Re: [block.git conflicts] Re: [PATCH 37/44] block: convert to advancing variants of iov_iter_get_pages{,_alloc}()

[Keith Busch <kbusch@kernel.org>]

On Fri, Jul 01, 2022 at 06:40:40PM +0100, Al Viro wrote:
> -static void bio_put_pages(struct page **pages, size_t size, size_t off)
> -{
> -	size_t i, nr = DIV_ROUND_UP(size + (off & ~PAGE_MASK), PAGE_SIZE);
> -
> -	for (i = 0; i < nr; i++)
> -		put_page(pages[i]);
> -}
> -
>  static int bio_iov_add_page(struct bio *bio, struct page *page,
>  		unsigned int len, unsigned int offset)
>  {
> @@ -1228,11 +1220,11 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  	 * the iov data will be picked up in the next bio iteration.
>  	 */
>  	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> -	if (size > 0)
> -		size = ALIGN_DOWN(size, bdev_logical_block_size(bio->bi_bdev));
>  	if (unlikely(size <= 0))
>  		return size ? size : -EFAULT;
> +	nr_pages = DIV_ROUND_UP(offset + size, PAGE_SIZE);
>  
> +	size = ALIGN_DOWN(size, bdev_logical_block_size(bio->bi_bdev));

This isn't quite right. The result of the ALIGN_DOWN could be 0, so whatever
page we got before would be leaked since unused pages are only released on an
add_page error. I was about to reply with a patch that fixes this, but here's
the one that I'm currently testing:

---
diff --git a/block/bio.c b/block/bio.c
index 933ea3210954..c4a1ce39c65c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1151,14 +1151,6 @@ void bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
 	bio_set_flag(bio, BIO_CLONED);
 }
 
-static void bio_put_pages(struct page **pages, size_t size, size_t off)
-{
-	size_t i, nr = DIV_ROUND_UP(size + (off & ~PAGE_MASK), PAGE_SIZE);
-
-	for (i = 0; i < nr; i++)
-		put_page(pages[i]);
-}
-
 static int bio_iov_add_page(struct bio *bio, struct page *page,
 		unsigned int len, unsigned int offset)
 {
@@ -1208,9 +1200,10 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
 	struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
 	struct page **pages = (struct page **)bv;
+	unsigned len, i = 0;
 	ssize_t size, left;
-	unsigned len, i;
 	size_t offset;
+	int ret;
 
 	/*
 	 * Move page array up in the allocated memory for the bio vecs as far as
@@ -1228,14 +1221,19 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	 * the iov data will be picked up in the next bio iteration.
 	 */
 	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
-	if (size > 0)
+	if (size > 0) {
+		nr_pages = DIV_ROUND_UP(size + offset, PAGE_SIZE);
 		size = ALIGN_DOWN(size, bdev_logical_block_size(bio->bi_bdev));
-	if (unlikely(size <= 0))
-		return size ? size : -EFAULT;
+	} else
+		nr_pages = 0;
+
+	if (unlikely(size <= 0)) {
+		ret = size ? size : -EFAULT;
+		goto out;
+	}
 
-	for (left = size, i = 0; left > 0; left -= len, i++) {
+	for (left = size; left > 0; left -= len, i++) {
 		struct page *page = pages[i];
-		int ret;
 
 		len = min_t(size_t, PAGE_SIZE - offset, left);
 		if (bio_op(bio) == REQ_OP_ZONE_APPEND)
@@ -1244,15 +1242,19 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		else
 			ret = bio_iov_add_page(bio, page, len, offset);
 
-		if (ret) {
-			bio_put_pages(pages + i, left, offset);
-			return ret;
-		}
+		if (ret)
+			goto out;
 		offset = 0;
 	}
 
 	iov_iter_advance(iter, size);
-	return 0;
+out:
+	while (i < nr_pages) {
+		put_page(pages[i]);
+		i++;
+	}
+
+	return ret;
 }
 
 /**
--