[PATCH v2 3/3] bio: convert get_user_pages_fast() --> pin_user_pages_fast()

[John Hubbard <jhubbard@nvidia.com>]

Change generic block/bio Direct IO routines, to acquire FOLL_PIN user
pages via the recently added routines:

    iov_iter_pin_user_pages()
    iov_iter_pin_user_pages_alloc()
    pin_user_page()

This effectively converts several file systems (ext4, for example) that
use the common Direct IO routines.

Change the corresponding page release calls from put_page() to
unpin_user_page().

Change bio_release_pages() to handle FOLL_PIN pages. In fact, that
is now the *only* type of pages it handles now.

Design notes
============

Quite a few approaches have been considered over the years. This one is
inspired by Christoph Hellwig's July, 2019 observation that there are
only 5 ITER_ types, and we can simplify handling of them for Direct IO
[1]. Accordingly, this patch implements the following pseudocode:

Direct IO behavior:

    ITER_IOVEC:
        pin_user_pages_fast();
        break;

    ITER_PIPE:
        for each page:
             pin_user_page();
        break;

    ITER_KVEC:    // already elevated page refcount, leave alone
    ITER_BVEC:    // already elevated page refcount, leave alone
    ITER_DISCARD: // discard
        return -EFAULT or -ENVALID;

...which works for callers that already have sorted out which case they
are in. Such as, Direct IO in the block/bio layers.

Now, this does leave ITER_KVEC and ITER_BVEC unconverted, but on the
other hand, it's not clear that these are actually affected in the real
world, by the get_user_pages()+filesystem interaction problems of [2].
If it turns out to matter, then those can be handled too, but it's just
more refactoring and surgery to do so.

Page acquisition: The iov_iter_get_pages*() routines
above are at just the right level in the call stack: the callers already
know which system to use, and so it's a small change to just drop in the
replacement routines. And it's a fan-in/fan-out point: block/bio call
sites for Direct IO funnel their page acquisitions through the
iov_iter_get_pages*() routines, and there are many other callers of
those. And we can't convert all of the callers at once--too many
subsystems are involved, and it would be a too large and too risky
patch.

Page release: there are already separate release routines: put_page()
vs. unpin_user_page(), so it's already done there.

[1] https://lore.kernel.org/kvm/20190724061750.GA19397@infradead.org/

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 block/bio.c          | 24 ++++++++++++------------
 block/blk-map.c      |  6 +++---
 fs/direct-io.c       | 28 ++++++++++++++--------------
 fs/iomap/direct-io.c |  2 +-
 4 files changed, 30 insertions(+), 30 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index a9931f23d933..f54e9414e6d9 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -955,7 +955,7 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
 	bio_for_each_segment_all(bvec, bio, iter_all) {
 		if (mark_dirty && !PageCompound(bvec->bv_page))
 			set_page_dirty_lock(bvec->bv_page);
-		put_page(bvec->bv_page);
+		unpin_user_page(bvec->bv_page);
 	}
 }
 EXPORT_SYMBOL_GPL(bio_release_pages);
@@ -986,9 +986,9 @@ static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
  * @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.
- * For multi-segment *iter, this function only adds pages from the
- * next non-empty segment of the iov iterator.
+ * pages will have to be released using put_page() or unpin_user_page() when
+ * done. For multi-segment *iter, this function only adds pages from the next
+ * non-empty segment of the iov iterator.
  */
 static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
@@ -1009,7 +1009,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
 	pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
 
-	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+	size = iov_iter_pin_user_pages(iter, pages, LONG_MAX, nr_pages, &offset);
 	if (unlikely(size <= 0))
 		return size ? size : -EFAULT;
 
@@ -1020,7 +1020,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 
 		if (__bio_try_merge_page(bio, page, len, offset, &same_page)) {
 			if (same_page)
-				put_page(page);
+				unpin_user_page(page);
 		} else {
 			if (WARN_ON_ONCE(bio_full(bio, len)))
                                 return -EINVAL;
@@ -1056,7 +1056,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
 	BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
 	pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
 
-	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
+	size = iov_iter_pin_user_pages(iter, pages, LONG_MAX, nr_pages, &offset);
 	if (unlikely(size <= 0))
 		return size ? size : -EFAULT;
 
@@ -1069,7 +1069,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
 				max_append_sectors, &same_page) != len)
 			return -EINVAL;
 		if (same_page)
-			put_page(page);
+			unpin_user_page(page);
 		offset = 0;
 	}
 
@@ -1113,8 +1113,8 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		} else {
 			if (is_bvec)
 				ret = __bio_iov_bvec_add_pages(bio, iter);
-			else
-				ret = __bio_iov_iter_get_pages(bio, iter);
+		else
+			ret = __bio_iov_iter_get_pages(bio, iter);
 		}
 	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
 
@@ -1326,8 +1326,8 @@ void bio_set_pages_dirty(struct bio *bio)
  * the BIO and re-dirty the pages in process context.
  *
  * It is expected that bio_check_pages_dirty() will wholly own the BIO from
- * here on.  It will run one put_page() against each page and will run one
- * bio_put() against the BIO.
+ * here on.  It will run one unpin_user_page() against each page
+ * and will run one bio_put() against the BIO.
  */
 
 static void bio_dirty_fn(struct work_struct *work);
diff --git a/block/blk-map.c b/block/blk-map.c
index 6e804892d5ec..7a095b4947ea 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -275,7 +275,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
 		size_t offs, added = 0;
 		int npages;
 
