xfs: support in-memory buffer cache targets

Allow the buffer cache to target in-memory files by making it possible
to have a buftarg that maps pages from private shmem files.  As the
prevous patch alludes, the in-memory buftarg contains its own cache,
points to a shmem file, and does not point to a block_device.

The next few patches will make it possible to construct an xfs_btree in
pageable memory by using this buftarg.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 189 insertions, 0 deletions

diff --git a/fs/xfs/xfs_buf_mem.c b/fs/xfs/xfs_buf_mem.c
new file mode 100644
index 000000000000..be71ba1a3d7b
--- /dev/null
+++ b/fs/xfs/xfs_buf_mem.c
@@ -0,0 +1,189 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2023-2024 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#include "xfs.h"
+#include "xfs_fs.h"
+#include "xfs_buf.h"
+#include "xfs_buf_mem.h"
+#include "xfs_trace.h"
+#include <linux/shmem_fs.h>
+
+/*
+ * Buffer Cache for In-Memory Files
+ * ================================
+ *
+ * Online fsck wants to create ephemeral ordered recordsets.  The existing
+ * btree infrastructure can do this, but we need the buffer cache to target
+ * memory instead of block devices.
+ *
+ * When CONFIG_TMPFS=y, shmemfs is enough of a filesystem to meet those
+ * requirements.  Therefore, the xmbuf mechanism uses an unlinked shmem file to
+ * store our staging data.  This file is not installed in the file descriptor
+ * table so that user programs cannot access the data, which means that the
+ * xmbuf must be freed with xmbuf_destroy.
+ *
+ * xmbufs assume that the caller will handle all required concurrency
+ * management; standard vfs locks (freezer and inode) are not taken.  Reads
+ * and writes are satisfied directly from the page cache.
+ *
+ * The only supported block size is PAGE_SIZE, and we cannot use highmem.
+ */
+
+/*
+ * shmem files used to back an in-memory buffer cache must not be exposed to
+ * userspace.  Upper layers must coordinate access to the one handle returned
+ * by the constructor, so establish a separate lock class for xmbufs to avoid
+ * confusing lockdep.
+ */
+static struct lock_class_key xmbuf_i_mutex_key;
+
+/*
+ * Allocate a buffer cache target for a memory-backed file and set up the
+ * buffer target.
+ */
+int
+xmbuf_alloc(
+	struct xfs_mount	*mp,
+	const char		*descr,
+	struct xfs_buftarg	**btpp)
+{
+	struct file		*file;
+	struct inode		*inode;
+	struct xfs_buftarg	*btp;
+	int			error;
+
+	btp = kzalloc(struct_size(btp, bt_cache, 1), GFP_KERNEL);
+	if (!btp)
+		return -ENOMEM;
+
+	file = shmem_kernel_file_setup(descr, 0, 0);
+	if (IS_ERR(file)) {
+		error = PTR_ERR(file);
+		goto out_free_btp;
+	}
+	inode = file_inode(file);
+
+	/* private file, private locking */
+	lockdep_set_class(&inode->i_rwsem, &xmbuf_i_mutex_key);
+
+	/*
+	 * We don't want to bother with kmapping data during repair, so don't
+	 * allow highmem pages to back this mapping.
+	 */
+	mapping_set_gfp_mask(inode->i_mapping, GFP_KERNEL);
+
+	/* ensure all writes are below EOF to avoid pagecache zeroing */
+	i_size_write(inode, inode->i_sb->s_maxbytes);
+
+	trace_xmbuf_create(btp);
+
+	error = xfs_buf_cache_init(btp->bt_cache);
+	if (error)
+		goto out_file;
+
+	/* Initialize buffer target */
+	btp->bt_mount = mp;
+	btp->bt_dev = (dev_t)-1U;
+	btp->bt_bdev = NULL; /* in-memory buftargs have no bdev */
+	btp->bt_file = file;
+	btp->bt_meta_sectorsize = XMBUF_BLOCKSIZE;
+	btp->bt_meta_sectormask = XMBUF_BLOCKSIZE - 1;
+
+	error = xfs_init_buftarg(btp, XMBUF_BLOCKSIZE, descr);
+	if (error)
+		goto out_bcache;
+
+	*btpp = btp;
+	return 0;
+
+out_bcache:
+	xfs_buf_cache_destroy(btp->bt_cache);
+out_file:
+	fput(file);
+out_free_btp:
+	kfree(btp);
+	return error;
+}
+
+/* Free a buffer cache target for a memory-backed buffer cache. */
+void
+xmbuf_free(
+	struct xfs_buftarg	*btp)
+{
+	ASSERT(xfs_buftarg_is_mem(btp));
+	ASSERT(percpu_counter_sum(&btp->bt_io_count) == 0);
+
+	trace_xmbuf_free(btp);
+
+	xfs_destroy_buftarg(btp);
+	xfs_buf_cache_destroy(btp->bt_cache);
+	fput(btp->bt_file);
+	kfree(btp);
+}
+
+/* Directly map a shmem page into the buffer cache. */
+int
+xmbuf_map_page(
+	struct xfs_buf		*bp)
+{
+	struct inode		*inode = file_inode(bp->b_target->bt_file);
+	struct folio		*folio = NULL;
+	struct page		*page;
+	loff_t                  pos = BBTOB(xfs_buf_daddr(bp));
+	int			error;
+
+	ASSERT(xfs_buftarg_is_mem(bp->b_target));
+
+	if (bp->b_map_count != 1)
+		return -ENOMEM;
+	if (BBTOB(bp->b_length) != XMBUF_BLOCKSIZE)
+		return -ENOMEM;
+	if (offset_in_page(pos) != 0) {
+		ASSERT(offset_in_page(pos));
+		return -ENOMEM;
+	}
+
+	error = shmem_get_folio(inode, pos >> PAGE_SHIFT, &folio, SGP_CACHE);
+	if (error)
+		return error;
+
+	if (filemap_check_wb_err(inode->i_mapping, 0)) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return -EIO;
+	}
+
+	page = folio_file_page(folio, pos >> PAGE_SHIFT);
+
+	/*
+	 * Mark the page dirty so that it won't be reclaimed once we drop the
+	 * (potentially last) reference in xmbuf_unmap_page.
+	 */
+	set_page_dirty(page);
+	unlock_page(page);
+
+	bp->b_addr = page_address(page);
+	bp->b_pages = bp->b_page_array;
+	bp->b_pages[0] = page;
+	bp->b_page_count = 1;
+	return 0;
+}
+
+/* Unmap a shmem page that was mapped into the buffer cache. */
+void
+xmbuf_unmap_page(
+	struct xfs_buf		*bp)
+{
+	struct page		*page = bp->b_pages[0];
+
+	ASSERT(xfs_buftarg_is_mem(bp->b_target));
+
+	put_page(page);
+
+	bp->b_addr = NULL;
+	bp->b_pages[0] = NULL;
+	bp->b_pages = NULL;
+	bp->b_page_count = 0;
+}