[RFC,V2,10/11] Enable writing encrypted files in blocksize less than pagesize setup

On Wed, Feb 21, 2018 at 03:27:29PM +0530, Chandan Rajendra wrote:
> On Wednesday, February 21, 2018 6:24:54 AM IST Eric Biggers wrote:
> > On Mon, Feb 12, 2018 at 03:13:46PM +0530, Chandan Rajendra wrote:
> > > This commit splits the functionality of fscrypt_encrypt_block(). The
> > > allocation of fscrypt context and cipher text page is moved to a new
> > > function fscrypt_prep_ciphertext_page().
> > > 
> > > ext4_bio_write_page() is modified to appropriately make use of the above
> > > two functions.
> > > 
> > > Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
> > 
> > Well, this patch also modifies ext4_bio_write_page() to support the blocksize <
> > pagesize case.  The commit message makes it sound like it's just refactoring.
> > 
> > > diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> > > index 0a4a1e7..1e869d5 100644
> > > --- a/fs/ext4/page-io.c
> > > +++ b/fs/ext4/page-io.c
> > > @@ -419,9 +419,12 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> > >  	struct inode *inode = page->mapping->host;
> > >  	unsigned block_start;
> > >  	struct buffer_head *bh, *head;
> > > +	u64 blk_nr;
> > > +	gfp_t gfp_flags = GFP_NOFS;
> > >  	int ret = 0;
> > >  	int nr_submitted = 0;
> > >  	int nr_to_submit = 0;
> > > +	int blocksize = (1 << inode->i_blkbits);
> > >  
> > >  	BUG_ON(!PageLocked(page));
> > >  	BUG_ON(PageWriteback(page));
> > > @@ -475,15 +478,11 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> > >  		nr_to_submit++;
> > >  	} while ((bh = bh->b_this_page) != head);
> > >  
> > > -	bh = head = page_buffers(page);
> > > -
> > > -	if (ext4_encrypted_inode(inode) && S_ISREG(inode->i_mode) &&
> > > -	    nr_to_submit) {
> > > -		gfp_t gfp_flags = GFP_NOFS;
> > > -
> > > -	retry_encrypt:
> > > -		data_page = fscrypt_encrypt_block(inode, page, PAGE_SIZE, 0,
> > > -						page->index, gfp_flags);
> > > +	if (ext4_encrypted_inode(inode) && S_ISREG(inode->i_mode)
> > > +		&& nr_to_submit) {
> > > +	retry_prep_ciphertext_page:
> > > +		data_page = fscrypt_prep_ciphertext_page(inode, page,
> > > +							gfp_flags);
> > >  		if (IS_ERR(data_page)) {
> > >  			ret = PTR_ERR(data_page);
> > >  			if (ret == -ENOMEM && wbc->sync_mode == WB_SYNC_ALL) {
> > > @@ -492,17 +491,28 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> > >  					congestion_wait(BLK_RW_ASYNC, HZ/50);
> > >  				}
> > >  				gfp_flags |= __GFP_NOFAIL;
> > > -				goto retry_encrypt;
> > > +				goto retry_prep_ciphertext_page;
> > >  			}
> > >  			data_page = NULL;
> > >  			goto out;
> > >  		}
> > >  	}
> > >  
> > > +	blk_nr = page->index << (PAGE_SHIFT - inode->i_blkbits);
> > > +
> > >  	/* Now submit buffers to write */
> > > +	bh = head = page_buffers(page);
> > >  	do {
> > >  		if (!buffer_async_write(bh))
> > >  			continue;
> > > +
> > > +		if (ext4_encrypted_inode(inode) && S_ISREG(inode->i_mode)) {
> > > +			ret = fscrypt_encrypt_block(inode, page, data_page, blocksize,
> > > +						bh_offset(bh), blk_nr, gfp_flags);
> > > +			if (ret)
> > > +				break;
> > > +		}
> > > +
> > >  		ret = io_submit_add_bh(io, inode,
> > >  				       data_page ? data_page : page, bh);
> > >  		if (ret) {
> > > @@ -515,12 +525,12 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> > >  		}
> > >  		nr_submitted++;
> > >  		clear_buffer_dirty(bh);
> > > -	} while ((bh = bh->b_this_page) != head);
> > > +	} while (++blk_nr, (bh = bh->b_this_page) != head);
> > >  
> > >  	/* Error stopped previous loop? Clean up buffers... */
> > >  	if (ret) {
> > >  	out:
> > > -		if (data_page)
> > > +		if (data_page && bh == head)
> > >  			fscrypt_restore_control_page(data_page);
> > >  		printk_ratelimited(KERN_ERR "%s: ret = %d\n", __func__, ret);
> > >  		redirty_page_for_writepage(wbc, page);
> > 
> > I'm wondering why you didn't move the crypto stuff in ext4_bio_write_page() into
> > a separate function like I had suggested?  It's true we don't have to encrypt
> > all the blocks in the page at once, but it would make the crypto stuff more
> > self-contained.
> 
> Eric, Are you suggesting that the entire block of code that has invocations to
> fscrypt_prep_ciphertext_page() and fscrypt_encrypt_block() be moved to a
> separate function that gets defined in fscrypt module?

I just had in mind that it would be a separate function in ext4.

> 
> If yes, In Ext4, We have the invocation of io_submit_add_bh() being
> interleaved with calls to fscrypt_encrypt_block(). 
> 

Well yes that's what your patch does.  But we could instead just encrypt all the
blocks at once, right?  It would be a bit less efficient since we'd have to
iterate through the buffer_head list twice, but the advantage is that we end up
with ~105 lines ext4_bio_write_page() instead of 130, since the crypto stuff
would be more self-contained.  Here's an example, given as a diff from master
(beware, it's compile-tested only):

[RFC,V2,10/11] Enable writing encrypted files in blocksize less than pagesize setup

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