linux/fs/xfs/xfs_buf_mem.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright (c) 2023-2024 Oracle.  All Rights Reserved.
 * Author: Darrick J. Wong <[email protected]>
 */
#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>
#include "xfs_log_format.h"
#include "xfs_trans.h"
#include "xfs_buf_item.h"
#include "xfs_error.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)
{ … }

/* Free a buffer cache target for a memory-backed buffer cache. */
void
xmbuf_free(
	struct xfs_buftarg	*btp)
{ … }

/* Directly map a shmem page into the buffer cache. */
int
xmbuf_map_page(
	struct xfs_buf		*bp)
{ … }

/* Unmap a shmem page that was mapped into the buffer cache. */
void
xmbuf_unmap_page(
	struct xfs_buf		*bp)
{ … }

/* Is this a valid daddr within the buftarg? */
bool
xmbuf_verify_daddr(
	struct xfs_buftarg	*btp,
	xfs_daddr_t		daddr)
{ … }

/* Discard the page backing this buffer. */
static void
xmbuf_stale(
	struct xfs_buf		*bp)
{ … }

/*
 * Finalize a buffer -- discard the backing page if it's stale, or run the
 * write verifier to detect problems.
 */
int
xmbuf_finalize(
	struct xfs_buf		*bp)
{ … }

/*
 * Detach this xmbuf buffer from the transaction by any means necessary.
 * All buffers are direct-mapped, so they do not need bwrite.
 */
void
xmbuf_trans_bdetach(
	struct xfs_trans	*tp,
	struct xfs_buf		*bp)
{ … }