ubifs: sync() causes writes even if nothing is changed

[Artem Bityutskiy]

On Wed, 2010-10-13 at 18:30 +0200, Hans J. Koch wrote:
> Running this command:
> 
> # while true ; do sync; sleep 1; done
> 
> causes two eraseblocks being erased every second, although there
> are no writes to the ubifs filesystem. I hacked some printks into
> my NAND driver that print page_address and column for each erase.
> With that, I get this output every second:
> 
> ...
> [   63.701765] erase p=0x0000ae40 c=0xffffffff
> [   63.706534] erase p=0xffffffff c=0xffffffff
> [   63.725492] erase p=0x0000ae80 c=0xffffffff
> [   63.730260] erase p=0xffffffff c=0xffffffff
> ...
> 
> From a quick glance at the ubifs code, this might come out of the
> garbage collector that is triggered on every sync() and writes
> something even if nothing has changed.

With nandsim I only can see one erase, but this is anyway suboptimal.
The below patch should fix the issue, please, test if you can. I've also
pushed it to ubifs-2.6.git.