linux-ext4 - Re: [RFC PATCH V2 07/11] fscrypt_zeroout_range: Encrypt all zeroed out blocks of a page

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Date:   Thu, 05 Apr 2018 18:37:24 +0530
From:   Chandan Rajendra <chandan@...ux.vnet.ibm.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Eric Biggers <ebiggers3@...il.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-fscrypt@...r.kernel.org
Subject: Re: [RFC PATCH V2 07/11] fscrypt_zeroout_range: Encrypt all zeroed out blocks of a page

On Thursday, April 5, 2018 6:17:45 PM IST Theodore Y. Ts'o wrote:
> On Thu, Apr 05, 2018 at 12:33:26PM +0530, Chandan Rajendra wrote:
> > 
> > I encountered a problem when refactoring the code to get fscrypt layer to
> > encrypt all the blocks of a page by internally calling
> > fscrypt_encrypt_block().
> > 
> > It is the filesystem which knows which subset of blocks mapped by a page that
> > needs to be encrypted.
> 
> That's not quite correct.  All blocks in a file are either always
> encrypted, or not.  So that's not really the problem.

Sorry, I wasn't clear enough with my explaination. I actually meant to say
that not all blocks mapped by a page might be dirty and hence only a subset
of the dirty blocks might need to be written to the disk. I understand that
in such cases we could still invoke fscrypt_encrypt_page() and encrypt the
contents of all the blocks (irrespective of whether a block is dirty or not).
I wanted to optimize this and avoid the encryption of "clean blocks".

> 
> > For example, ext4_bio_write_page() marks such blocks
> > with "Async Write" flag and later in another pass, it encrypts and also adds
> > these blocks to a bio.
> 
> The tricky bits with ext4_bio_write_page() all are in handling the
> case where page_size > block_size.  In that case, where there are multiple
> file system blocks covering a page, we need to know the on-disk
> block numbers are for the blocks covering the page, and the encryption
> is intertwined with the I/O submission path, which is file system
> specific -- mainly because how the completion callback and the
> parameters which need to be passed *into* the the callback function is
> file system specific.
> 
> However, none of that is needed or relevant to the encryption
> operation.  It's an accident of code development history that
> fscrypt_encrypt_page was placed where it was.
> 
> That is, none of work done in the first pass (starting with the
> comment "In the first loop we prepare and mark buffers to submit....")
> is needed to be done before we call fscrypt_encrypt_page().  That call:
> 
> 	data_page = fscrypt_encrypt_page(inode, page, PAGE_SIZE, 0,
> 					page->index, gfp_flags);
> 
> ... could easily be moved to the beginning of ext4_bio_write_page().
> 
> I can do that to make the function easier to understand, but that
> particular cleanup is merely cosmetic.  It doesn't what you would need
> to do order to make fscrypt_encrypt_page() iterate over the page as it
> calls fscrypt_encrypt_buffer().
> 

-- 
chandan