[PATCH v2] fs/sync.c: sync_file_range(2) may use WB_SYNC_ALL writeback

[Amir Goldstein]

Commit 23d0127096cb ("fs/sync.c: make sync_file_range(2) use WB_SYNC_NONE
writeback") claims that sync_file_range(2) syscall was "created for
userspace to be able to issue background writeout and so waiting for
in-flight IO is undesirable there" and changes the writeback (back) to
WB_SYNC_NONE.

This claim is only partially true. It is true for users that use the flag
SYNC_FILE_RANGE_WRITE by itself, as does PostgreSQL, the user that was
the reason for changing to WB_SYNC_NONE writeback.

However, that claim is not true for users that use that flag combination
SYNC_FILE_RANGE_{WAIT_BEFORE|WRITE|_WAIT_AFTER}.
Those users explicitly requested to wait for in-flight IO as well as to
writeback of dirty pages.

Re-brand that flag combination as SYNC_FILE_RANGE_WRITE_AND_WAIT
and use the helper filemap_write_and_wait_range(), that uses WB_SYNC_ALL
writeback, to perform the full range sync request.

Fixes: 23d0127096cb ("fs/sync.c: make sync_file_range(2) use WB_SYNC_NONE")
Acked-by: Jan Kara <jack@xxxxxxxx>
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/sync.c               | 25 ++++++++++++++++++-------
 include/uapi/linux/fs.h |  3 +++
 2 files changed, 21 insertions(+), 7 deletions(-)

Andrew,

Since you were the one to merge Jan's patch that this Fixes,
I figured it would be best to send the fix through your tree.

You may find the discussion on V1 here:
https://lore.kernel.org/lkml/20190409114922.30095-1-amir73il@xxxxxxxxx/

Thanks,
Amir.

Changes since v1:
- Remove non-guaranties of the API from commit message
- Added ACK by Jan

diff --git a/fs/sync.c b/fs/sync.c
index b54e0541ad89..5cf6fdbae4de 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -18,8 +18,8 @@
 #include <linux/backing-dev.h>
 #include "internal.h"
 
-#define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \
-			SYNC_FILE_RANGE_WAIT_AFTER)
+#define VALID_FLAGS (SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WRITE_AND_WAIT | \
+		     SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WAIT_AFTER)
 
 /*
  * Do the filesystem syncing work. For simple filesystems
@@ -235,9 +235,9 @@ SYSCALL_DEFINE1(fdatasync, unsigned int, fd)
 }
 
 /*
- * sys_sync_file_range() permits finely controlled syncing over a segment of
+ * ksys_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.
+ * zero then ksys_sync_file_range() will operate from offset out to EOF.
  *
  * The flag bits are:
  *
@@ -254,7 +254,7 @@ SYSCALL_DEFINE1(fdatasync, unsigned int, fd)
  * 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
+ * in the range which were dirty on entry to ksys_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
@@ -266,10 +266,13 @@ SYSCALL_DEFINE1(fdatasync, unsigned int, fd)
  * 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:
+ * SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER
+ * (a.k.a. SYNC_FILE_RANGE_WRITE_AND_WAIT):
  * 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.
+ * ksys_sync_file_range() are written to disk.  It should be noted that disk
+ * caches are not flushed by this call, so there are no guarantees here that the
+ * data will be available on disk after a crash.
  *
  *
  * SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will detect any
@@ -338,6 +341,14 @@ int ksys_sync_file_range(int fd, loff_t offset, loff_t nbytes,
 
 	mapping = f.file->f_mapping;
 	ret = 0;
+	if ((flags & SYNC_FILE_RANGE_WRITE_AND_WAIT) ==
+		     SYNC_FILE_RANGE_WRITE_AND_WAIT) {
+		/* Unlike SYNC_FILE_RANGE_WRITE alone uses WB_SYNC_ALL */
+		ret = filemap_write_and_wait_range(mapping, offset, endbyte);
+		if (ret < 0)
+			goto out_put;
+	}
+
 	if (flags & SYNC_FILE_RANGE_WAIT_BEFORE) {
 		ret = file_fdatawait_range(f.file, offset, endbyte);
 		if (ret < 0)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 121e82ce296b..59c71fa8c553 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -320,6 +320,9 @@ struct fscrypt_key {
 #define SYNC_FILE_RANGE_WAIT_BEFORE	1
 #define SYNC_FILE_RANGE_WRITE		2
 #define SYNC_FILE_RANGE_WAIT_AFTER	4
+#define SYNC_FILE_RANGE_WRITE_AND_WAIT	(SYNC_FILE_RANGE_WRITE | \
+					 SYNC_FILE_RANGE_WAIT_BEFORE | \
+					 SYNC_FILE_RANGE_WAIT_AFTER)
 
 /*
  * Flags for preadv2/pwritev2:
-- 
2.17.1