Last year I wrote a piece about swap file strategies in general (Unix/Linux), but on further investigation I discovered Linux has a twist. On Unix there has never been any metadata written to a swap device – it’s just a sequence of blocks, and that’s what I’ve always told people. It turns out that with modern Linux, there is.
Before Linux will use a file or block device for swapping you need to use the mkswap command on it (there’s no Unix equivalent). The man page doesn’t exactly explain what it does as the low level, and I was curious. Unix hasn’t needed metadata on swap (or page) files from 1971 to the present day, so what is Linux doing differently?
Dumping a zeroed swap device before and after running mkswap on it revealed the following had been added:
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000400 01 00 00 00 ff ff 03 00 00 00 00 00 ad 40 e5 4b |.............@.K|
00000410 87 1a 46 d2 b0 36 ee 60 6d 40 08 d1 00 00 00 00 |..F..6.`m@......|
00000420 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000ff0 00 00 00 00 00 00 53 57 41 50 53 50 41 43 45 32 |......SWAPSPACE2|
00001000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
40000000
So it’s definitely doing something. As you can see the first 1K remains unchanged, which is presumably to avoid messing up partition tables and suchlike. There’s some binary data at 0x400 (1K if you don’t speak hex) and then right on the 4K boundary there’s what appears to be a signature, “SWAPSPACE2”.
Time to go digging into the source code (not so easy with a Linux, as it’s not normally present). It turns out that this is mapped by a union of two structures, and confirmed my suspisions:
union swap_header {
struct {
char reserved[PAGE_SIZE - 10];
char magic[10];
} magic;
struct {
char bootbits[1024];
__u32 version;
__u32 last_page;
__u32 nr_badpages;
__u32 padding[117];
__u32 badpages[1];
} info;
};If you’re not clear with ‘C’ structures, this is basically saying the header is PAGE_SIZE long, with the last ten bytes being used as a signature (called “magic” – the SWAPSPACE2 in the dump). PAGE_SIZE is, logically, the size of the page the kernel is using for virtual memory, so the header occupies exactly one slot. It’s found in arch/x86/include/asm/page.h, and is the result of a shift calculation based on another tunable, PAGE_SHIFT and results in 4K on most systems but might be adjusted to suit the MMU on non-Intel platforms.
The second structure in the swap_header, which is overlaid on the first because it’s a union, skips the first 1K, calling it “bootbits”, which is fair enough. There might be a bootstrap loader or partition table there. In fact there may well be if the whole drive is being used for swap. It’s not necessary to put a header on a raw block device but I suspect some Linux tools like to see one.
Then comes the version number, which appears to always be ‘1’, and the address of the last page in the device in case the kernel doesn’t know. Finally there’s the size bad page table, which according to the structure is one entry long. After padding it up to 0x600 there’s a bad page table, four bytes per entry, listing blocks on the page device that can’t be used. Since the introduction of IDE/SCSI drives around 1990, the disk handled bad block mapping so this is only really relevant to ST506 type interfaces, which indicates it must have been in the design of Linux since the very beginning – and even then it would have been for legacy drives.
There’s another twist I found in the code – the first sixteen bytes of the padding (0x40C to 0x41B) may contain a UUID (in binary, not ASCII) and 0x41C to 0x42B might contain a volume label if you set one using the -L option on mkswap (which is the source I looked at to find out what was going on).
If you try to run swapon on a device or file that doesn’t have this header it will bork. I had a look a the kernel, but I don’t think it cares – however I dare say some userland utilities do (including the aforementioned swapon).
Does this matter? Well it means you’re being short-changed by 4K of swap space on each device, but that’s hardly a big deal. The advantage is that it can detect and refuse to swap on a drive or partition you didn’t intend for swapping. The important thing is to remember to put the header on if you’re using Linux instead of Unix.