfile_line(const char *line)
 static void usage(const char *prog)
 {
 	fprintf(stderr, "Usage:\n"
-		"\t%s [-t <timestamp>] [-c] [-o <output_path>] <cpio_list>\n"
-		"\n"
+		"\t%s [-t <timestamp>] [-c] [-o <output_path>] [-a <data_align>] <cpio_list>\n\n"
 		"<cpio_list> is a file containing newline separated entries that\n"
 		"describe the files to be included in the initramfs archive:\n"
 		"\n"
@@ -590,7 +604,10 @@ static void usage(const char *prog)
 		"The default is to use the current time for all files, but\n"
 		"preserve modification time for regular files.\n"
 		"-c: calculate and store 32-bit checksums for file data.\n"
-		"<output_path>: write cpio to this file instead of stdout\n",
+		"<output_path>: write cpio to this file instead of stdout\n"
+		"<data_align>: attempt to align file data by zero-padding the\n"
+		"filename field up to data_align. Must be a multiple of 4.\n"
+		"Alignment is best-effort; PATH_MAX limits filename padding.\n",
 		prog);
 }
 
@@ -632,7 +649,7 @@ int main (int argc, char *argv[])
 
 	default_mtime = time(NULL);
 	while (1) {
-		int opt = getopt(argc, argv, "t:cho:");
+		int opt = getopt(argc, argv, "t:cho:a:");
 		char *invalid;
 
 		if (opt == -1)
@@ -661,6 +678,15 @@ int main (int argc, char *argv[])
 				exit(1);
 			}
 			break;
+		case 'a':
+			dalign = strtoul(optarg, &invalid, 10);
+			if (!*optarg || *invalid || (dalign & 3)) {
+				fprintf(stderr, "Invalid data_align: %s\n",
+						optarg);
+				usage(argv[0]);
+				exit(1);
+			}
+			break;
 		case 'h':
 		case '?':
 			usage(argv[0]);
-- 
2.43.0

Re: [PATCH v2 7/7] gen_init_cpio: add -a <data_align> as reflink optimization

[Nicolas Schier]

