Re: [PATCH v3 1/7] fscrypt: Add functions for direct I/O support

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 17, 2020 at 01:45:34AM +0000, Satya Tangirala wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Introduce fscrypt_dio_supported() to check whether a direct I/O request
> is unsupported due to encryption constraints, and
> fscrypt_limit_io_pages() to check how many pages may be added to a bio
> being prepared for direct I/O.
> 
> The IV_INO_LBLK_32 fscrypt policy introduced the possibility that DUNs
> in logically continuous file blocks might wrap from 0xffffffff to 0.
> Since this was particularly difficult to handle when block_size !=
> PAGE_SIZE, fscrypt only supports blk-crypto en/decryption with
> the IV_INO_LBLK_32 policy when block_size == PAGE_SIZE, and ensures that
> the DUN never wraps around within any submitted bio.
> fscrypt_limit_io_pages() can be used to determine the number of logically
> contiguous blocks/pages that may be added to the bio without causing the
> DUN to wrap around within the bio. This is an alternative to calling
> fscrypt_mergeable_bio() on each page in a range of logically contiguous
> pages.

This is a bit hard to read, especially the second paragraph.  How about:


"Introduce fscrypt_dio_supported() to check whether a direct I/O request
is unsupported due to encryption constraints.

Also introduce fscrypt_limit_io_pages() to limit how many pages can be
added to a bio being prepared for direct I/O.  This is needed for the
iomap direct I/O implementation to avoid DUN wraparound in the middle of
a bio (which is possible with the IV_INO_LBLK_32 IV generation method).
Elsewhere fscrypt_mergeable_bio() is used for this, but iomap operates
on logical ranges directly and thus needs doesn't have a chance to call
fscrypt_mergeable_bio() on every block or page.  So we need a function
which limits a logical range in one go."


In particular, the detail about PAGE_SIZE better belongs in the code, I think.

> +/**
> + * fscrypt_limit_io_pages() - limit I/O pages to avoid discontiguous DUNs
> + * @inode: the file on which I/O is being done
> + * @pos: the file position (in bytes) at which the I/O is being done
> + * @nr_pages: the number of pages we want to submit starting at @pos
> + *
> + * Determine the limit to the number of pages that can be submitted in the bio
> + * targeting @pos without causing a data unit number (DUN) discontinuity.
> + *
> + * For IV generation methods that can't cause DUN wraparounds
> + * within logically continuous data blocks, the maximum number of pages is
> + * simply @nr_pages. For those IV generation methods that *might* cause DUN
> + * wraparounds, the returned number of pages is the largest possible number of
> + * pages (less than @nr_pages) that can be added to the bio without causing a
> + * DUN wraparound within the bio.

How about replacing the second paragraph here with:
 
 * This is normally just @nr_pages, as normally the DUNs just increment along
 * with the logical blocks.  (Or the file is not encrypted.)
 *
 * In rare cases, fscrypt can be using an IV generation method that allows the
 * DUN to wrap around within logically continuous blocks, and that wraparound
 * will occur.  If this happens, a value less than @nr_pages will be returned so
 * that the wraparound doesn't occur in the middle of the bio.  Note that we
 * only support block_size == PAGE_SIZE (and page-aligned DIO) in such cases.