Re: [PATCH 3/9] f2fs: rework write preallocations

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jul 27, 2021 at 11:23:03AM +0800, Chao Yu wrote:
> > > 
> > > Do you have a proper explanation for why preallocations shouldn't be done in
> 
> See commit f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode"),
> f2fs_map_blocks() logic was changed to force allocating a new block address no matter
> previous block address was existed if it is called from write path of DIO. So, in such
> condition, if we preallocate new block address in f2fs_file_write_iter(), we will
> suffer the problem which my trace indicates.
> 
> > > this case?  Note that preallocations are still done for buffered writes, which
> > > may be out-of-place as well; how are those different?
> Got your concern.
> 
> For buffered IO, we use F2FS_GET_BLOCK_PRE_AIO, in this mode, we just preserve
> filesystem block count and tag NEW_ADDR in dnode block, so, it's fine, double
> new block address allocation won't happen during data page writeback.
> 
> For direct IO, we use F2FS_GET_BLOCK_PRE_DIO, in this mode, we will allocate
> physical block address, after preallocation, if we fallback to buffered IO, we
> may suffer double new block address allocation issue... IIUC.
> 
> Well, can we relocate preallocation into f2fs_direct_IO() after all cases which
> may cause fallbacking DIO to buffered IO?
> 

That's somewhat helpful, but I've been doing some more investigation and now I'm
even more confused.  How can f2fs support non-overwrite DIO writes at all
(meaning DIO writes in LFS mode as well as DIO writes to holes in non-LFS mode),
given that it has no support for unwritten extents?  AFAICS, as-is users can
easily leak uninitialized disk contents on f2fs by issuing a DIO write that
won't complete fully (or might not complete fully), then reading back the blocks
that got allocated but not written to.

I think that f2fs will have to take the ext2 approach of not allowing
non-overwrite DIO writes at all...

- Eric