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

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 30, 2021 at 10:46:16PM -0400, Theodore Ts'o wrote:
> 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

Jaegeuk and Chao, any idea how feasible it would be for f2fs to do this?

- Eric