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>
---
Andrew,
Oops found a braino in my patch. Here is v3.
You may find the discussion on V1 here:
https://lore.kernel.org/lkml/20190409114922.30095-1-amir73il@xxxxxxxxx/
Thanks,
Amir.
Changes since v2:
- Return after filemap_write_and_wait_range()
Changes since v1:
- Remove non-guaranties of the API from commit message
- Added ACK by Jan
fs/sync.c | 24 +++++++++++++++++-------
include/uapi/linux/fs.h | 3 +++
2 files changed, 20 insertions(+), 7 deletions(-)
diff --git a/fs/sync.c b/fs/sync.c
index b54e0541ad89..3a923652d720 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 +341,13 @@ 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);
+ 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
