Re: [PATCH v3 3/7] iomap: support direct I/O with fscrypt using blk-crypto

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 17, 2020 at 01:45:36AM +0000, Satya Tangirala wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Wire up iomap direct I/O with the fscrypt additions for direct I/O,
> and set bio crypt contexts on bios when appropriate.
> 
> Make iomap_dio_bio_actor() call fscrypt_limit_io_pages() to ensure that
> DUNs remain contiguous within a bio, since it works directly with logical
> ranges and can't call fscrypt_mergeable_bio() on each page.

This commit message is still confusing.

How about the following:

"Wire up iomap direct I/O with the fscrypt additions for direct I/O.
This allows ext4 to support direct I/O on encrypted files when inline
encryption is enabled.

This change consists of two parts:

- Set a bio_crypt_ctx on bios for encrypted files, so that the file
  contents get encrypted (or decrypted).

- Ensure that encryption data unit numbers (DUNs) are contiguous within
  each bio.  Use the new function fscrypt_limit_io_pages() for this,
  since the iomap code works directly with logical ranges and thus
  doesn't have a chance to call fscrypt_mergeable_bio() on each page.

Note that fscrypt_limit_io_pages() is normally a no-op, as normally the
DUNs simply increment along with the logical blocks.  But it's needed to
handle an edge case in one of the fscrypt IV generation methods."

> @@ -183,11 +184,14 @@ static void
>  iomap_dio_zero(struct iomap_dio *dio, struct iomap *iomap, loff_t pos,
>  		unsigned len)
>  {
> +	struct inode *inode = file_inode(dio->iocb->ki_filp);
>  	struct page *page = ZERO_PAGE(0);
>  	int flags = REQ_SYNC | REQ_IDLE;
>  	struct bio *bio;
>  
>  	bio = bio_alloc(GFP_KERNEL, 1);
> +	fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
> +				  GFP_KERNEL);
>  	bio_set_dev(bio, iomap->bdev);
>  	bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
>  	bio->bi_private = dio;

iomap_dio_zero() is only used on partial filesystem blocks.  But, we
only allow direct I/O on encrypted files when the I/O is
filesystem-block-aligned.

So this part appears to be unnecessary.

How about replacing it with:

	/* encrypted direct I/O is guaranteed to be fs-block aligned */
	WARN_ON_ONCE(fscrypt_needs_contents_encryption(inode));

> @@ -253,6 +257,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		ret = nr_pages;
>  		goto out;
>  	}
> +	nr_pages = fscrypt_limit_io_pages(inode, pos, nr_pages);
>  
>  	if (need_zeroout) {
>  		/* zero out from the start of the block to the write offset */
> @@ -270,6 +275,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		}
>  
>  		bio = bio_alloc(GFP_KERNEL, nr_pages);
> +		fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
> +					  GFP_KERNEL);
>  		bio_set_dev(bio, iomap->bdev);
>  		bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
>  		bio->bi_write_hint = dio->iocb->ki_hint;
> @@ -307,6 +314,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		copied += n;
>  
>  		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> +		nr_pages = fscrypt_limit_io_pages(inode, pos, nr_pages);
>  		iomap_dio_submit_bio(dio, iomap, bio, pos);
>  		pos += n;
>  	} while (nr_pages);

I think the part at the end is wrong.

We want to limit the *next* bio, not the current one.

So 'pos' needs to be updated first.

I think it should be:

                iomap_dio_submit_bio(dio, iomap, bio, pos);
                pos += n;
                nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
                nr_pages = fscrypt_limit_io_pages(inode, pos, nr_pages);