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diff --git a/include/linux/netfs.h b/include/linux/netfs.h
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+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Network filesystem support services.
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * See:
+ *
+ * Documentation/filesystems/netfs_library.rst
+ *
+ * for a description of the network filesystem interface declared here.
+ */
+
+#ifndef _LINUX_NETFS_H
+#define _LINUX_NETFS_H
+
+#include <linux/workqueue.h>
+#include <linux/fs.h>
+#include <linux/pagemap.h>
+
+/*
+ * Overload PG_private_2 to give us PG_fscache - this is used to indicate that
+ * a page is currently backed by a local disk cache
+ */
+#define PageFsCache(page) PagePrivate2((page))
+#define SetPageFsCache(page) SetPagePrivate2((page))
+#define ClearPageFsCache(page) ClearPagePrivate2((page))
+#define TestSetPageFsCache(page) TestSetPagePrivate2((page))
+#define TestClearPageFsCache(page) TestClearPagePrivate2((page))
+
+/**
+ * set_page_fscache - Set PG_fscache on a page and take a ref
+ * @page: The page.
+ *
+ * Set the PG_fscache (PG_private_2) flag on a page and take the reference
+ * needed for the VM to handle its lifetime correctly. This sets the flag and
+ * takes the reference unconditionally, so care must be taken not to set the
+ * flag again if it's already set.
+ */
+static inline void set_page_fscache(struct page *page)
+{
+ set_page_private_2(page);
+}
+
+/**
+ * end_page_fscache - Clear PG_fscache and release any waiters
+ * @page: The page
+ *
+ * Clear the PG_fscache (PG_private_2) bit on a page and wake up any sleepers
+ * waiting for this. The page ref held for PG_private_2 being set is released.
+ *
+ * This is, for example, used when a netfs page is being written to a local
+ * disk cache, thereby allowing writes to the cache for the same page to be
+ * serialised.
+ */
+static inline void end_page_fscache(struct page *page)
+{
+ end_page_private_2(page);
+}
+
+/**
+ * wait_on_page_fscache - Wait for PG_fscache to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_fscache (aka PG_private_2) to be cleared on a page.
+ */
+static inline void wait_on_page_fscache(struct page *page)
+{
+ wait_on_page_private_2(page);
+}
+
+/**
+ * wait_on_page_fscache_killable - Wait for PG_fscache to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_fscache (aka PG_private_2) to be cleared on a page or until a
+ * fatal signal is received by the calling task.
+ *
+ * Return:
+ * - 0 if successful.
+ * - -EINTR if a fatal signal was encountered.
+ */
+static inline int wait_on_page_fscache_killable(struct page *page)
+{
+ return wait_on_page_private_2_killable(page);
+}
+
+enum netfs_read_source {
+ NETFS_FILL_WITH_ZEROES,
+ NETFS_DOWNLOAD_FROM_SERVER,
+ NETFS_READ_FROM_CACHE,
+ NETFS_INVALID_READ,
+} __mode(byte);
+
+typedef void (*netfs_io_terminated_t)(void *priv, ssize_t transferred_or_error,
+ bool was_async);
+
+/*
+ * Resources required to do operations on a cache.
+ */
+struct netfs_cache_resources {
+ const struct netfs_cache_ops *ops;
+ void *cache_priv;
+ void *cache_priv2;
+};
+
+/*
+ * Descriptor for a single component subrequest.
+ */
+struct netfs_read_subrequest {
+ struct netfs_read_request *rreq; /* Supervising read request */
+ struct list_head rreq_link; /* Link in rreq->subrequests */
+ loff_t start; /* Where to start the I/O */
+ size_t len; /* Size of the I/O */
+ size_t transferred; /* Amount of data transferred */
+ refcount_t usage;
+ short error; /* 0 or error that occurred */
+ unsigned short debug_index; /* Index in list (for debugging output) */
+ enum netfs_read_source source; /* Where to read from */
+ unsigned long flags;
+#define NETFS_SREQ_WRITE_TO_CACHE 0 /* Set if should write to cache */
+#define NETFS_SREQ_CLEAR_TAIL 1 /* Set if the rest of the read should be cleared */
+#define NETFS_SREQ_SHORT_READ 2 /* Set if there was a short read from the cache */
+#define NETFS_SREQ_SEEK_DATA_READ 3 /* Set if ->read() should SEEK_DATA first */
+#define NETFS_SREQ_NO_PROGRESS 4 /* Set if we didn't manage to read any data */
+};
+
+/*
+ * Descriptor for a read helper request. This is used to make multiple I/O
+ * requests on a variety of sources and then stitch the result together.
