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

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jul 27, 2021 at 04:30:16PM +0800, Chao Yu wrote:
> On 2021/7/27 15:38, Eric Biggers wrote:
> > 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
> 
> I'm trying to pick up DAX support patch created by Qiuyang from huawei, and it
> looks it faces the same issue, so it tries to fix this by calling sb_issue_zeroout()
> in f2fs_map_blocks() before it returns.

I really hope you don't, because zeroing the region before memcpy'ing it
is absurd.  I don't know if f2fs can do that (xfs can't really) without
pinning resources during a potentially lengthy memcpy operation, but you
/could/ allocate the space in ->iomap_begin, attach some record of that
to iomap->private, and only commit the mapping update in ->iomap_end.

--D

> > 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...
> Yes,
> 
> Another option is to enhance f2fs metadata's scalability which needs to update layout
> of dnode block or SSA block, after that we can record the status of unwritten data block
> there... it's a big change though...
> 
> Thanks,
> 
> > 
> > - Eric
> >