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repart: add --grain-size= option for partition alignment

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Add a --grain-size= CLI option to override the default 4 KiB partition
alignment grain. Setting --grain-size=1M matches the alignment used by
fdisk/parted and fixes misaligned partitions after small fixed-size
partitions like the 16 KiB verity-sig partition.

Also fix context_place_partitions() to re-align the start offset after
each partition, not just once per free area. Without this, a small
partition would cause all subsequent partitions in the same free area
to start at an unaligned offset.
This commit is contained in:
Nandakumar Raghavan
2026-03-18 10:39:39 +00:00
committed by Luca Boccassi
parent 3236700a67
commit 36d129a7ad
2 changed files with 53 additions and 6 deletions
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man/systemd-repart.xml
+13
View File
@@ -535,6 +535,19 @@
<xi:include href="version-info.xml" xpointer="v253"/></listitem>
</varlistentry>
<varlistentry>
<term><option>--grain-size=<replaceable>BYTES</replaceable></option></term>
<listitem><para>This option controls the partition alignment granularity used when placing
partitions. It takes a power-of-2 value that is at least the sector size. All partition start
offsets will be rounded up to a multiple of this value. Defaults to
<constant>MAX(4096, sector_size)</constant>, matching the conventional 4 KiB alignment. Setting
this to <literal>1M</literal> ensures proper alignment for modern storage devices even after
small fixed-size partitions such as a verity signature partition.</para>
<xi:include href="version-info.xml" xpointer="v261"/></listitem>
</varlistentry>
<varlistentry>
<term><option>--architecture=<replaceable>ARCH</replaceable></option></term>
src/repart/repart.c
+40 -6
View File
@@ -200,6 +200,7 @@ static size_t arg_n_defer_partitions = 0;
static bool arg_defer_partitions_empty = false;
static bool arg_defer_partitions_factory_reset = false;
static uint64_t arg_sector_size = 0;
static uint64_t arg_grain_size = 0;
static ImagePolicy *arg_image_policy = NULL;
static Architecture arg_architecture = _ARCHITECTURE_INVALID;
static int arg_offline = -1;
@@ -617,6 +618,10 @@ static const char *progress_phase_table[_PROGRESS_PHASE_MAX] = {
[PROGRESS_REREADING_TABLE] = "rereading-table",
};
static uint64_t determine_grain_size(uint64_t sector_size) {
return MAX(arg_grain_size > 0 ? arg_grain_size : 4096U, sector_size);
}
DEFINE_PRIVATE_STRING_TABLE_LOOKUP(empty_mode, EmptyMode);
DEFINE_PRIVATE_STRING_TABLE_LOOKUP(append_mode, AppendMode);
DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING_WITH_BOOLEAN(encrypt_mode, EncryptMode, ENCRYPT_KEY_FILE);
@@ -1701,7 +1706,7 @@ static void context_place_partitions(Context *context) {
for (size_t i = 0; i < context->n_free_areas; i++) {
FreeArea *a = context->free_areas[i];
_unused_ uint64_t left;
uint64_t left;
uint64_t start;
if (a->after) {
@@ -1717,6 +1722,8 @@ static void context_place_partitions(Context *context) {
left = a->size;
LIST_FOREACH(partitions, p, context->partitions) {
uint64_t gap;
if (p->allocated_to_area != a)
continue;
@@ -1730,6 +1737,21 @@ static void context_place_partitions(Context *context) {
assert(left >= p->new_padding);
start += p->new_padding;
left -= p->new_padding;
/* Re-align start to the grain after each partition, so that the next
* partition placed into this free area also starts on a grain boundary.
