[PATCH v2 06/21] iov_iter: Make iov_iter_get_pages*() wrap iov_iter_extract_pages()

[David Howells <dhowells@redhat.com>]

Make iov_iter_get_pages*() wrap iov_iter_extract_pages() for kernel
iterator types (e.g. ITER_BVEC, ITER_FOLIOQ, ITER_XARRAY).  The pages
obtained have their refcounts incremented afterwards if they're not slab
pages.  ITER_KVEC is left returning -EFAULT.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-block@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 lib/iov_iter.c | 164 ++++++-------------------------------------------
 1 file changed, 19 insertions(+), 145 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af7..cac7d7364bc2 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -910,118 +910,34 @@ static int want_pages_array(struct page ***res, size_t size,
 	return count;
 }
 
-static ssize_t iter_folioq_get_pages(struct iov_iter *iter,
+/*
+ * Wrap iov_iter_extract_pages() and then pin the non-slab pages we got back.
+ * This only works for non-user iterator types as get_pages uses get_user_pages
+ * not pin_user_pages.
+ */
+static ssize_t iter_get_kernel_pages(struct iov_iter *iter,
 				     struct page ***ppages, size_t maxsize,
 				     unsigned maxpages, size_t *_start_offset)
 {
-	const struct folio_queue *folioq = iter->folioq;
 	struct page **pages;
-	unsigned int slot = iter->folioq_slot;
-	size_t extracted = 0, count = iter->count, iov_offset = iter->iov_offset;
+	ssize_t ret, done;
 
-	if (slot >= folioq_nr_slots(folioq)) {
-		folioq = folioq->next;
-		slot = 0;
-		if (WARN_ON(iov_offset != 0))
-			return -EIO;
-	}
+	ret = iov_iter_extract_pages(iter, ppages, maxsize, maxpages,
+				     0, _start_offset);
+	if (ret <= 0)
+		return ret;
 
-	maxpages = want_pages_array(ppages, maxsize, iov_offset & ~PAGE_MASK, maxpages);
-	if (!maxpages)
-		return -ENOMEM;
-	*_start_offset = iov_offset & ~PAGE_MASK;
 	pages = *ppages;
+	for (done = ret + *_start_offset; done > 0; done -= PAGE_SIZE) {
+		struct folio *folio = page_folio(*pages);
 
-	for (;;) {
-		struct folio *folio = folioq_folio(folioq, slot);
-		size_t offset = iov_offset, fsize = folioq_folio_size(folioq, slot);
-		size_t part = PAGE_SIZE - offset % PAGE_SIZE;
-
-		if (offset < fsize) {
-			part = umin(part, umin(maxsize - extracted, fsize - offset));
-			count -= part;
-			iov_offset += part;
-			extracted += part;
-
-			*pages = folio_page(folio, offset / PAGE_SIZE);
-			get_page(*pages);
-			pages++;
-			maxpages--;
-		}
-
-		if (maxpages == 0 || extracted >= maxsize)
-			break;
-
-		if (iov_offset >= fsize) {
-			iov_offset = 0;
-			slot++;
-			if (slot == folioq_nr_slots(folioq) && folioq->next) {
-				folioq = folioq->next;
-				slot = 0;
-			}
-		}
-	}
-
-	iter->count = count;
-	iter->iov_offset = iov_offset;
-	iter->folioq = folioq;
-	iter->folioq_slot = slot;
-	return extracted;
-}
-
-static ssize_t iter_xarray_populate_pages(struct page **pages, struct xarray *xa,
-					  pgoff_t index, unsigned int nr_pages)
-{
-	XA_STATE(xas, xa, index);
-	struct folio *folio;
-	unsigned int ret = 0;
-
-	rcu_read_lock();
-	for (folio = xas_load(&xas); folio; folio = xas_next(&xas)) {
-		if (xas_retry(&xas, folio))
-			continue;
-
-		/* Has the folio moved or been split? */
-		if (unlikely(folio != xas_reload(&xas))) {
-			xas_reset(&xas);
-			continue;
-		}
-
-		pages[ret] = folio_file_page(folio, xas.xa_index);
-		folio_get(folio);
-		if (++ret == nr_pages)
-			break;
+		if (!folio_test_slab(folio))
+			folio_get(folio);
+		pages++;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
-static ssize_t iter_xarray_get_pages(struct iov_iter *i,
-				     struct page ***pages, size_t maxsize,
-				     unsigned maxpages, size_t *_start_offset)
-{
-	unsigned nr, offset, count;
-	pgoff_t index;
-	loff_t pos;
-
-	pos = i->xarray_start + i->iov_offset;
-	index = pos >> PAGE_SHIFT;
-	offset = pos & ~PAGE_MASK;
-	*_start_offset = offset;
-
-	count = want_pages_array(pages, maxsize, offset, maxpages);
-	if (!count)
-		return -ENOMEM;
-	nr = iter_xarray_populate_pages(*pages, i->xarray, index, count);
-	if (nr == 0)
-		return 0;
-
-	maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
-	i->iov_offset += maxsize;
-	i->count -= maxsize;
-	return maxsize;
-}
-
 /* must be done on non-empty ITER_UBUF or ITER_IOVEC one */
 static unsigned long first_iovec_segment(const struct iov_iter *i, size_t *size)
 {
@@ -1044,22 +960,6 @@ static unsigned long first_iovec_segment(const struct iov_iter *i, size_t *size)
 	BUG(); // if it had been empty, we wouldn't get called
 }
 
-/* must be done on non-empty ITER_BVEC one */
-static struct page *first_bvec_segment(const struct iov_iter *i,
-				       size_t *size, size_t *start)
-{
-	struct page *page;
-	size_t skip = i->iov_offset, len;
-
-	len = i->bvec->bv_len - skip;
-	if (*size > len)
-		*size = len;
-	skip += i->bvec->bv_offset;
-	page = i->bvec->bv_page + skip / PAGE_SIZE;
-	*start = skip % PAGE_SIZE;
-	return page;
-}
-
 static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
 		   unsigned int maxpages, size_t *start)
@@ -1095,36 +995,10 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
 		iov_iter_advance(i, maxsize);
 		return maxsize;
 	}
-	if (iov_iter_is_bvec(i)) {
-		struct page **p;
-		struct page *page;
 
-		page = first_bvec_segment(i, &maxsize, start);
-		n = want_pages_array(pages, maxsize, *start, maxpages);
-		if (!n)
-			return -ENOMEM;
-		p = *pages;
-		for (int k = 0; k < n; k++) {
-			struct folio *folio = page_folio(page + k);
-			p[k] = page + k;
-			if (!folio_test_slab(folio))
-				folio_get(folio);
-		}
-		maxsize = min_t(size_t, maxsize, n * PAGE_SIZE - *start);
-		i->count -= maxsize;
-		i->iov_offset += maxsize;
-		if (i->iov_offset == i->bvec->bv_len) {
-			i->iov_offset = 0;
-			i->bvec++;
-			i->nr_segs--;
-		}
-		return maxsize;
-	}
-	if (iov_iter_is_folioq(i))
-		return iter_folioq_get_pages(i, pages, maxsize, maxpages, start);
-	if (iov_iter_is_xarray(i))
-		return iter_xarray_get_pages(i, pages, maxsize, maxpages, start);
-	return -EFAULT;
+	if (iov_iter_is_kvec(i))
+		return -EFAULT;
+	return iter_get_kernel_pages(i, pages, maxsize, maxpages, start);
 }
 
 ssize_t iov_iter_get_pages2(struct iov_iter *i, struct page **pages,