Re: FAT filesystem write and long names stability

[Sascha Hauer <s.hauer@pengutronix.de>]

On Thu, Dec 15, 2011 at 10:28:38AM +0100, Robert Jarzmik wrote:
> Robert Jarzmik <robert.jarzmik@free.fr> writes:
> 
> > Sascha Hauer <s.hauer@pengutronix.de> writes:
> >> It would be great if you could give it some testing as obviously my test
> >> patterns didn't reveal the 'create folder' bug you described.
> > Tomorrow or wednesday, I'll make the few tests I always do (making a
> > directory, copying a file, deleting a file, etc ...) and report.
> This is what my tests give back:
> 
> **** With FAT caching (barebox.git/next + rjk mioa701 support):
> barebox:/sdcard time bmp /sdcard/help.bmp 
> time: 6895ms
> barebox:/sdcard time cp /sdcard/help.bmp /sdcard/toto.tmp
> time: 13278ms
> barebox:/sdcard ls -l help.bmp
> -rwxrwxrwx     230454 help.bmp
> barebox:/sdcard 

There is something wrong. These times are far from being acceptable.

Here's what I get with copying a 2MiB file from SD Card:

barebox@Freescale i.MX51 PDK:/ time cp /fat/x /x
time: 273ms

> 
> **** Without FAT caching (merged pu/block into barebox.git/next + rjk mioa701 support)
> barebox:/sdcard time bmp /sdcard/help.bmp
> time: 9219ms
> barebox:/sdcard time cp /sdcard/help.bmp /sdcard/titi.tmp
> BUG: failure at common/block.c:248/block_

Hm, I don't understand where this can come from.

	data = block_get(blk, block);
	if (!data)
		BUG();

So this bug is triggered when block_get fails:

static void *block_get(struct block_device *blk, int block)
{
	void *outdata;
	int ret;

	if (block >= blk->num_blocks)
		return NULL;

	outdata = block_get_cached(blk, block);
	if (outdata)
		return outdata;

	ret = block_cache(blk, block);
	if (ret)
		return NULL;

	outdata = block_get_cached(blk, block);
	if (!outdata)
		BUG();

	return outdata;
}

So block_get fails when

a) You access some block outside the device (which should have been
   caught earlier
b) block_cache fails. (This indeed can fail when the underlying hardware
   fails to read the block, so the BUG should be replaced with a simple
   error return)

> 
> The "BUG" thing prevents me from going further. But it seems the cache in main
> tree speeds up things. I also think that creating directories without files
> serves no purpose I can think of, and is an acceptable tradeoff for the improved
> performance.

It's not only the creating-directory-without-files thing, The FAT can
become corrupted in current mainline, so we have to do something.

Sascha


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