* This matters when the grain is larger than the default (e.g. 1 MiB via
* --grain-size=) and a small partition like verity-sig (16 KiB) precedes
* a larger one: without this, the successor would start at an unaligned
* offset. */
gap = round_up_size(start, context->grain_size) - start;
if (gap > left) {
log_warning("Not enough space left in free area to re-align partition start to grain size, "
"next partition may start at an unaligned offset.");
gap = 0;
}
start += gap;
left -= gap;
}
}
}
@@ -3579,7 +3601,7 @@ static int context_load_fallback_metrics(Context *context) {
assert(context);
context->sector_size = arg_sector_size > 0 ? arg_sector_size : 512;
context->grain_size = MAX(context->sector_size, 4096U);
context->grain_size = determine_grain_size(context->sector_size);
context->default_fs_sector_size = arg_sector_size > 0 ? arg_sector_size : DEFAULT_FILESYSTEM_SECTOR_SIZE;
return 1; /* Starting from scratch */
}
@@ -3672,7 +3694,7 @@ static int context_load_partition_table(Context *context) {
/* Use the fallback values if we have no better idea */
context->sector_size = fdisk_get_sector_size(c);
context->default_fs_sector_size = fs_secsz;
context->grain_size = MAX(context->sector_size, 4096U);
context->grain_size = determine_grain_size(context->sector_size);
return /* from_scratch= */ true;
}
@@ -3701,9 +3723,9 @@ static int context_load_partition_table(Context *context) {
if (secsz < 512 || !ISPOWEROF2(secsz))
return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Sector size %lu is not a power of two larger than 512? Refusing.", secsz);
/* Use at least 4K, and ensure it's a multiple of the sector size, regardless if that is smaller or
* larger */
grainsz = MAX(secsz, 4096U);
/* Determine the grain size: by default at least 4K and a multiple of the sector size, but may be
* overridden via --grain-size=. */
grainsz = determine_grain_size(secsz);
log_debug("Sector size of device is %lu bytes. Using default filesystem sector size of %" PRIu64 " and grain size of %" PRIu64 ".", secsz, fs_secsz, grainsz);
@@ -9069,6 +9091,7 @@ static int help(void) {
" --offline=BOOL Whether to build the image offline\n"
" --discard=BOOL Whether to discard backing blocks for new partitions\n"
" --sector-size=SIZE Set the logical sector size for the image\n"
" --grain-size=BYTES Set the grain size for partition alignment\n"
" --architecture=ARCH Set the generic architecture for the image\n"
" --size=BYTES Grow loopback file to specified size\n"
" --seed=UUID 128-bit seed UUID to derive all UUIDs from\n"
@@ -9199,6 +9222,7 @@ static int parse_argv(int argc, char *argv[]) {
ARG_DEFER_PARTITIONS_EMPTY,
ARG_DEFER_PARTITIONS_FACTORY_RESET,
ARG_SECTOR_SIZE,
ARG_GRAIN_SIZE,
ARG_SKIP_PARTITIONS,
ARG_ARCHITECTURE,
ARG_OFFLINE,
@@ -9248,6 +9272,7 @@ static int parse_argv(int argc, char *argv[]) {
{ "defer-partitions-empty", required_argument, NULL, ARG_DEFER_PARTITIONS_EMPTY },
{ "defer-partitions-factory-reset", required_argument, NULL, ARG_DEFER_PARTITIONS_FACTORY_RESET },
{ "sector-size", required_argument, NULL, ARG_SECTOR_SIZE },
{ "grain-size", required_argument, NULL, ARG_GRAIN_SIZE },
{ "architecture", required_argument, NULL, ARG_ARCHITECTURE },
{ "offline", required_argument, NULL, ARG_OFFLINE },
{ "copy-from", required_argument, NULL, ARG_COPY_FROM },
@@ -9570,6 +9595,15 @@ static int parse_argv(int argc, char *argv[]) {
break;
case ARG_GRAIN_SIZE:
r = parse_size(optarg, 1024, &arg_grain_size);
if (r < 0)
return log_error_errno(r, "Failed to parse --grain-size= parameter: %s", optarg);
if (arg_grain_size < 512 || !ISPOWEROF2(arg_grain_size))
return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Grain size must be a power of 2 >= 512.");
break;
case ARG_ARCHITECTURE:
r = architecture_from_string(optarg);
if (r < 0)