[PATCH] prctl.2: doc PR_SET/GET_IO_FLUSHER - V4

[Mike Christie <mchristi@redhat.com>]

This patch documents the PR_SET_IO_FLUSHER and PR_GET_IO_FLUSHER
prctl commands added to the linux kernel for 5.6 in commit:

commit 8d19f1c8e1937baf74e1962aae9f90fa3aeab463
Author: Mike Christie <mchristi@redhat.com>
Date:   Mon Nov 11 18:19:00 2019 -0600

    prctl: PR_{G,S}ET_IO_FLUSHER to support controlling memory reclaim

Signed-off-by: Mike Christie <mchristi@redhat.com>
---

V4:
- Remove internal details.
- Add CAP_SYS_RESOURCE requirement info.

V3:
- Replace emulation device example.

V2:
- My initial patch for this was very bad. This version is almost 100%
taken word for word from Dave Chinner's review comments.

Signed-off-by: Mike Christie <mchristi@redhat.com>

 man2/prctl.2 | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/man2/prctl.2 b/man2/prctl.2
index 720ec04e4..3c3355439 100644
--- a/man2/prctl.2
+++ b/man2/prctl.2
@@ -1381,6 +1381,31 @@ system call on Tru64).
 for information on versions and architectures.)
 Return unaligned access control bits, in the location pointed to by
 .IR "(unsigned int\ *) arg2" .
+.TP
+.B PR_SET_IO_FLUSHER (Since Linux 5.6)
+If a user process is involved in the block layer or file system IO path,
+and can allocate memory while processing IO requests it must set
+\fIarg2\fP to 1. This will put the process in the IO_FLUSHER state which
+allows it special treatment to make progress when allocating memory.
+If \fIarg2\fP is 0, the process will clear the IO_FLUSHER state, and
+the default behavior will be used.
+
+The calling process must have the
+.BR CAP_SYS_RESOURCE
+capability.
+
+Examples of IO_FLUSHER applications are FUSE daemons, SCSI device
+emulation daemons, and daemons that perform error handling like multipath
+path recovery applications.
+
+.TP
+.B PR_GET_IO_FLUSHER (Since Linux 5.6)
+Return as the function result 1 if the caller is in the IO_FLUSHER state and
+0 if not.
+
+The calling process must have the
+.BR CAP_SYS_RESOURCE
+capability.
 .SH RETURN VALUE
 On success,
 .BR PR_GET_DUMPABLE ,
@@ -1395,6 +1420,7 @@ On success,
 .BR PR_GET_SPECULATION_CTRL ,
 .BR PR_MCE_KILL_GET ,
 .BR PR_CAP_AMBIENT + PR_CAP_AMBIENT_IS_SET ,
+.BR PR_GET_IO_FLUSHER ,
 and (if it returns)
 .BR PR_GET_SECCOMP
 return the nonnegative values described above.
-- 
2.21.0