Re: [PATCH v4 3/7] iomap: support direct I/O with fscrypt using blk-crypto

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jul 22, 2020 at 04:26:25PM -0700, Eric Biggers wrote:
> On Wed, Jul 22, 2020 at 03:34:04PM -0700, Eric Biggers wrote:
> > So, something like this:
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 44bad4bb8831..2816194db46c 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3437,6 +3437,15 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> >  	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
> >  			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> >  
> > +	/*
> > +	 * When inline encryption is enabled, sometimes I/O to an encrypted file
> > +	 * has to be broken up to guarantee DUN contiguity.  Handle this by
> > +	 * limiting the length of the mapping returned.
> > +	 */
> > +	if (!(flags & IOMAP_REPORT))
> > +		map.m_len = fscrypt_limit_io_blocks(inode, map.m_lblk,
> > +						    map.m_len);
> > +
> >  	if (flags & IOMAP_WRITE)
> >  		ret = ext4_iomap_alloc(inode, &map, flags);
> >  	else
> > 
> > 
> > That also avoids any confusion between pages and blocks, which is nice.
> 
> Correction: for fiemap, ext4 actually uses ext4_iomap_begin_report() instead of
> ext4_iomap_begin().  So we don't need to check for !IOMAP_REPORT.
> 
> Also it could make sense to limit map.m_len after ext4_iomap_alloc() rather than
> before, so that we don't limit the length of the extent that gets allocated but
> rather just the length that gets returned to iomap.

Naïve question here -- if the decision to truncate the bio depends on
the file block offset, can you achieve the same thing by capping the
length of the iovec prior to iomap_dio_rw?

Granted that probably only makes sense if the LBLK IV thing is only
supposed to be used infrequently, and having to opencode a silly loop
might be more hassle than it's worth...

--D

> - Eric