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

Darrick J. Wong
2020-07-22T23:33:17Z

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