On Thu, Nov 21, 2019 at 05:15:35PM +0100, Jan Kara wrote:
> iomap_dio_bio_actor() copies iter to a local variable and then limits it
> to a file extent we have mapped. When IO is submitted,
> iomap_dio_bio_actor() advances the original iter while the copied iter
> is advanced inside bio_iov_iter_get_pages(). This logic is non-obvious
> especially because both iters still point to same shared structures
> (such as pipe info) so if iov_iter_advance() changes anything in the
> shared structure, this scheme breaks. Let's just truncate and reexpand
> the original iter as needed instead of playing games with copying iters
> and keeping them in sync.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/iomap/direct-io.c | 25 ++++++++++++-------------
> 1 file changed, 12 insertions(+), 13 deletions(-)
>
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 30189652c560..01a4264bce37 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -201,12 +201,12 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> unsigned int blkbits = blksize_bits(bdev_logical_block_size(iomap->bdev));
> unsigned int fs_block_size = i_blocksize(inode), pad;
> unsigned int align = iov_iter_alignment(dio->submit.iter);
> - struct iov_iter iter;
> struct bio *bio;
> bool need_zeroout = false;
> bool use_fua = false;
> int nr_pages, ret = 0;
> size_t copied = 0;
> + size_t orig_count = iov_iter_count(dio->submit.iter);
>
> if ((pos | length | align) & ((1 << blkbits) - 1))
> return -EINVAL;
> @@ -235,16 +235,14 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> use_fua = true;
> }
>
> - /*
> - * Operate on a partial iter trimmed to the extent we were called for.
> - * We'll update the iter in the dio once we're done with this extent.
> - */
> - iter = *dio->submit.iter;
> - iov_iter_truncate(&iter, length);
> + /* Operate on a partial iter trimmed to the extent we were called for */
> + iov_iter_truncate(dio->submit.iter, length);
Ok... so here we shorten the dio iterator to fit the mapping we got...
>
> - nr_pages = iov_iter_npages(&iter, BIO_MAX_PAGES);
> - if (nr_pages <= 0)
> + nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> + if (nr_pages <= 0) {
> + iov_iter_reexpand(dio->submit.iter, orig_count);
> return nr_pages;
...and if there aren't any pages, we revert the truncation and bail...
> + }
>
> if (need_zeroout) {
> /* zero out from the start of the block to the write offset */
> @@ -257,6 +255,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> size_t n;
> if (dio->error) {
> iov_iter_revert(dio->submit.iter, copied);
> + iov_iter_reexpand(dio->submit.iter, orig_count);
...if the bio failed, we walk the dio iterator backward the entire
amount that it had advanced, undo the length truncation and bail...
> return 0;
> }
>
> @@ -268,7 +267,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> bio->bi_private = dio;
> bio->bi_end_io = iomap_dio_bio_end_io;
>
> - ret = bio_iov_iter_get_pages(bio, &iter);
> + ret = bio_iov_iter_get_pages(bio, dio->submit.iter);
...here's where we walk the dio iter forward as part of attaching pages
to the bio...
> if (unlikely(ret)) {
> /*
> * We have to stop part way through an IO. We must fall
> @@ -294,13 +293,11 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> bio_set_pages_dirty(bio);
> }
>
> - iov_iter_advance(dio->submit.iter, n);
> -
> dio->size += n;
> pos += n;
> copied += n;
>
> - nr_pages = iov_iter_npages(&iter, BIO_MAX_PAGES);
> + nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> iomap_dio_submit_bio(dio, iomap, bio);
> } while (nr_pages);
>
> @@ -318,6 +315,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
> if (pad)
> iomap_dio_zero(dio, iomap, pos, fs_block_size - pad);
> }
> + /* Undo iter limitation to current extent */
> + iov_iter_reexpand(dio->submit.iter, orig_count - copied);
...and here we undo the length truncation, same as all the other exit
points. Assuming my understanding of the bookkeeping is correct,
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
(Would still like to see a proper regression test for fstests though...)
--D
> return copied ? copied : ret;
> }
>
> --
> 2.16.4
>
