[PATCH v12 10/10] block: convert bio_map_user_iov to use iov_iter_extract_pages

David Howells
2023-02-07T17:16:15Z

This will pin pages or leave them unaltered rather than getting a ref on
them as appropriate to the iterator.

The pages need to be pinned for DIO rather than having refs taken on them
to prevent VM copy-on-write from malfunctioning during a concurrent fork()
(the result of the I/O could otherwise end up being visible to/affected by
the child process).

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Jan Kara <jack@suse.cz>
cc: Matthew Wilcox <willy@infradead.org>
cc: Logan Gunthorpe <logang@deltatee.com>
cc: linux-block@vger.kernel.org
---

Notes:
ver #10)
- Drop bio_set_cleanup_mode(), open coding it instead.

ver #8)
- Split the patch up a bit [hch].
- We should only be using pinned/non-pinned pages and not ref'd pages,
so adjust the comments appropriately.

ver #7)
- Don't treat BIO_PAGE_REFFED/PINNED as being the same as FOLL_GET/PIN.

ver #5)
- Transcribe the FOLL_* flags returned by iov_iter_extract_pages() to
BIO_* flags and got rid of bi_cleanup_mode.
- Replaced BIO_NO_PAGE_REF to BIO_PAGE_REFFED in the preceding patch.

block/blk-map.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/block/blk-map.c b/block/blk-map.c
index f1f70b50388d..0f1593e144da 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -281,22 +281,21 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,

if (blk_queue_pci_p2pdma(rq->q))
extraction_flags |= ITER_ALLOW_P2PDMA;
+ if (iov_iter_extract_will_pin(iter))
+ bio_set_flag(bio, BIO_PAGE_PINNED);

- bio_set_flag(bio, BIO_PAGE_REFFED);
while (iov_iter_count(iter)) {
- struct page **pages, *stack_pages[UIO_FASTIOV];
+ struct page *stack_pages[UIO_FASTIOV];
+ struct page **pages = stack_pages;
ssize_t bytes;
size_t offs;
int npages;

- if (nr_vecs <= ARRAY_SIZE(stack_pages)) {
- pages = stack_pages;
- bytes = iov_iter_get_pages(iter, pages, LONG_MAX,
- nr_vecs, &offs, extraction_flags);
- } else {
- bytes = iov_iter_get_pages_alloc(iter, &pages,
- LONG_MAX, &offs, extraction_flags);
- }
+ if (nr_vecs > ARRAY_SIZE(stack_pages))
+ pages = NULL;
+
+ bytes = iov_iter_extract_pages(iter, &pages, LONG_MAX,
+ nr_vecs, extraction_flags, &offs);
if (unlikely(bytes <= 0)) {
ret = bytes ? bytes : -EFAULT;
goto out_unmap;
@@ -318,7 +317,7 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
if (!bio_add_hw_page(rq->q, bio, page, n, offs,
max_sectors, &same_page)) {
if (same_page)
- put_page(page);
+ bio_release_page(bio, page);
break;
}

@@ -330,7 +329,7 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
* release the pages we didn't map into the bio, if any
*/
while (j < npages)
- put_page(pages[j++]);
+ bio_release_page(bio, pages[j++]);
if (pages != stack_pages)
kvfree(pages);
/* couldn't stuff something into bio? */