-		bytes = iov_iter_get_pages_alloc(iter, &pages, LONG_MAX, &offs);
+		bytes = iov_iter_pin_user_pages_alloc(iter, &pages, LONG_MAX, &offs);
 		if (unlikely(bytes <= 0)) {
 			ret = bytes ? bytes : -EFAULT;
 			goto out_unmap;
@@ -298,7 +298,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
 				if (!bio_add_hw_page(q, bio, page, n, offs,
 						     max_sectors, &same_page)) {
 					if (same_page)
-						put_page(page);
+						unpin_user_page(page);
 					break;
 				}
 
@@ -312,7 +312,7 @@ static struct bio *bio_map_user_iov(struct request_queue *q,
 		 * release the pages we didn't map into the bio, if any
 		 */
 		while (j < npages)
-			put_page(pages[j++]);
+			unpin_user_page(pages[j++]);
 		kvfree(pages);
 		/* couldn't stuff something into bio? */
 		if (bytes)
diff --git a/fs/direct-io.c b/fs/direct-io.c
index 183299892465..b01c8d003bd3 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -170,7 +170,7 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
 {
 	ssize_t ret;
 
-	ret = iov_iter_get_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,
+	ret = iov_iter_pin_user_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,
 				&sdio->from);
 
 	if (ret < 0 && sdio->blocks_available && (dio->op == REQ_OP_WRITE)) {
@@ -182,7 +182,7 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
 		 */
 		if (dio->page_errors == 0)
 			dio->page_errors = ret;
-		get_page(page);
+		pin_user_page(page);
 		dio->pages[0] = page;
 		sdio->head = 0;
 		sdio->tail = 1;
@@ -472,7 +472,7 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
 static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio)
 {
 	while (sdio->head < sdio->tail)
-		put_page(dio->pages[sdio->head++]);
+		unpin_user_page(dio->pages[sdio->head++]);
 }
 
 /*
@@ -739,7 +739,7 @@ static inline int dio_bio_add_page(struct dio_submit *sdio)
 		 */
 		if ((sdio->cur_page_len + sdio->cur_page_offset) == PAGE_SIZE)
 			sdio->pages_in_io--;
-		get_page(sdio->cur_page);
+		pin_user_page(sdio->cur_page);
 		sdio->final_block_in_bio = sdio->cur_page_block +
 			(sdio->cur_page_len >> sdio->blkbits);
 		ret = 0;
@@ -853,13 +853,13 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
 	 */
 	if (sdio->cur_page) {
 		ret = dio_send_cur_page(dio, sdio, map_bh);
-		put_page(sdio->cur_page);
+		unpin_user_page(sdio->cur_page);
 		sdio->cur_page = NULL;
 		if (ret)
 			return ret;
 	}
 
-	get_page(page);		/* It is in dio */
+	pin_user_page(page);		/* It is in dio */
 	sdio->cur_page = page;
 	sdio->cur_page_offset = offset;
 	sdio->cur_page_len = len;
@@ -874,7 +874,7 @@ submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
 		ret = dio_send_cur_page(dio, sdio, map_bh);
 		if (sdio->bio)
 			dio_bio_submit(dio, sdio);
-		put_page(sdio->cur_page);
+		unpin_user_page(sdio->cur_page);
 		sdio->cur_page = NULL;
 	}
 	return ret;
@@ -974,7 +974,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 
 				ret = get_more_blocks(dio, sdio, map_bh);
 				if (ret) {
-					put_page(page);
+					unpin_user_page(page);
 					goto out;
 				}
 				if (!buffer_mapped(map_bh))
@@ -1019,7 +1019,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 
 				/* AKPM: eargh, -ENOTBLK is a hack */
 				if (dio->op == REQ_OP_WRITE) {
-					put_page(page);
+					unpin_user_page(page);
 					return -ENOTBLK;
 				}
 
@@ -1032,7 +1032,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 				if (sdio->block_in_file >=
 						i_size_aligned >> blkbits) {
 					/* We hit eof */
-					put_page(page);
+					unpin_user_page(page);
 					goto out;
 				}
 				zero_user(page, from, 1 << blkbits);
@@ -1072,7 +1072,7 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 						  sdio->next_block_for_io,
 						  map_bh);
 			if (ret) {
-				put_page(page);
+				unpin_user_page(page);
 				goto out;
 			}
 			sdio->next_block_for_io += this_chunk_blocks;
@@ -1087,8 +1087,8 @@ static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 				break;
 		}
 
-		/* Drop the ref which was taken in get_user_pages() */
-		put_page(page);
+		/* Drop the ref which was taken in pin_user_pages() */
+		unpin_user_page(page);
 	}
 out:
 	return ret;
@@ -1327,7 +1327,7 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 		ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
 		if (retval == 0)
 			retval = ret2;
-		put_page(sdio.cur_page);
+		unpin_user_page(sdio.cur_page);
 		sdio.cur_page = NULL;
 	}
 	if (sdio.bio)
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..390f611528ea 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -194,7 +194,7 @@ iomap_dio_zero(struct iomap_dio *dio, struct iomap *iomap, loff_t pos,
 	bio->bi_private = dio;
 	bio->bi_end_io = iomap_dio_bio_end_io;
 
-	get_page(page);
+	pin_user_page(page);
 	__bio_add_page(bio, page, len, 0);
 	bio_set_op_attrs(bio, REQ_OP_WRITE, flags);
 	iomap_dio_submit_bio(dio, iomap, bio, pos);
-- 
2.28.0