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

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Mike,

On 4/2/20 4:08 AM, Mike Christie wrote:
> 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>

I've applied this patch, but I have some questions:

(a) What happens to the IO_FLUSHER setting in the child of fork(2)?
(b) What happens to the IO_FLUSHER setting during an execve(2)?
    (Is it preserved, reset, something else?)

Thanks,

Michael

> ---
> 
> 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.
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/