[RFC PATCH 03/31] netfs: Provide facility to alloc buffer in a bvecq

[David Howells <dhowells@redhat.com>]

Provide facility to allocate a series of bvecq structs and to attach
sufficient pages to that series to provide a buffer for the specified
amount of space.  This can be used to do things like creating an encryption
buffer in cifs and it can then be attached to an ITER_BVECQ iterator and
passed to a socket.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Shyam Prasad N <sprasad@microsoft.com>
cc: Tom Talpey <tom@talpey.com>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/Makefile     |   1 +
 fs/netfs/buffer.c     | 101 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |   4 ++
 3 files changed, 106 insertions(+)
 create mode 100644 fs/netfs/buffer.c

diff --git a/fs/netfs/Makefile b/fs/netfs/Makefile
index b43188d64bd8..afab6603bd98 100644
--- a/fs/netfs/Makefile
+++ b/fs/netfs/Makefile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 netfs-y := \
+	buffer.o \
 	buffered_read.o \
 	buffered_write.o \
 	direct_read.o \
diff --git a/fs/netfs/buffer.c b/fs/netfs/buffer.c
new file mode 100644
index 000000000000..1e4ed2746e95
--- /dev/null
+++ b/fs/netfs/buffer.c
@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Buffering helpers for bvec_queues
+ *
+ * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+#include "internal.h"
+
+void dump_bvecq(const struct bvecq *bq)
+{
+	int b = 0;
+
+	for (; bq; bq = bq->next, b++) {
+		printk("BQ[%u] %u/%u\n", b, bq->nr_segs, bq->max_segs);
+		for (int s = 0; s < bq->nr_segs; s++) {
+			const struct bio_vec *bv = &bq->bv[s];
+			printk("BQ[%u:%u] %10lx %04x %04x %u\n",
+			       b, s,
+			       bv->bv_page ? page_to_pfn(bv->bv_page) : 0,
+			       bv->bv_offset, bv->bv_len,
+			       page_count(bv->bv_page));
+		}
+	}
+}
+
+/**
+ * netfs_alloc_bvecq_buffer - Allocate buffer space into a bvec queue
+ * @size: Target size of the buffer.
+ * @pre_slots: Number of preamble slots to set aside
+ * @gfp: The allocation constraints.
+ */
+struct bvecq *netfs_alloc_bvecq_buffer(size_t size, unsigned int pre_slots, gfp_t gfp)
+{
+	struct bvecq *head = NULL, *tail = NULL, *p = NULL;
+	size_t count = DIV_ROUND_UP(size, PAGE_SIZE);
+	int max_segs = 32;
+
+	_enter("%zx,%zx,%u", size, count, pre_slots);
+
+	do {
+		struct page **pages;
+		int want, got;
+
+		p = kzalloc(struct_size(p, bv, max_segs), gfp);
+		if (!p)
+			goto oom;
+		if (tail) {
+			tail->next = p;
+			p->prev = tail;
+		} else {
+			head = p;
+		}
+		tail = p;
+		pages = (struct page **)&p->bv[max_segs];
+		pages -= max_segs - pre_slots;
+
+		want = umin(count, max_segs - pre_slots);
+		got = alloc_pages_bulk(gfp, want, pages);
+		if (got < want) {
+			for (int i = 0; i < got; i++)
+				__free_page(pages[i]);
+			goto oom;
+		}
+
+		tail->max_segs = max_segs;
+		tail->nr_segs = pre_slots + got;
+		for (int i = 0; i < got; i++) {
+			int j = pre_slots + i;
+			set_page_count(pages[i], 1);
+			bvec_set_page(&tail->bv[j], pages[i], PAGE_SIZE, 0);
+		}
+
+		count -= got;
+		pre_slots = 0;
+	} while (count > 0);
+
+	return head;
+oom:
+	netfs_free_bvecq_buffer(head);
+	return NULL;
+}
+EXPORT_SYMBOL(netfs_alloc_bvecq_buffer);
+
+/**
+ * netfs_free_bvecq_buffer - Free a bvec queue
+ * @bq: The start of the folio queue to free
+ *
+ * Free up a chain of bvecqs and the pages it points to.
+ */
+void netfs_free_bvecq_buffer(struct bvecq *bq)
+{
+	struct bvecq *next;
+
+	for (; bq; bq = next) {
+		for (int seg = 0; seg < bq->nr_segs; seg++)
+			__free_page(bq->bv[seg].bv_page);
+		next = bq->next;
+		kfree(bq);
+	}
+}
+EXPORT_SYMBOL(netfs_free_bvecq_buffer);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index f43f075852c0..8756129b7472 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -23,6 +23,7 @@
 enum netfs_sreq_ref_trace;
 typedef struct mempool_s mempool_t;
 struct folio_queue;
+struct bvecq;
 
 /**
  * folio_start_private_2 - Start an fscache write on a folio.  [DEPRECATED]
@@ -462,6 +463,9 @@ int netfs_alloc_folioq_buffer(struct address_space *mapping,
 			      struct folio_queue **_buffer,
 			      size_t *_cur_size, ssize_t size, gfp_t gfp);
 void netfs_free_folioq_buffer(struct folio_queue *fq);
+void dump_bvecq(const struct bvecq *bq);
+struct bvecq *netfs_alloc_bvecq_buffer(size_t size, unsigned int pre_slots, gfp_t gfp);
+void netfs_free_bvecq_buffer(struct bvecq *bq);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode