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Date:   Fri, 18 Dec 2020 11:53:58 +0000
From:   Satya Tangirala <satyat@google.com>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     "Theodore Y . Ts'o" <tytso@mit.edu>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Eric Biggers <ebiggers@kernel.org>, Chao Yu <chao@kernel.org>,
        linux-kernel@vger.kernel.org, linux-fscrypt@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [PATCH 0/3] add support for metadata encryption to F2FS
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On Fri, Dec 18, 2020 at 05:02:23PM +0800, Chao Yu wrote:
> On 2020/12/17 23:44, Satya Tangirala wrote:
> > On Sat, Oct 10, 2020 at 05:53:06PM +0800, Chao Yu wrote:
> > > Why not using nid as DUN, then GC could migrate encrypted node block directly via
> > > meta inode's address space like we do for encrypted data block, rather than
> > > decrypting node block to node page and then encrypting node page with DUN of new
> > > blkaddr it migrates to.
> > > 
> > The issue is, the bi_crypt_context in a bio holds a single DUN value,
> > which is the DUN for the first data unit in the bio. blk-crypto assumes
> > that the DUN of each subsequent data unit can be computed by simply
> > incrementing the DUN. So physically contiguous data units can only be put
> > into the same bio if they also have contiguous DUNs. I don't know much
> > about nids, but if the nid is invariant w.r.t the physical block location,
> > then there might be more fragmentation of bios in regular read/writes
> 
> Correct, considering performance of in batch node flush, it will be better to
> use pba as IV value.
> 
> But, what's the plan about supporting software encryption for metadata? Current
> f2fs write flow will handle all operations which may encounter failure before
> allocating block address for node, if we do allocation first, and then use pba
> as IV to encrypt node block, it will be a little complicated to revert allocation
> if we fail to encrypt node block.
> 
Software encryption for metadata is supported through the blk-crypto
framework - so encryption will happen in the block layer, not the
filesystem layer. So there's nothing extra/special we need to do if
there's an encryption failure - an encryption failure is no different
from a read/write failure in a lower layer from f2fs' perspective.