libfdisk: support alignment to non power of 2

Let's create a disk with 33553920 bytes optimal I/O size:

	# modprobe scsi_debug dev_size_mb=1000 opt_blks=65535

and try to create partition:

	 echo -e 'n\n\n\n\n+512M\np\nq\n'  | fdisk /dev/sdc

old version:

	 Device     Boot Start     End Sectors  Size Id Type
	 /dev/sdc1       65535 1114110 1048576  512M 83 Linux

The next partition will be expected on sector 1114110 + 1, but it's
not aligned to optimal I/O:

        ((1114110 + 1) * 512) % 33553920 = 8192

fixed version:

 Device     Boot Start     End Sectors  Size Id Type
 /dev/sdc1       65535 1114094 1048560  512M 83 Linux

        ((1114094 + 1) * 512) % 33553920 = 0

Note that the same problem with alignment calculation has been fixed
in Linux kernel by commit b8839b8c55f3fdd60dc36abcda7e0266aff7985c
(Oct 2014).

The patch also improves fdisk_align_lba_in_range() to not align sizes
smaller than grain (default 1MiB) to make it possible to create really
small partitions.

Reported-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 24 insertions, 7 deletions

diff --git a/libfdisk/src/alignment.c b/libfdisk/src/alignment.c
index 1baee57a8..c658a5bee 100644
--- a/libfdisk/src/alignment.c
+++ b/libfdisk/src/alignment.c
@@ -36,16 +36,17 @@
 /*
  * Alignment according to logical granularity (usually 1MiB)
  */
-static int lba_is_aligned(struct fdisk_context *cxt, fdisk_sector_t lba)
+static int lba_is_aligned(struct fdisk_context *cxt, uintmax_t lba)
 {
 	unsigned long granularity = max(cxt->phy_sector_size, cxt->min_io_size);
 	uintmax_t offset;
 
 	if (cxt->grain > granularity)
 		granularity = cxt->grain;
-	offset = (lba * cxt->sector_size) & (granularity - 1);
 
-	return !((granularity + cxt->alignment_offset - offset) & (granularity - 1));
+	offset = (lba * cxt->sector_size) % granularity;
+
+	return !((granularity + cxt->alignment_offset - offset) % granularity);
 }
 
 /*
@@ -54,9 +55,9 @@ static int lba_is_aligned(struct fdisk_context *cxt, fdisk_sector_t lba)
 static int lba_is_phy_aligned(struct fdisk_context *cxt, fdisk_sector_t lba)
 {
 	unsigned long granularity = max(cxt->phy_sector_size, cxt->min_io_size);
-	uintmax_t offset = (lba * cxt->sector_size) & (granularity - 1);
+	uintmax_t offset = (lba * cxt->sector_size) % granularity;
 
-	return !((granularity + cxt->alignment_offset - offset) & (granularity - 1));
+	return !((granularity + cxt->alignment_offset - offset) % granularity);
 }
 
 /**
@@ -111,9 +112,15 @@ fdisk_sector_t fdisk_align_lba(struct fdisk_context *cxt, fdisk_sector_t lba, in
 	}
 
 	if (lba != res)
-		DBG(CXT, ul_debugobj(cxt, "LBA %ju -aligned-to-> %ju",
+		DBG(CXT, ul_debugobj(cxt, "LBA %ju -aligned-%s-> %ju [grain=%jus]",
 				(uintmax_t) lba,
-				(uintmax_t) res));
+				direction == FDISK_ALIGN_UP ? "up" :
+				direction == FDISK_ALIGN_DOWN ? "down" : "near",
+				(uintmax_t) res,
+				cxt->grain / cxt->sector_size));
+	else
+		DBG(CXT, ul_debugobj(cxt, "LBA %ju -unchanged-", lba));
+
 	return res;
 }
 
@@ -135,6 +142,15 @@ fdisk_sector_t fdisk_align_lba_in_range(struct fdisk_context *cxt,
 
 	start = fdisk_align_lba(cxt, start, FDISK_ALIGN_UP);
 	stop = fdisk_align_lba(cxt, stop, FDISK_ALIGN_DOWN);
+
+	if (lba > start && lba < stop
+	    && (lba - start) < (cxt->grain / cxt->sector_size)) {
+
+		DBG(CXT, ul_debugobj(cxt, "LBA: area smaller than grain, don't align"));
+		res = lba;
+		goto done;
+	}
+
 	lba = fdisk_align_lba(cxt, lba, FDISK_ALIGN_NEAREST);
 
 	if (lba < start)
@@ -144,6 +160,7 @@ fdisk_sector_t fdisk_align_lba_in_range(struct fdisk_context *cxt,
 	else
 		res = lba;
 
+done:
 	DBG(CXT, ul_debugobj(cxt, "LBA %ju range:<%ju..%ju>, result: %ju",
 				(uintmax_t) lba,
 				(uintmax_t) start,