[PATCH v12 5/8] netfs: Provide a func to load the readahead buffers into a bvecq chain

[David Howells <dhowells@redhat.com>]

In netfslib, provide a function to load the readahead buffers into a bvecq
chain by preallocating the bvecq chain and then iterating directly over the
pagecache to extract all the folios.  This builds on the preceding patch to
note the folio count in the readahead_control struct.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/bvecq.c      | 60 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/bvecq.h |  1 +
 2 files changed, 61 insertions(+)

diff --git a/fs/netfs/bvecq.c b/fs/netfs/bvecq.c
index 1493e028bb17..2edc0045c245 100644
--- a/fs/netfs/bvecq.c
+++ b/fs/netfs/bvecq.c
@@ -592,3 +592,63 @@ size_t bvecq_slice(struct bvecq_pos *pos, size_t max_size,
 	pos->offset = offset;
 	return size;
 }
+
+/**
+ * bvecq_load_from_ra - Allocate a bvecq chain and load from readahead
+ * @pos: Blank position object to attach the new chain to.
+ * @ractl: The readahead control context.
+ *
+ * Decant the set of folios to be read from the readahead context into a bvecq
+ * chain.  Each folio occupies one bio_vec element.
+ *
+ * Return: Amount of data loaded or -ENOMEM on allocation failure.
+ */
+ssize_t bvecq_load_from_ra(struct bvecq_pos *pos, struct readahead_control *ractl)
+{
+	XA_STATE(xas, &ractl->mapping->i_pages, ractl->_index);
+	struct folio *folio;
+	struct bvecq *bq;
+	unsigned int slot = 0;
+	size_t loaded = 0;
+
+	bq = bvecq_alloc_chain(ractl->_nr_folios, GFP_KERNEL, false);
+	if (!bq)
+		return -ENOMEM;
+
+	pos->bvecq  = bq;
+	pos->slot   = 0;
+	pos->offset = 0;
+
+	rcu_read_lock();
+
+	xas_for_each(&xas, folio, ractl->_index + ractl->_nr_pages - 1) {
+		size_t len;
+
+		if (xas_retry(&xas, folio))
+			continue;
+		VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
+
+		len = folio_size(folio);
+		bvec_set_folio(&bq->bv[slot], folio, len, 0);
+		loaded += len;
+		slot++;
+		trace_netfs_folio(folio, netfs_folio_trace_read);
+
+		if (slot >= bq->max_slots) {
+			bvecq_filled_to(bq, slot);
+			bq = bq->next;
+			if (!bq)
+				break;
+			slot = 0;
+		}
+	}
+
+	rcu_read_unlock();
+
+	if (bq)
+		bvecq_filled_to(bq, slot);
+
+	ractl->_index += ractl->_nr_pages;
+	ractl->_nr_pages = 0;
+	return loaded;
+}
diff --git a/include/linux/bvecq.h b/include/linux/bvecq.h
index f1513f507cfb..389f6407cb84 100644
--- a/include/linux/bvecq.h
+++ b/include/linux/bvecq.h
@@ -77,6 +77,7 @@ void bvecq_pos_advance(struct bvecq_pos *pos, size_t amount);
 ssize_t bvecq_zero(struct bvecq_pos *pos, size_t amount);
 size_t bvecq_slice(struct bvecq_pos *pos, size_t max_size,
 		   unsigned int max_slots, unsigned int *_nr_slots);
+ssize_t bvecq_load_from_ra(struct bvecq_pos *pos, struct readahead_control *ractl);
 
 /**
  * bvecq_alloc_buffer - Allocate a bvecq chain and populate with buffers