Re: [RFC PATCH V3 07/12] mpage_readpage[s]: Introduce post process callback parameters

[Eric Biggers <ebiggers3@gmail.com>]

Hi Chandan,

On Tue, May 29, 2018 at 08:34:21AM +0530, Chandan Rajendra wrote:
> On Tuesday, May 29, 2018 1:04:37 AM IST Theodore Y. Ts'o wrote:
> > On Mon, May 28, 2018 at 11:05:52AM +0530, Chandan Rajendra wrote:
> > > > Can you describe more of what you are doing here; specifically, you
> > > > deleted all of fs/ext4/readpage.c --- was this because you moved
> > > > functionality back into fs/mpage.c?  Did you make sure all of the
> > > > local changes in fs/ext4/readpage was moved back to fs/mpage.c?
> > > > 
> > > > If the goal is to refactor code to remove the need for
> > > > fs/ext4/readpage.c, you should probably make that be the first patch
> > > > as a prerequisite patch.  And we then need to make sure we don't
> > > > accidentally break anyone else who might be using fs/mpage.c.  Saying
> > > > a bit more about why you think the refactor is a good thing would also
> > > > be useful.
> > > 
> > > I will split this patch into two as suggested by you. Also, I will update 
> > > the commit messages.
> > 
> > Note that I was planning on making changes to fs/ext4/readpage.c as
> > part of integrating fsverity[1][2] support into ext4.  Basically, I
> > need to do something like [3] to fs/ext4/readpage.c.
> > 
> > [1] https://www.spinics.net/lists/linux-fsdevel/msg121182.html
> > [2] https://www.youtube.com/watch?v=GlEWcVuRbNA
> > [3] https://git.kernel.org/pub/scm/linux/kernel/git/mhalcrow/linux.git/commit/?h=fs-verity-dev&id=827faba05972517f49fa2f2aaf272150f5766af2
> > 
> > Which is why I'm really interested in your reasoning for why you
> > propose to drop fs/ext4/readpage.c.  :-)
> > 
> 
> The first patchset to support encryption in subpage-blocksize scenario copied
> the block_read_full_page() from fs/buffer.c to ext4/readpage.c and had made
> changes required to support encryption in that function. However, the
> conclusion was to not create copies of existing code but rather add support
> for decryption inside generic mpage_readpage[s] functions. Hence this patchset
> implements the required decryption logic in the generic mpage_readpage[s]
> functions. Since this makes the code in ext4/readpage.c redundant, I had
> decided to delete the ext4/readpage.c.
> 

Strictly speaking, I don't think anything has been "concluded" yet.  The issue,
as I saw it, was that your original patchset just copy-and-pasted lots more
generic code from fs/buffer.c into ext4, without consideration of alternatives
that would allow the code to be shared, such as adding a postprocessing callback
to mpage_readpage{,s}().  My hope was that you would thoughtfully consider the
alternatives and make a decision of what was the best solution, and then explain
that decision as part of your patchset -- not just implement some solution
without much explanation, which makes it very difficult for people to decide
whether it's the best solution or not.

And yes, now that fs-verity is planned to be a thing too, we should stop
thinking of the problem as specifically "how to support decryption", but rather
how to support the ability to post-process the data using potentially multiple
length-preserving postprocessing steps such as decryption,
integrity/authenticity verification, etc.

I'll take a closer look at this patch when I have a chance, but as Ted pointed
out it really needs to be split out into multiple patches.  Just as a
preliminary comment, it looks like you are directly calling into fs/crypto/ from
fs/buffer.c, e.g. fscrypt_enqueue_decrypt_bio().  I don't understand that.  If
you're doing that (which would start requiring that fscrypt be built-in, not
modular) then there should be no need for the filesystem to pass a
postprocessing callback to the generic code, as you could just check
S_ISREG(inode->i_mode) && IS_ENCRYPTED(inode) in generic code to tell whether
decryption needs to be done.  The whole point of the postprocessing callback
would be to allow the generic read code to be used without it having to be aware
of all the specific types of post-read processing that filesystems may want.

Thanks!

- Eric