+ */
+struct netfs_read_request {
+ struct work_struct work;
+ struct inode *inode; /* The file being accessed */
+ struct address_space *mapping; /* The mapping being accessed */
+ struct netfs_cache_resources cache_resources;
+ struct list_head subrequests; /* Requests to fetch I/O from disk or net */
+ void *netfs_priv; /* Private data for the netfs */
+ unsigned int debug_id;
+ unsigned int cookie_debug_id;
+ atomic_t nr_rd_ops; /* Number of read ops in progress */
+ atomic_t nr_wr_ops; /* Number of write ops in progress */
+ size_t submitted; /* Amount submitted for I/O so far */
+ size_t len; /* Length of the request */
+ short error; /* 0 or error that occurred */
+ loff_t i_size; /* Size of the file */
+ loff_t start; /* Start position */
+ pgoff_t no_unlock_page; /* Don't unlock this page after read */
+ refcount_t usage;
+ unsigned long flags;
+#define NETFS_RREQ_INCOMPLETE_IO 0 /* Some ioreqs terminated short or with error */
+#define NETFS_RREQ_WRITE_TO_CACHE 1 /* Need to write to the cache */
+#define NETFS_RREQ_NO_UNLOCK_PAGE 2 /* Don't unlock no_unlock_page on completion */
+#define NETFS_RREQ_DONT_UNLOCK_PAGES 3 /* Don't unlock the pages on completion */
+#define NETFS_RREQ_FAILED 4 /* The request failed */
+#define NETFS_RREQ_IN_PROGRESS 5 /* Unlocked when the request completes */
+ const struct netfs_read_request_ops *netfs_ops;
+};
+
+/*
+ * Operations the network filesystem can/must provide to the helpers.
+ */
+struct netfs_read_request_ops {
+ bool (*is_cache_enabled)(struct inode *inode);
+ void (*init_rreq)(struct netfs_read_request *rreq, struct file *file);
+ int (*begin_cache_operation)(struct netfs_read_request *rreq);
+ void (*expand_readahead)(struct netfs_read_request *rreq);
+ bool (*clamp_length)(struct netfs_read_subrequest *subreq);
+ void (*issue_op)(struct netfs_read_subrequest *subreq);
+ bool (*is_still_valid)(struct netfs_read_request *rreq);
+ int (*check_write_begin)(struct file *file, loff_t pos, unsigned len,
+ struct page *page, void **_fsdata);
+ void (*done)(struct netfs_read_request *rreq);
+ void (*cleanup)(struct address_space *mapping, void *netfs_priv);
+};
+
+/*
+ * Table of operations for access to a cache. This is obtained by
+ * rreq->ops->begin_cache_operation().
+ */
+struct netfs_cache_ops {
+ /* End an operation */
+ void (*end_operation)(struct netfs_cache_resources *cres);
+
+ /* Read data from the cache */
+ int (*read)(struct netfs_cache_resources *cres,
+ loff_t start_pos,
+ struct iov_iter *iter,
+ bool seek_data,
+ netfs_io_terminated_t term_func,
+ void *term_func_priv);
+
+ /* Write data to the cache */
+ int (*write)(struct netfs_cache_resources *cres,
+ loff_t start_pos,
+ struct iov_iter *iter,
+ netfs_io_terminated_t term_func,
+ void *term_func_priv);
+
+ /* Expand readahead request */
+ void (*expand_readahead)(struct netfs_cache_resources *cres,
+ loff_t *_start, size_t *_len, loff_t i_size);
+
+ /* Prepare a read operation, shortening it to a cached/uncached
+ * boundary as appropriate.
+ */
+ enum netfs_read_source (*prepare_read)(struct netfs_read_subrequest *subreq,
+ loff_t i_size);
+
+ /* Prepare a write operation, working out what part of the write we can
+ * actually do.
+ */
+ int (*prepare_write)(struct netfs_cache_resources *cres,
+ loff_t *_start, size_t *_len, loff_t i_size);
+};
+
+struct readahead_control;
+extern void netfs_readahead(struct readahead_control *,
+ const struct netfs_read_request_ops *,
+ void *);
+extern int netfs_readpage(struct file *,
+ struct page *,
+ const struct netfs_read_request_ops *,
+ void *);
+extern int netfs_write_begin(struct file *, struct address_space *,
+ loff_t, unsigned int, unsigned int, struct page **,
+ void **,
+ const struct netfs_read_request_ops *,
+ void *);
+
+extern void netfs_subreq_terminated(struct netfs_read_subrequest *, ssize_t, bool);
+extern void netfs_stats_show(struct seq_file *);
+
+#endif /* _LINUX_NETFS_H */