[PATCH V2] lib/iov_iter.c: extract virt-contiguous pages in iov_iter_extract_bvec_pages

[Ming Lei]

Actually iov_iter_extract_pages() requires that there isn't gap in the
extracted pages, so 'offset' only exists in the 1st page, then these
pages can be mapped to one virtual(contiguous) address.

All iov_iter_bvec() users only want to extract virt-contiguous pages from
iov_iter_extract_pages() instead physical-contiguous pages.

Change iov_iter_extract_bvec_pages() to extract virt-contiguous pages via
bvec helper.

This way can fill much more pages one time, instead of (often)one page from
iov_iter_extract_pages() each time.

The change is reasonable & safe since oher kind of iterators(UBUF, KVEC, ...)
do return non physically-contiguous pages.

Fixes: a7e689dd1c06 ("block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages")
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
V2:
	- add fixes
	- improve commit log

 include/linux/bvec.h |  6 ++++++
 lib/iov_iter.c       | 47 +++++++++++++++++++++++---------------------
 2 files changed, 31 insertions(+), 22 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index f41c7f0ef91e..98e1a4ad09e0 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -184,6 +184,12 @@ static inline void bvec_iter_advance_single(const struct bio_vec *bv,
 		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
 	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
 
+#define for_each_bvec_max(bvl, bio_vec, iter, start, nr_bvecs)		\
+	for (iter = (start);						\
+	     (iter).bi_size && iter.bi_idx < nr_bvecs &&		\
+		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
+	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
+
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\
 {									\
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 97003155bfac..6e00f6da5259 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1677,8 +1677,8 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
 }
 
 /*
- * Extract a list of contiguous pages from an ITER_BVEC iterator.  This does
- * not get references on the pages, nor does it get a pin on them.
+ * Extract a list of virtually contiguous pages from an ITER_BVEC iterator.
+ * This does not get references on the pages, nor does it get a pin on them.
  */
 static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
 					   struct page ***pages, size_t maxsize,
@@ -1686,35 +1686,38 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
 					   iov_iter_extraction_t extraction_flags,
 					   size_t *offset0)
 {
-	struct page **p, *page;
-	size_t skip = i->iov_offset, offset, size;
-	int k;
+	size_t skip = i->iov_offset, size = 0;
+	struct bvec_iter bi;
+	struct bio_vec bv;
+	int k = 0;
 
-	for (;;) {
-		if (i->nr_segs == 0)
-			return 0;
-		size = min(maxsize, i->bvec->bv_len - skip);
-		if (size)
-			break;
+	if (i->nr_segs == 0)
+		return 0;
+
+	if (i->iov_offset == i->bvec->bv_len) {
 		i->iov_offset = 0;
 		i->nr_segs--;
 		i->bvec++;
 		skip = 0;
 	}
+	bi.bi_size = maxsize + skip;
+	bi.bi_bvec_done = skip;
 
-	skip += i->bvec->bv_offset;
-	page = i->bvec->bv_page + skip / PAGE_SIZE;
-	offset = skip % PAGE_SIZE;
-	*offset0 = offset;
+	maxpages = want_pages_array(pages, maxsize, skip, maxpages);
 
-	maxpages = want_pages_array(pages, size, offset, maxpages);
-	if (!maxpages)
-		return -ENOMEM;
-	p = *pages;
-	for (k = 0; k < maxpages; k++)
-		p[k] = page + k;
+	for_each_bvec_max(bv, i->bvec, bi, bi, i->nr_segs) {
+		if (k >= maxpages)
+			break;
+		if (!k)
+			*offset0 = bv.bv_offset;
+		else if (bv.bv_offset)
+			break;
+		(*pages)[k++] = bv.bv_page;
+		size += bv.bv_len;
+		if (bv.bv_offset + bv.bv_len != PAGE_SIZE)
+			break;
+	}
 
-	size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
 	iov_iter_advance(i, size);
 	return size;
 }
-- 
2.46.0