Re: [PATCH] f2fs: remove broken support for allocating DIO writes

["Theodore Ts'o" <tytso@mit.edu>]

On Fri, Jul 30, 2021 at 12:17:26PM -0700, Eric Biggers wrote:
> > Currently, non-overwrite DIO writes are fundamentally unsafe on f2fs as
> > they require preallocating blocks, but f2fs doesn't support unwritten
> > blocks and therefore has to preallocate the blocks as regular blocks.
> > f2fs has no way to reliably roll back such preallocations, so as a
> > result, f2fs will leak uninitialized blocks to users if a DIO write
> > doesn't fully complete.

There's another way of solving this problem which doesn't require
supporting unwritten blocks.  What a file system *could* do is to
allocate the blocks, but *not* update the on-disk data structures ---
so the allocation happens in memory only, so you know that the
physical blocks won't get used for another files, and then issue the
data block writes.  On the block I/O completion, trigger a workqueue
function which updates the on-disk metadata to assign physical blocks
to the inode.

That way if you crash before the data I/O has a chance to complete,
the on-disk logical block -> physical block map hasn't been updated
yet, and so you don't need to worry about leaking uninitialized blocks.

Cheers,

					- Ted