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

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On Tue, Aug 17, 2021 at 11:57:46AM -0700, Jaegeuk Kim wrote:
> On 08/17, Christoph Hellwig wrote:
> > On Mon, Aug 16, 2021 at 07:03:21PM -0700, Eric Biggers wrote:
> > > Freeing preallocated blocks on error would be better than nothing, although note
> > > that the preallocated blocks may have filled an arbitrary sequence of holes --
> > > so simply truncating past EOF would *not* be sufficient.
> > > 
> > > But really filesystems need to be designed to never expose uninitialized data,
> > > even if I/O errors or a sudden power failure occurs.  It is unfortunate that
> > > f2fs apparently wasn't designed with that goal in mind.
> > > 
> > > In any case, I don't think we can proceed with any other f2fs direct I/O
> > > improvements until this data leakage bug can be solved one way or another.  If
> > > my patch to remove support for allocating writes isn't acceptable and the
> > > desired solution is going to require some more invasive f2fs surgery, are you or
> > > Chao going to work on it?  I'm not sure there's much I can do here.
> > 
> > Btw, this is generally a problem for buffered I/O as well, although the
> > window for exposing uninitialized blocks on a crash tends to be smaller.
> 
> How about adding a warning message when we meet an error with preallocated
> unwritten blocks? In the meantime, can we get the Eric's patches for iomap
> support? I feel that we only need to modify the preallocation and error
> handling parts?

A warning message would do nothing to prevent uninitialized blocks from being
leaked to userspace.

- Eric



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