linux-kernel - RE: [PATCH] sysinfo: include availram field in sysinfo struct

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Date:   Thu, 6 Jan 2022 17:41:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pintu Agarwal' <pintu.ping@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>
CC:     Pintu Kumar <quic_pintu@...cinc.com>,
        open list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "christian.brauner@...ntu.com" <christian.brauner@...ntu.com>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "legion@...nel.org" <legion@...nel.org>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "chris.hyser@...cle.com" <chris.hyser@...cle.com>,
        "ccross@...gle.com" <ccross@...gle.com>,
        "pcc@...gle.com" <pcc@...gle.com>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "caoxiaofeng@...ong.com" <caoxiaofeng@...ong.com>,
        "david@...hat.com" <david@...hat.com>
Subject: RE: [PATCH] sysinfo: include availram field in sysinfo struct

From: Pintu Agarwal
> Sent: 06 January 2022 16:50
> 
> On Thu, 6 Jan 2022 at 21:41, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> >
> > On Thu, Jan 06, 2022 at 09:04:10PM +0530, Pintu Kumar wrote:
> > > The sysinfo member does not have any "available ram" field and
> > > the bufferram field is not much helpful either, to get a rough
> > > estimate of available ram needed for allocation.
> > >
> > > One needs to parse MemAvailable field separately from /proc/meminfo
> > > to get this info instead of directly getting if from sysinfo itself.
> > >
> > > Thus, this patch introduce a new field as availram in sysinfo
> > > so that all the info total/free/available can be retrieved from
> > > one place itself.
> > >
> > > There are couple of places in kernel as well where this can be improved.
> > > For example:
> > > In fs/proc/meminfo.c:
> > > meminfo_proc_show:
> > >    si_meminfo(&i);
> > >    available = si_mem_available();
> > > Now with this change the second call be avoided.
> > > Thus, we can directly do:
> > > show_val_kb(m, "MemAvailable:   ", i.availram);
> > >
> > > Note, this also requires update in procfs for free and other commands.
> > > Like in free command as well we frist call sysinfo then again parse
> > > /proc/meminfo to get available field.
> > > This can be avoided too with higher kernel version.
> > >
> > > A sample output with single sysinfo call is shown below:
> > > Total RAM: 248376 kB
> > >  Free RAM: 231540 kB
> > > Avail RAM: 230448 kB
> > >
> > > Signed-off-by: Pintu Kumar <quic_pintu@...cinc.com>
> > > Signed-off-by: Pintu Agarwal <pintu.ping@...il.com>
> > > ---
> > >  include/uapi/linux/sysinfo.h | 1 +
> > >  kernel/sys.c                 | 4 ++++
> > >  mm/page_alloc.c              | 2 ++
> > >  3 files changed, 7 insertions(+)
> > >
> > > diff --git a/include/uapi/linux/sysinfo.h b/include/uapi/linux/sysinfo.h
> > > index 435d5c2..6e77e90 100644
> > > --- a/include/uapi/linux/sysinfo.h
> > > +++ b/include/uapi/linux/sysinfo.h
> > > @@ -12,6 +12,7 @@ struct sysinfo {
> > >       __kernel_ulong_t freeram;       /* Available memory size */
> > >       __kernel_ulong_t sharedram;     /* Amount of shared memory */
> > >       __kernel_ulong_t bufferram;     /* Memory used by buffers */
> > > +     __kernel_ulong_t availram;      /* Memory available for allocation */
> > >       __kernel_ulong_t totalswap;     /* Total swap space size */
> > >       __kernel_ulong_t freeswap;      /* swap space still available */
> > >       __u16 procs;                    /* Number of current processes */
> >
> > Hi! Sorry, but I don't understand -- the sysinfo structure seems to
> > be part of user API, no? Don't we break it up here?
> 
> Yes, the corresponding user space header /usr/include/linux/sysinfo.h
> also needs to be updated.
> When we generate the kernel header it will be updated automatically.

You can't add a field in the middle of a UAPI structure.
It breaks compatibility for old binaries.

Depending on the interface definition you may be able to add one at the end.

	David

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