Re: why fscrypt_mergeable_bio use logical block number?

[Eric Biggers <ebiggers@kernel.org>]

On Sun, Apr 17, 2022 at 02:34:17AM +0000, 常凤楠 wrote:
> 
> 
> > -----Original Message-----
> > From: Eric Biggers <ebiggers@kernel.org>
> > Sent: Saturday, April 16, 2022 2:50 AM
> > To: 常凤楠 <changfengnan@vivo.com>
> > Cc: tytso@mit.edu; jaegeuk@kernel.org; linux-fscrypt@vger.kernel.org
> > Subject: Re: why fscrypt_mergeable_bio use logical block number?
> > 
> > On Fri, Apr 15, 2022 at 08:18:11AM +0000, 常凤楠 wrote:
> > > Hi:
> > > 	When I dig into a problem, I found: bio merge may reduce a lot when
> > > 	enable inlinecrypt, the root cause is fscrypt_mergeable_bio use logical
> > > 	block number rather than physical block number. I had read the UFSHCI,
> > > 	but not see any description about why data unit number shoud use
> > logical
> > > 	block number. I want to know why, Is the anyone can explain this?
> > >
> > > Thanks.
> > > Fengnan Chang.
> > 
> > The main reason that fscrypt generates IVs using the file logical block number
> > instead of the sector number is because f2fs needs to move data blocks
> > around without the key.  That would be impossible with sector number
> > based encryption.
> So if use sector number to generate IVs, after f2fs move data blocks in gc, we can
> not decrypt correctly, am I right ?

Yes, that's correct.

> > 
> > There are other reasons too.  Always using the file logical block number
> > keeps the various fscrypt options consistent, it works well with per-file keys, it
> > doesn't break filesystem resizing, and it avoids having the interpretation of
> > the filesystem depend on its on-disk location which would be a layering
> > violation.  But the need to support f2fs is the main one.
> > 
> > Note that by default, fscrypt uses a different key for every file, and in that case
> > the only way that using the file logical block number instead of the sector
> > number would prevent merging is if data was being read/written from a file
> > that is physically but not logically contiguous.  That's not a very common
> > case.
> This is exactly the problem I have faced. We read/write a file in physically contiguous but not logically.
> So I'm wander if we can keep physically contiguous always, can we skip check wheatear logical block 
> number is contiguous when bio merge?

No, inline encryption requires contiguous DUNs.

- Eric