[PATCH] ubifs: Add new mount option to force fdatasync before rename

[Richard Weinberger]

Hi!

Am 28.09.2015 um 20:19 schrieb Nikhilesh Reddy:
> The rename operation in UBIFS is synchronous (or nearly synchronous)
> while the write operation is not. This can result in zero length files when
> renaming of files followed by an abrupt power down or a crash.
> 
> For example:
> 1) Say a file a.txt exists with size 1KB.
> 2) Create a file b.tmp (open)
> 3) Update the data in b.tmp with new values (write and close)
> 4) rename b.tmp to a.txt
> 5) Abrupt power down or crash
> 
> This above scenario can result in a.txt becoming a file of zero length and
> giving the impression of a.txt being truncated.
> This scenario can ofcourse be prevented by calling fsync or fdatasync
> before the rename operation.

I gave this a try and hacked up something to emulate a powercut *exactly* after
rename() in UBIFS.

fd = open("b.tmp", ...)
write(fd, "foo", ...)
close(fd)
rename("b.tmp", "a.txt")
^---- powercut

After remounting UBIFS both a.txt and b.tmp are present
but b.tmp is truncated. Not a.txt as you said.

Can you please double check?
I want to make sure that we're talking about the same things.

Thanks,
//richard