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From:   Chandan Rajendra <chandan@linux.ibm.com>
To:     Chao Yu <chao@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-fscrypt@vger.kernel.org, hch@infradead.org, tytso@mit.edu,
        ebiggers@kernel.org, adilger.kernel@dilger.ca,
        chandanrmail@gmail.com, jaegeuk@kernel.org
Subject: Re: [f2fs-dev] [PATCH V4 5/8] f2fs: Use read_callbacks for decrypting file data
Date:   Mon, 19 Aug 2019 19:03:23 +0530
Organization: IBM
In-Reply-To: <bb3dc624-1249-2418-f9da-93da8c11e7f5@kernel.org>
References: <20190816061804.14840-1-chandan@linux.ibm.com> <20190816061804.14840-6-chandan@linux.ibm.com> <bb3dc624-1249-2418-f9da-93da8c11e7f5@kernel.org>
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On Sunday, August 18, 2019 7:15:42 PM IST Chao Yu wrote:
> Hi Chandan,
> 
> On 2019-8-16 14:18, Chandan Rajendra wrote:
> > F2FS has a copy of "post read processing" code using which encrypted
> > file data is decrypted. This commit replaces it to make use of the
> > generic read_callbacks facility.
> 
> I remember that previously Jaegeuk had mentioned f2fs will support compression
> later, and it needs to reuse 'post read processing' fwk.
> 
> There is very initial version of compression feature in below link:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/chao/linux.git/log/?h=compression
> 
> So my concern is how can we uplift the most common parts of this fwk into vfs,
> and meanwhile keeping the ability and flexibility when introducing private
> feature/step in specified filesytem(now f2fs)?
> 
> According to current f2fs compression's requirement, maybe we can expand to
> 
> - support callback to let filesystem set the function for the flow in
> decompression/verity/decryption step.
> - support to use individual/common workqueue according the parameter.
> 
> Any thoughts?
>

Hi,

F2FS can be made to use fscrypt's queue for decryption and hence can reuse
"read callbacks" code for decrypting data.

For decompression, we could have a STEP_MISC where we invoke a FS provided
callback function for FS specific post read processing? 

Something like the following can be implemented in read_callbacks(),
	  case STEP_MISC:
		  if (ctx->enabled_steps & (1 << STEP_MISC)) {
			  /*
			    ctx->fs_misc() must process bio in a workqueue
			    and later invoke read_callbacks() with
			    bio->bi_private's value as an argument.
			  */
			  ctx->fs_misc(ctx->bio);
			  return;
		  }
		  ctx->cur_step++;

The fs_misc() callback can be passed in by the filesystem when invoking
read_callbacks_setup_bio().

-- 
chandan