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

From: Chandan Rajendra <chandan@linux.vnet.ibm.com>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-fscrypt@vger.kernel.org, tytso@mit.edu
Subject: Re: [RFC PATCH V2 10/11] Enable writing encrypted files in blocksize less than pagesize setup
Date: Wed, 21 Feb 2018 15:27:29 +0530	[thread overview]
Message-ID: <2490066.ZFX8CK6sZb@localhost.localdomain> (raw)
In-Reply-To: <20180221005454.GB252219@gmail.com>

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?

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

-- 
chandan