There's no point documenting this syscall at any point in time,
because it changed constantly. A post-mortem summary I believe to be
comprehensive is included below.
The #include <sys/kdaemon.h> was removed in glibc 2.23:
commit eed3e1eb79bcfa9b52609fd875fa2d522e2d6bce
Author: Joseph Myers <joseph@xxxxxxxxxxxxxxxx>
Date: Mon Dec 14 22:52:15 2015 +0000
Make obsolete syscall wrappers into compat symbols (bug 18472).
* bdflush: in Linux 2.6, does nothing if present.
[...]
generated for such aliases. Those five syscalls are then made into
compat symbols (obsoleted in glibc 2.23, so future ports won't have
these symbols at all), with the header <sys/kdaemon.h> declaring
bdflush being removed. When we move to 3.2 as minimum kernel version,
the same can be done for nfsservctl (removed in Linux 3.1) as well.
[...]
Appears in 1.1.3
(func=0 turns the calling process into the bdflush daemon,
func=1 return sync_old_buffers();,
func>=2 is parameters):
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
Import 1.1.3
+/* This is the interface to bdflush. As we get more sophisticated, we can
+ * pass tuning parameters to this "process", to adjust how it behaves. If you
+ * invoke this again after you have done this once, you would simply modify
+ * the tuning parameters. We would want to verify each parameter, however,
+ * to make sure that it is reasonable. */
+
+asmlinkage int sys_bdflush(int func, int data)
+{
bdflush() is just a kernel thread, and func 0 is just return 0; since 1.3.50:
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
Import 1.3.50
- /* Basically func 0 means start, 1 means read param 1, 2 means write param 1, etc */
+ /* Basically func 1 means read param 1, 2 means write param 1, etc */
if (func >= 2) {
i = (func-2) >> 1;
if (i < 0 || i >= N_PARAM)
@@ -1930,13 +1845,32 @@ asmlinkage int sys_bdflush(int func, long data)
bdf_prm.data[i] = data;
return 0;
};
+
+ /* Having func 0 used to launch the actual bdflush and then never
+ return (unless explicitly killed). We return zero here to
+ remain semi-compatible with present update(8) programs. */
+
+ return 0;
+}
+
+/* This is the actual bdflush daemon itself. It used to be started from
+ * the syscall above, but now we launch it ourselves internally with
+ * kernel_thread(...) directly after the first thread in init/main.c */
+
+int bdflush(void * unused) {
func 1 is actually exit(0) since 2.3.23pre1:
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
Import 2.3.23pre1
if (func == 1) {
+ /* do_exit directly and let kupdate to do its work alone. */
+ do_exit(0);
+#if 0 /* left here as it's the only example of lazy-mm-stuff used from
+ a syscall that doesn't care about the current mm context. */
fund!=0 is a return 0 since 2.5.12:
Author: Andrew Morton <akpm@xxxxxxxxxx>
Date: Mon Apr 29 23:52:10 2002 -0700
[PATCH] writeback from address spaces
[ I reversed the order in which writeback walks the superblock's
dirty inodes. It sped up dbench's unlink phase greatly. I'm
such a sleaze ]
The core writeback patch. Switches file writeback from the dirty
buffer LRU over to address_space.dirty_pages.
- The buffer LRU is removed
- The buffer hash is removed (uses blockdev pagecache lookups)
- The bdflush and kupdate functions are implemented against
address_spaces, via pdflush.
[...]
Deprecated since 2.5.52:
Author: Andrew Morton <akpm@xxxxxxxxx>
Date: Sat Dec 14 03:16:29 2002 -0800
[PATCH] deprecate use of bdflush()
Patch from Robert Love <rml@xxxxxxxxx>
We can never get rid of it if we do not deprecate it - so do so and
print a stern warning to those who still run bdflush daemons.
Removed outright in 5.15-rc1:
commit b48c7236b13cb5ef1b5fdf744aa8841df0f7b43a
Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Date: Tue Jun 29 15:11:44 2021 -0500
exit/bdflush: Remove the deprecated bdflush system call
The bdflush system call has been deprecated for a very long time.
Recently Michael Schmitz tested[1] and found that the last known
caller of of the bdflush system call is unaffected by it's removal.
Since the code is not needed delete it.