On Thu, Aug 14, 2025 at 03:18:05PM +1000, David Disseldorp wrote:
> As described in buffer-format.rst, the existing initramfs.c extraction
> logic works fine if the cpio filename field is padded out with trailing
> zeros, with a caveat that the padded namesize can't exceed PATH_MAX.
> 
> Add filename zero-padding logic to gen_init_cpio, which can be triggered
> via the new -a <data_align> parameter. Performance and storage
> utilization is improved for Btrfs and XFS workloads, as copy_file_range
> can reflink the entire source file into a filesystem block-size aligned
> destination offset within the cpio archive.
> 
> Btrfs benchmarks run on 6.15.8-1-default (Tumbleweed) x86_64 host:
>   > truncate --size=2G /tmp/backing.img
>   > /sbin/mkfs.btrfs /tmp/backing.img
>   ...
>   Sector size:        4096        (CPU page size: 4096)
>   ...
>   > sudo mount /tmp/backing.img mnt
>   > sudo chown $USER mnt
>   > cd mnt
>   mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
>   ...
>   mnt> echo "file /foo foo 0755 0 0" > list
>   mnt> perf stat -r 10 gen_init_cpio -o unaligned_btrfs list
>   ...
>             0.023496 +- 0.000472 seconds time elapsed  ( +-  2.01% )
> 
>   mnt> perf stat -r 10 gen_init_cpio -o aligned_btrfs -a 4096 list
>   ...
>            0.0010010 +- 0.0000565 seconds time elapsed  ( +-  5.65% )
> 
>   mnt> /sbin/xfs_io -c "fiemap -v" unaligned_btrfs
>   unaligned_btrfs:
>    EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>      0: [0..40967]:      695040..736007   40968   0x1
>   mnt> /sbin/xfs_io -c "fiemap -v" aligned_btrfs
>   aligned_btrfs:
>    EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>      0: [0..7]:          26768..26775         8   0x0
>      1: [8..40967]:      269056..310015   40960 0x2000
>      2: [40968..40975]:  26776..26783         8   0x1
>   mnt> /sbin/btrfs fi du unaligned_btrfs aligned_btrfs
>        Total   Exclusive  Set shared  Filename
>     20.00MiB    20.00MiB       0.00B  unaligned_btrfs
>     20.01MiB     8.00KiB    20.00MiB  aligned_btrfs
> 
> XFS benchmarks run on same host:
>   > sudo umount mnt && rm /tmp/backing.img
>   > truncate --size=2G /tmp/backing.img
>   > /sbin/mkfs.xfs /tmp/backing.img
>   ...
>            =                       reflink=1    ...
>   data     =                       bsize=4096   blocks=524288, imaxpct=25
>   ...
>   > sudo mount /tmp/backing.img mnt
>   > sudo chown $USER mnt
>   > cd mnt
>   mnt> dd if=/dev/urandom of=foo bs=1M count=20 && cat foo >/dev/null
>   ...
>   mnt> echo "file /foo foo 0755 0 0" > list
>   mnt> perf stat -r 10 gen_init_cpio -o unaligned_xfs list
>   ...
>             0.011069 +- 0.000469 seconds time elapsed  ( +-  4.24% )
> 
>   mnt> perf stat -r 10 gen_init_cpio -o aligned_xfs -a 4096 list
>   ...
>             0.001273 +- 0.000288 seconds time elapsed  ( +- 22.60% )
> 
>   mnt> /sbin/xfs_io -c "fiemap -v" unaligned_xfs
>    unaligned_xfs:
>    EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>      0: [0..40967]:      106176..147143   40968   0x0
>      1: [40968..65023]:  147144..171199   24056 0x801
>   mnt> /sbin/xfs_io -c "fiemap -v" aligned_xfs
>    aligned_xfs:
>    EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>      0: [0..7]:          120..127             8   0x0
>      1: [8..40967]:      192..41151       40960 0x2000
>      2: [40968..40975]:  236728..236735       8   0x0
>      3: [40976..106495]: 236736..302255   65520 0x801
> 
> The alignment is best-effort; a stderr message is printed if alignment
> can't be achieved due to PATH_MAX overrun, with fallback to non-padded
> filename. This allows it to still be useful for opportunistic alignment,
> e.g. on aarch64 Btrfs with 64K block-size. Alignment failure messages
> provide an indicator that reordering of the cpio-manifest may be
> beneficial.
> 
> Archive read performance for reflinked initramfs images may suffer due
> to the effects of fragmentation, particularly on spinning disks. To
> mitigate excessive fragmentation, files with lengths less than
> data_align aren't padded.
> 
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
>  usr/gen_init_cpio.c | 50 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 38 insertions(+), 12 deletions(-)

Thanks!  Testing with a massively oversized initramfs (600MB) was fun:
from 2:44 down to 38s.


Questions that pop up in my mind:
Now, how can we make other benefit from this?  Might it make sense to
introduce a kconfig variable for initramfs alignment -- even though this
is just a build-time optimisation of few seconds?


Kind regards,
Nicolas

Re: [PATCH v2 7/7] gen_init_cpio: add -a <data_align> as reflink optimization

[David Disseldorp]

Oops, I somehow missed this comment...

On Mon, 18 Aug 2025 21:23:36 +0200, Nicolas Schier wrote:
...
> Thanks!  Testing with a massively oversized initramfs (600MB) was fun:
> from 2:44 down to 38s.

Thanks for the benchmarking :)

> Questions that pop up in my mind:
> Now, how can we make other benefit from this?  Might it make sense to
> introduce a kconfig variable for initramfs alignment -- even though this
> is just a build-time optimisation of few seconds?

