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>
---
First two applied.
As for the errors: those are equally as varying as the behaviours,
and a general summary of "some of them" or "all of them at some point"
is equally as meaningless and useless as the main DESCRIPTION was.
If you're unfortunate enough to care about bdflush(2) (no one is),
the same applies for the errors (though, if I'm reading
https://lkml.kernel.org/r/36123b5d-daa0-6c2b-f2d4-a942f069fd54@xxxxxxxxx
right, update(8) du jour didn't care about them either,
so much so it still works with the syscall removed).
man/man2/bdflush.2 | 104 +++++++++++---------------------------------
man/man2/syscalls.2 | 3 +-
2 files changed, 28 insertions(+), 79 deletions(-)
diff --git a/man/man2/bdflush.2 b/man/man2/bdflush.2
index 8627a42..5b5ed16 100644
--- a/man/man2/bdflush.2
+++ b/man/man2/bdflush.2
@@ -12,91 +12,39 @@ .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 );
.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 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
-.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 ().
-.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.
+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
+Sometimes, if
+.I func
+was even,
+.I data
+actually represented a pointer.
+.P
+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 7a7d6d7..56bbb59 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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