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block: add bio_iov_iter_get_pages()

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This is a helper that pins down a range from an iov_iter and adds it to
a bio without requiring a separate memory allocation for the page array.
It will be used for upcoming direct I/O implementations for block devices
and iomap based file systems.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
[hch: ported to the iov_iter interface, renamed and added comments.
      All blame should be directed to me and all fame should go to Kent
      after this!]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Kent Overstreet 2016-10-31 11:59:24 -06:00 committed by Jens Axboe
parent 13edd5e731
commit 2cefe4dbaa
2 changed files with 50 additions and 0 deletions
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49
block/bio.c
View File

@ -847,6 +847,55 @@ done:
}
EXPORT_SYMBOL(bio_add_page);
/**
* 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
* pages will have to be released using put_page() when done.
*/
int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
{
unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
struct page **pages = (struct page **)bv;
size_t offset, diff;
ssize_t size;
size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
if (unlikely(size <= 0))
return size ? size : -EFAULT;
nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
/*
* Deep magic below: We need to walk the pinned pages backwards
* because we are abusing the space allocated for the bio_vecs
* for the page array. Because the bio_vecs are larger than the
* page pointers by definition this will always work. But it also
* means we can't use bio_add_page, so any changes to it's semantics
* need to be reflected here as well.
*/
bio->bi_iter.bi_size += size;
bio->bi_vcnt += nr_pages;
diff = (nr_pages * PAGE_SIZE - offset) - size;
while (nr_pages--) {
bv[nr_pages].bv_page = pages[nr_pages];
bv[nr_pages].bv_len = PAGE_SIZE;
bv[nr_pages].bv_offset = 0;
}
bv[0].bv_offset += offset;
bv[0].bv_len -= offset;
if (diff)
bv[bio->bi_vcnt - 1].bv_len -= diff;
iov_iter_advance(iter, size);
return 0;
}
EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
struct submit_bio_ret {
struct completion event;
int error;

1
include/linux/bio.h
View File

@ -427,6 +427,7 @@ void bio_chain(struct bio *, struct bio *);
extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
unsigned int, unsigned int);
int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
struct rq_map_data;
extern struct bio *bio_map_user_iov(struct request_queue *,
const struct iov_iter *, gfp_t);