On Wed, Jul 25, 2018 at 11:15:09PM +0200, Martin Wilck wrote:
> bio_iov_iter_get_pages() currently only adds pages for the
> next non-zero segment from the iov_iter to the bio. That's
> suboptimal for callers, which typically try to pin as many
> pages as fit into the bio. This patch converts the current
> bio_iov_iter_get_pages() into a static helper, and introduces
> a new helper that allocates as many pages as
>
> 1) fit into the bio,
> 2) are present in the iov_iter,
> 3) and can be pinned by MM.
>
> Error is returned only if zero pages could be pinned. Because of
> 3), a zero return value doesn't necessarily mean all pages have been
> pinned. Callers that have to pin every page in the iov_iter must still
> call this function in a loop (this is currently the case).
>
> This change matters most for __blkdev_direct_IO_simple(), which calls
> bio_iov_iter_get_pages() only once. If it obtains less pages than requested,
> it returns a "short write" or "short read", and __generic_file_write_iter()
> falls back to buffered writes, which may lead to data corruption.
>
> Fixes: 72ecad22d9f1 ("block: support a full bio worth of IO for
> simplified bdev direct-io")
> Signed-off-by: Martin Wilck <[email protected]>
> ---
> block/bio.c | 35 ++++++++++++++++++++++++++++++++---
> 1 file changed, 32 insertions(+), 3 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 489a430..925033d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -903,14 +903,16 @@ int bio_add_page(struct bio *bio, struct page *page,
> EXPORT_SYMBOL(bio_add_page);
>
> /**
> - * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> + * __bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> * @bio: bio to add pages to
> * @iter: iov iterator describing the region to be mapped
> *
> - * Pins as many pages from *iter and appends them to @bio's bvec array. The
> + * Pins pages from *iter and appends them to @bio's bvec array. The
> * pages will have to be released using put_page() when done.
> + * For multi-segment *iter, this function only adds pages from the
> + * the next non-empty segment of the iov iterator.
> */
> -int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> +static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> {
> unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt, idx;
> struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
> @@ -947,6 +949,33 @@ int bio_iov_iter_get_pages(struct bio *bio, struct
> iov_iter *iter)
> iov_iter_advance(iter, size);
> return 0;
> }
> +
> +/**
> + * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> + * @bio: bio to add pages to
> + * @iter: iov iterator describing the region to be mapped
> + *
> + * Pins pages from *iter and appends them to @bio's bvec array. The
> + * pages will have to be released using put_page() when done.
> + * The function tries, but does not guarantee, to pin as many pages as
> + * fit into the bio, or are requested in *iter, whatever is smaller.
> + * If MM encounters an error pinning the requested pages, it stops.
> + * Error is returned only if 0 pages could be pinned.
> + */
> +int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> +{
> + unsigned short orig_vcnt = bio->bi_vcnt;
> +
> + do {
> + int ret = __bio_iov_iter_get_pages(bio, iter);
> +
> + if (unlikely(ret))
> + return bio->bi_vcnt > orig_vcnt ? 0 : ret;
> +
> + } while (iov_iter_count(iter) && !bio_full(bio));
When 'ret' isn't zero, and some partial progress has been made, seems less pages
might be obtained than requested too. Is that something we need to worry about?
Thanks,
Ming
