[PATCH] sys_sync_file_range()

Remove the recently-added LINUX_FADV_ASYNC_WRITE and LINUX_FADV_WRITE_WAIT
fadvise() additions, do it in a new sys_sync_file_range() syscall instead.
Reasons:

- It's more flexible.  Things which would require two or three syscalls with
  fadvise() can be done in a single syscall.

- Using fadvise() in this manner is something not covered by POSIX.

The patch wires up the syscall for x86.

The sycall is implemented in the new fs/sync.c.  The intention is that we can
move sys_fsync(), sys_fdatasync() and perhaps sys_sync() into there later.

Documentation for the syscall is in fs/sync.c.

A test app (sync_file_range.c) is in
http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz.

The available-to-GPL-modules do_sync_file_range() is for knfsd: "A COMMIT can
say NFS_DATA_SYNC or NFS_FILE_SYNC.  I can skip the ->fsync call for
NFS_DATA_SYNC which is hopefully the more common."

Note: the `async' writeout mode SYNC_FILE_RANGE_WRITE will turn synchronous if
the queue is congested.  This is trivial to fix: add a new flag bit, set
wbc->nonblocking.  But I'm not sure that we want to expose implementation
details down to that level.

Note: it's notable that we can sync an fd which wasn't opened for writing.
Same with fsync() and fdatasync()).

Note: the code takes some care to handle attempts to sync file contents
outside the 16TB offset on 32-bit machines.  It makes such attempts appear to
succeed, for best 32-bit/64-bit compatibility.  Perhaps it should make such
requests fail...

Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 164 insertions, 0 deletions

diff --git a/fs/sync.c b/fs/sync.c
new file mode 100644
index 000000000000..8616006d2094
--- /dev/null
+++ b/fs/sync.c
@@ -0,0 +1,164 @@
+/*
+ * High-level sync()-related operations
+ */
+
+#include <linux/kernel.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/writeback.h>
+#include <linux/syscalls.h>
+#include <linux/linkage.h>
+#include <linux/pagemap.h>
+
+#define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \
+                        SYNC_FILE_RANGE_WAIT_AFTER)
+
+/*
+ * sys_sync_file_range() permits finely controlled syncing over a segment of
+ * a file in the range offset .. (offset+nbytes-1) inclusive.  If nbytes is
+ * zero then sys_sync_file_range() will operate from offset out to EOF.
+ *
+ * The flag bits are:
+ *
+ * SYNC_FILE_RANGE_WAIT_BEFORE: wait upon writeout of all pages in the range
+ * before performing the write.
+ *
+ * SYNC_FILE_RANGE_WRITE: initiate writeout of all those dirty pages in the
+ * range which are not presently under writeback.
+ *
+ * SYNC_FILE_RANGE_WAIT_AFTER: wait upon writeout of all pages in the range
+ * after performing the write.
+ *
+ * Useful combinations of the flag bits are:
+ *
+ * SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE: ensures that all pages
+ * in the range which were dirty on entry to sys_sync_file_range() are placed
+ * under writeout.  This is a start-write-for-data-integrity operation.
+ *
+ * SYNC_FILE_RANGE_WRITE: start writeout of all dirty pages in the range which
+ * are not presently under writeout.  This is an asynchronous flush-to-disk
+ * operation.  Not suitable for data integrity operations.
+ *
+ * SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER): wait for
+ * completion of writeout of all pages in the range.  This will be used after an
+ * earlier SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE operation to wait
+ * for that operation to complete and to return the result.
+ *
+ * SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER:
+ * a traditional sync() operation.  This is a write-for-data-integrity operation
+ * which will ensure that all pages in the range which were dirty on entry to
+ * sys_sync_file_range() are committed to disk.
+ *
+ *
+ * SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will detect any
+ * I/O errors or ENOSPC conditions and will return those to the caller, after
+ * clearing the EIO and ENOSPC flags in the address_space.
+ *
+ * It should be noted that none of these operations write out the file's
+ * metadata.  So unless the application is strictly performing overwrites of
+ * already-instantiated disk blocks, there are no guarantees here that the data
+ * will be available after a crash.
+ */
+asmlinkage long sys_sync_file_range(int fd, loff_t offset, loff_t nbytes,
+                                        int flags)
+{
+        int ret;
+        struct file *file;
+        loff_t endbyte;                 /* inclusive */
+        int fput_needed;
+        umode_t i_mode;
+
+        ret = -EINVAL;
+        if (flags & ~VALID_FLAGS)
+                goto out;
+
+        endbyte = offset + nbytes;
+
+        if ((s64)offset < 0)
+                goto out;
+        if ((s64)endbyte < 0)
+                goto out;
+        if (endbyte < offset)
+                goto out;
+
+        if (sizeof(pgoff_t) == 4) {
+                if (offset >= (0x100000000ULL << PAGE_CACHE_SHIFT)) {
+                        /*
+                         * The range starts outside a 32 bit machine's
+                         * pagecache addressing capabilities.  Let it "succeed"
+                         */
+                        ret = 0;
+                        goto out;
+                }
+                if (endbyte >= (0x100000000ULL << PAGE_CACHE_SHIFT)) {
+                        /*
+                         * Out to EOF
+                         */
+                        nbytes = 0;
+                }
+        }
+
+        if (nbytes == 0)
+                endbyte = -1;
+        else
+                endbyte--;              /* inclusive */
+
+        ret = -EBADF;
+        file = fget_light(fd, &fput_needed);
+        if (!file)
+                goto out;
+
+        i_mode = file->f_dentry->d_inode->i_mode;
+        ret = -ESPIPE;
+        if (!S_ISREG(i_mode) && !S_ISBLK(i_mode) && !S_ISDIR(i_mode) &&
+                        !S_ISLNK(i_mode))
+                goto out_put;
+
+        ret = do_sync_file_range(file, offset, endbyte, flags);
+out_put:
+        fput_light(file, fput_needed);
+out:
+        return ret;
+}
+
+/*
+ * `endbyte' is inclusive
+ */
+int do_sync_file_range(struct file *file, loff_t offset, loff_t endbyte,
+                        int flags)
+{
+        int ret;
+        struct address_space *mapping;
+
+        mapping = file->f_mapping;
+        if (!mapping) {
+                ret = -EINVAL;
+                goto out;
+        }
+
+        ret = 0;
+        if (flags & SYNC_FILE_RANGE_WAIT_BEFORE) {
+                ret = wait_on_page_writeback_range(mapping,
+                                        offset >> PAGE_CACHE_SHIFT,
+                                        endbyte >> PAGE_CACHE_SHIFT);
+                if (ret < 0)
+                        goto out;
+        }
+
+        if (flags & SYNC_FILE_RANGE_WRITE) {
+                ret = __filemap_fdatawrite_range(mapping, offset, endbyte,
+                                                WB_SYNC_NONE);
+                if (ret < 0)
+                        goto out;
+        }
+
+        if (flags & SYNC_FILE_RANGE_WAIT_AFTER) {
+                ret = wait_on_page_writeback_range(mapping,
+                                        offset >> PAGE_CACHE_SHIFT,
+                                        endbyte >> PAGE_CACHE_SHIFT);
+        }
+out:
+        return ret;
+}
+EXPORT_SYMBOL_GPL(do_sync_file_range);