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

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

Mike,

Do you have a refresh of this patch, in the light of the comments?

Thanks,

Michael

On Fri, 14 Feb 2020 at 13:54, Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 13-02-20 12:23:36, 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>
> > Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
> > ---
> >
> > 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 | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> >
> > diff --git a/man2/prctl.2 b/man2/prctl.2
> > index 720ec04e4..58d77bf2e 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.
>
> I would refrain from describing the internal implementation. The
> important part is that this flag tells the kernel that IO_FLUSHER
> process gets a special treatment to workaround the deadlock.
>
> So anytime a process is involved in the IO path and the kernel cannot
> make a forward progress without it then the flag should be set.
>
> --
> Michal Hocko
> SUSE Labs



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