Fwd: swapon and swapoff testcases on busybox

[Luciano Miguel Ferreira Rocha]

On Tue, Mar 13, 2007 at 01:09:30PM +0530, Riaz Rahaman wrote:
> / # dd if=/dev/zero of=swappyfile bs=1 count=1 seek=500000
> 1+0 records in
> 1+0 records out

That creates a file with holes, and the kernel *doesn't* support that.
Swap files are already chaotic enough without supporting filesystem
block allocation on critically low memory cases.

> / # ./stat_riaz   --> a small binary that does a stat on swappyfile and

Don't forget mkswap.

Just tested here, a file with holes and one without, and swapon using
the simple program:
#include <unistd.h>
#include <asm/page.h>
#include <sys/swap.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
        while (*++argv) {
                if (swapon(*argv, 0))
                        perror(*argv);
                }
        }
        return 0;
}

# dd if=/dev/zero of=swp bs=1 count=1 seek=128000000 # with holes
# mkswap swp
Setting up swapspace version 1, size = 127995 kB
# ./swpo swp
swp: Invalid argument
# dd if=/dev/zero of=swp2 bs=100000 count=1280 # w/o holes
# mkswap swp2
Setting up swapspace version 1, size = 127995 kB
# ./swpo swp2
# grep swp /proc/swaps
/root/swp2                              file            124992  0 -3


> prints the below values
> stat_buf.st_blocks: 506
> stat_buf.st_size: 500001
> stat_buf.st_blksize: 512

# stat swp swp2 | grep Block
  Size: 128000001       Blocks: 32         IO Block: 4096   regular file
  Size: 128000000       Blocks: 250256     IO Block: 4096   regular file

Note that IO Block is *IO* Block, not block size:
$ man 2 stat
...
     The st_blocks field indicates the number of  blocks  allocated to  the
     file, 512-byte units.  (This may be smaller than st_size/512, for exam-
     ple, when the file has holes.)

     The st_blksize field gives the "preferred" blocksize for efficient file
     system  I/O.  (Writing to a file in smaller chunks may cause an ineffi-
     cient read-modify-rewrite.)
...

-- 
lfr
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