Re: [PATCH 2/5] direct-io: add support for fscrypt using blk-crypto

[Satya Tangirala <satyat@google.com>]

On Mon, Jul 13, 2020 at 11:36:19AM -0700, Eric Biggers wrote:
> On Fri, Jul 10, 2020 at 06:34:06AM +0100, Christoph Hellwig wrote:
> > On Thu, Jul 09, 2020 at 07:47:48PM +0000, Satya Tangirala wrote:
> > > From: Eric Biggers <ebiggers@google.com>
> > > 
> > > Set bio crypt contexts on bios by calling into fscrypt when required,
> > > and explicitly check for DUN continuity when adding pages to the bio.
> > > (While DUN continuity is usually implied by logical block contiguity,
> > > this is not the case when using certain fscrypt IV generation methods
> > > like IV_INO_LBLK_32).
> > 
> > I know it is asking you for more work, but instead of adding more
> > features to the legacy direct I/O code, could you just switch the user
> > of it (I guess this is for f2f2?) to the iomap one?
> 
> Eventually we should do that, as well as convert f2fs's fiemap, bmap, and llseek
> to use iomap.  However there's a nontrivial barrier to entry, at least for
> someone who isn't an expert in iomap, especially since f2fs currently doesn't
> use iomap at all and thus doesn't have an iomap_ops implementation.  And using
> ext4 as an example, there will be some subtle cases that need to be handled.
> 
> Satya says he's looking into it; we'll see what he can come up with and what the
> f2fs developers say.
> 
> If it turns out to be difficult and people think this patchset is otherwise
> ready, we probably shouldn't hold it up on that.  This is a very small patch,
> and Satya and I have to maintain it for years in downstream kernels anyway, so
> it will be used and tested regardless.  It would also be nice to allow userspace
> (e.g. xfstests) to assume that if the inlinecrypt mount option is supported,
> then direct I/O is supported too, without having to handle intermediate kernel
> releases where inlinecrypt was supported but not direct I/O.
> 
As Eric pointed out, it doesn't seem to be completely straightforward to
move f2fs to using iomap - I'm still looking into it, but for now I've
sent out v2 (and v3 just because I forgot to add a changelog to v2) with
the changes to fs/direct-io.c as is from v1, but (again, for the reasons
Eric points out) I think it'd be better not to hold this patch up for that.
> - Eric