[1] https://lkml.kernel.org/r/36123b5d-daa0-6c2b-f2d4-a942f069fd54@xxxxxxxxx
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx>
---
man/man2/bdflush.2 | 97 ++++++++++-----------------------------------
man/man2/syscalls.2 | 3 +-
2 files changed, 22 insertions(+), 78 deletions(-)
diff --git a/man/man2/bdflush.2 b/man/man2/bdflush.2
index 8627a42a2..c4c6400ff 100644
--- a/man/man2/bdflush.2
+++ b/man/man2/bdflush.2
@@ -12,91 +12,34 @@ .SH SYNOPSIS
.nf
.B #include <sys/kdaemon.h>
.P
-.BI "[[deprecated]] int bdflush(int " func ", long *" address );
-.BI "[[deprecated]] int bdflush(int " func ", long " data );
+.BI "int bdflush(int " func ", long " data );
+.RI "// if " func " is even, " data " actually represents a pointer)"
.fi
.SH DESCRIPTION
-.IR Note :
-Since Linux 2.6,
-.\" As noted in changes in the 2.5.12 source
-this system call is deprecated and does nothing.
-It is likely to disappear altogether in a future kernel release.
-Nowadays, the task performed by
-.BR bdflush ()
-is handled by the kernel
-.I pdflush
-thread.
+This system call was introduced in Linux 1.1.3,
+became effectively obsolete in Linux 1.3.50,
+mostly useless in Linux 2.3.23,
+entirely useless in Linux 2.5.12,
+officially deprecated in Linux 2.5.52,
+and removed outright in Linux 5.15.
.P
-.BR bdflush ()
-starts, flushes, or tunes the buffer-dirty-flush daemon.
-Only a privileged process (one with the
-.B CAP_SYS_ADMIN
-capability) may call
-.BR bdflush ().
+It used to turn the calling process into the
+.I bdflush
+daemon,
+or tune it,
+or flush the "old buffers".
+It then progressively lost all of that functionality.
.P
-If
-.I func
-is negative or 0, and no daemon has been started, then
-.BR bdflush ()
-enters the daemon code and never returns.
-.P
-If
-.I func
-is 1,
-some dirty buffers are written to disk.
-.P
-If
-.I func
-is 2 or more and is even (low bit is 0), then
-.I address
-is the address of a long word,
-and the tuning parameter numbered
-.RI "(" "func" "\-2)/2"
-is returned to the caller in that address.
-.P
-If
-.I func
-is 3 or more and is odd (low bit is 1), then
-.I data
-is a long word,
-and the kernel sets tuning parameter numbered
-.RI "(" "func" "\-3)/2"
-to that value.
-.P
-The set of parameters, their values, and their valid ranges
-are defined in the Linux kernel source file
-.IR fs/buffer.c .
-.SH RETURN VALUE
-If
-.I func
-is negative or 0 and the daemon successfully starts,
-.BR bdflush ()
-never returns.
-Otherwise, the return value is 0 on success and \-1 on failure, with
-.I errno
-set to indicate the error.
+See
+.I fs/buffer.c
+in the kernel version you're interested in to see what it actually does there.
.SH ERRORS
-.TP
-.B EBUSY
-An attempt was made to enter the daemon code after
-another process has already entered.
-.TP
-.B EFAULT
-.I address
-points outside your accessible address space.
-.TP
-.B EINVAL
-An attempt was made to read or write an invalid parameter number,
-or to write an invalid value to a parameter.
-.TP
-.B EPERM
-Caller does not have the
-.B CAP_SYS_ADMIN
-capability.
+.B ENOSYS
+(this system call is unimplemented)
.SH STANDARDS
Linux.
.SH HISTORY
-Since glibc 2.23, glibc no longer supports this obsolete system call.
+The header and prototype were removed in glibc 2.23.
.SH SEE ALSO
.BR sync (1),
.BR fsync (2),
diff --git a/man/man2/syscalls.2 b/man/man2/syscalls.2
index 795a3f7be..325ecf5ac 100644
--- a/man/man2/syscalls.2
+++ b/man/man2/syscalls.2
@@ -170,7 +170,8 @@ .SS System call list
\fBatomic_cmpxchg_32\fP(2) 2.6.34 m68k only
\fBbdflush\fP(2) 1.2 T{
Deprecated (does nothing)
-since 2.6
+since 2.6,
+removed in 5.15
T}
\fBbind\fP(2) 2.0 T{
See notes on \fBsocketcall\fP(2)
--
2.39.2
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