[PATCH] mm: swapfile: fix /proc/swaps heading and Size/Used/Priority alignment

[Randy Dunlap]

From: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>

Fix the heading and Size/Used/Priority field alignments in
/proc/swaps. If the Size and/or Used value is >= 10000000 (8 bytes),
then the alignment by using tab characters is broken.

This patch maintains the use of tabs for alignment. If spaces
are preferred, we can just use a Field Width specifier for the
bytes and inuse fields. That way those fields don't have to be
a multiple of 8 bytes in width. E.g., with a field width of 12,
both Size and Used would always fit on the first line of an
80-column wide terminal (only Priority would be on the second line).

There are actually 2 problems: heading alignment and field width.
On an xterm, if Used is 7 bytes in length, the tab does nothing, and
the display is like this, with no space/tab between the Used and
Priority fields. (ugh)

Filename				Type		Size	Used	Priority
/dev/sda8                               partition	16779260	2023012-1

To be clear, if one does 'cat /proc/swaps >/tmp/proc.swaps', it does
look different, like so:

Filename				Type		Size	Used	Priority
/dev/sda8                               partition	16779260	2086988	-1

Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 mm/swapfile.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

--- linux-next-20200521.orig/mm/swapfile.c
+++ linux-next-20200521/mm/swapfile.c
@@ -2753,20 +2753,24 @@ static int swap_show(struct seq_file *sw
 	struct swap_info_struct *si = v;
 	struct file *file;
 	int len;
+	unsigned int bytes, inuse;
 
 	if (si == SEQ_START_TOKEN) {
-		seq_puts(swap,"Filename\t\t\t\tType\t\tSize\tUsed\tPriority\n");
+		seq_puts(swap,"Filename\t\t\t\tType\t\tSize\t\tUsed\t\tPriority\n");
 		return 0;
 	}
 
+	bytes = si->pages << (PAGE_SHIFT - 10);
+	inuse = si->inuse_pages << (PAGE_SHIFT - 10);
+
 	file = si->swap_file;
 	len = seq_file_path(swap, file, " \t\n\\");
-	seq_printf(swap, "%*s%s\t%u\t%u\t%d\n",
+	seq_printf(swap, "%*s%s\t%u\t%s%u\t%s%d\n",
 			len < 40 ? 40 - len : 1, " ",
 			S_ISBLK(file_inode(file)->i_mode) ?
 				"partition" : "file\t",
-			si->pages << (PAGE_SHIFT - 10),
-			si->inuse_pages << (PAGE_SHIFT - 10),
+			bytes, bytes < 10000000 ? "\t" : "",
+			inuse, inuse < 10000000 ? "\t" : "",
 			si->prio);
 	return 0;
 }