Re: [PATCH 1/1] prctl.2: doc PR_SET/GET_IO_FLUSHER

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 [off-list ref]
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 <redacted>
---

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


 man2/prctl.2 | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/man2/prctl.2 b/man2/prctl.2
index 720ec04e4..b481d186b 100644
--- a/man2/prctl.2
+++ b/man2/prctl.2

@@ -1381,6 +1381,30 @@ 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)
+An IO_FLUSHER is a user process that the kernel uses to issue IO
+that cleans dirty page cache data and/or filesystem metadata. The
+kernel may need to clean this memory when under memory pressure in
+order to free it. This means there is potential for a memory reclaim
+recursion deadlock if the user process attempts to allocate memory
+and the kernel then blocks waiting for it to clean memory before it
+can make reclaim progress.
+
+The kernel avoids these recursion problems internally via a special
+process state that prevents recursive reclaim from issuing new IO.
+If \fIarg2\fP is 1, the \fPPR_SET_IO_FLUSHER\fP control allows a userspace
+process to set up this same process state and hence avoid the memory
+reclaim recursion deadlocks in the same manner the kernel avoids them.
+If \fIarg2\fP is 0, the process will clear the IO_FLUSHER state, and the
+default behavior will be used.
+
+Examples of IO_FLUSHER applications are FUSE daemons, zoned disk
+emulation daemons, etc."

+.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.
 .SH RETURN VALUE
 On success,
 .BR PR_GET_DUMPABLE ,

@@ -1395,6 +1419,7 @@ 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.