A kconfig option with non-aligned as default isn't a bad idea, but at
this stage I don't think it's worth adding. Mostly because reflinks are
very FS / arch / environment specific and the extra fragmentation may
have unintended consequences.

Thanks, David

Re: [PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support

[Nicolas Schier]

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On Thu, Aug 14, 2025 at 03:17:58PM +1000, David Disseldorp wrote:
> This patchset adds copy_file_range() support to gen_init_cpio. When
> combined with data segment alignment, large-file archiving performance
> is improved on Btrfs and XFS due to reflinks (see 7/7 benchmarks).
> 
> cpio data segment alignment is provided by "bending" the newc spec
> to zero-pad the filename field. GNU cpio and Linux initramfs extractors
> handle this fine as long as PATH_MAX isn't exceeded.
> 
> Changes since v1 RFC
> - add alignment patches 6-7
> - slightly rework commit and error messages
> - rename l->len to avoid 1/i confusion
> 
> David Disseldorp (7):
>       gen_init_cpio: write to fd instead of stdout stream
>       gen_init_cpio: support -o <output_path> parameter
>       gen_init_cpio: attempt copy_file_range for file data
>       gen_init_cpio: avoid duplicate strlen calls
>       gen_initramfs.sh: use gen_init_cpio -o parameter
>       docs: initramfs: file data alignment via name padding
>       gen_init_cpio: add -a <data_align> as reflink optimization
> 
>  .../driver-api/early-userspace/buffer-format.rst   |   5 +
>  usr/gen_init_cpio.c                                | 234 ++++++++++++++-------
>  usr/gen_initramfs.sh                               |   7 +-
>  3 files changed, 166 insertions(+), 80 deletions(-)
> 
> 

Thanks for the series!  I have found only a minor nick pick and some few
bike-shedding things.

Reviewed-by: Nicolas Schier <nsc@kernel.org>

Kind regards,
Nicolas

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Re: [PATCH v2 0/7] gen_init_cpio: add copy_file_range / reflink support

[David Disseldorp]

On Mon, 18 Aug 2025 20:40:23 +0200, Nicolas Schier wrote:

> On Thu, Aug 14, 2025 at 03:17:58PM +1000, David Disseldorp wrote:
> > This patchset adds copy_file_range() support to gen_init_cpio. When
> > combined with data segment alignment, large-file archiving performance
> > is improved on Btrfs and XFS due to reflinks (see 7/7 benchmarks).
> > 
> > cpio data segment alignment is provided by "bending" the newc spec
> > to zero-pad the filename field. GNU cpio and Linux initramfs extractors
> > handle this fine as long as PATH_MAX isn't exceeded.
> > 
> > Changes since v1 RFC
> > - add alignment patches 6-7
> > - slightly rework commit and error messages
> > - rename l->len to avoid 1/i confusion
> > 
> > David Disseldorp (7):
> >       gen_init_cpio: write to fd instead of stdout stream
> >       gen_init_cpio: support -o <output_path> parameter
> >       gen_init_cpio: attempt copy_file_range for file data
> >       gen_init_cpio: avoid duplicate strlen calls
> >       gen_initramfs.sh: use gen_init_cpio -o parameter
> >       docs: initramfs: file data alignment via name padding
> >       gen_init_cpio: add -a <data_align> as reflink optimization
> > 
> >  .../driver-api/early-userspace/buffer-format.rst   |   5 +
> >  usr/gen_init_cpio.c                                | 234 ++++++++++++++-------
> >  usr/gen_initramfs.sh                               |   7 +-
> >  3 files changed, 166 insertions(+), 80 deletions(-)
> > 
> >   
> 
> Thanks for the series!  I have found only a minor nick pick and some few
> bike-shedding things.
> 
> Reviewed-by: Nicolas Schier <nsc@kernel.org>

Thanks a lot for the review, Nicolas!
I'll post a v3 with your suggestions addressed, as I have one further
initramfs_test.c patch to padded filename test coverage.

Cheers, David