On 07/30, Eric Biggers wrote:
> On Tue, Jul 27, 2021 at 06:51:54PM -0700, Eric Biggers wrote:
> > From: Eric Biggers <[email protected]>
> >
> > 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?
> >
> > rm -f file
> > truncate -s 1000000 file
> > dd if=/dev/zero bs=999999 oflag=direct conv=notrunc of=file
> > od -tx1 file # shows uninitialized disk blocks
> >
> > Until a proper design for non-overwrite DIO writes on f2fs can be
> > designed and implemented, remove support for them and make them fall
> > back to buffered I/O. This is what other filesystems that don't support
> > unwritten blocks, e.g. ext2, also do, at least for non-extending DIO
> > writes. However, f2fs can't do extending DIO writes either, as f2fs
> > appears to have no mechanism for guaranteeing that leftover allocated
> > blocks past EOF will get truncated. (f2fs does have an orphan list, but
> > it's only used for deleting inodes, not truncating them.)
> >
> > This patch doesn't attempt to remove the F2FS_GET_BLOCK_{DIO,PRE_DIO}
> > cases in f2fs_map_blocks(); that can be cleaned up later.
> >
> > Fixes: bfad7c2d4033 ("f2fs: introduce a new direct_IO write path")
> > Cc: [email protected]
> > Signed-off-by: Eric Biggers <[email protected]>
> > ---
>
> Any opinion on this patch? This really needs to be fixed one way or another.
> Probably before the conversion to iomap, as this fix will need to be
> backported.
>
> - Eric
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