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

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 30, 2021 at 03:12:15PM -0700, Jaegeuk Kim wrote:
> On 07/30, Eric Biggers wrote:
> > On Tue, Jul 27, 2021 at 06:51:54PM -0700, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@google.com>
> > > 
> > > 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
> 
> Hmm, I'm still wondering why this becomes a problem. And, do we really need
> to roll back the preallocated blocks?
> 
> > > result, f2fs will leak uninitialized blocks to users if a DIO write
> > > doesn't fully complete.  This can be easily reproduced by issuing a DIO
> > > write that will fail due to misalignment, e.g.:
> 
> If there's any error, truncating blocks having NEW_ADDR could address this?
> 

My understanding is that the "NEW_ADDR" block address in f2fs means that space
was reserved for the block, but not allocated in any particular place yet.
Buffered writes reserve blocks in this way, but DIO writes cannot because DIO by
definition has to directly write to a specific on-disk location.  Therefore DIO
writes require that the blocks be preallocated for real.

- Eric