On Fri, Jul 17, 2020 at 01:45:34AM +0000, Satya Tangirala wrote: > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > 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.
