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

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Date:   Thu, 06 Jan 2022 13:20:05 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Pintu Agarwal <pintu.ping@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        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>,
        "christian.brauner\@ubuntu.com" <christian.brauner@...ntu.com>,
        "sfr\@canb.auug.org.au" <sfr@...b.auug.org.au>,
        "legion\@kernel.org" <legion@...nel.org>,
        "sashal\@kernel.org" <sashal@...nel.org>,
        "chris.hyser\@oracle.com" <chris.hyser@...cle.com>,
        "ccross\@google.com" <ccross@...gle.com>,
        "pcc\@google.com" <pcc@...gle.com>,
        "dave\@stgolabs.net" <dave@...olabs.net>,
        "caoxiaofeng\@yulong.com" <caoxiaofeng@...ong.com>,
        "david\@redhat.com" <david@...hat.com>
Subject: Re: [PATCH] sysinfo: include availram field in sysinfo struct

Pintu Agarwal <pintu.ping@...il.com> writes:

> On Thu, 6 Jan 2022 at 23:12, David Laight <David.Laight@...lab.com> wrote:
>>
>> 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.
>>
> oh okay thank you for your feedback. I will move to the end and check again.
> But my doubt is, whether I should move before this
> char _f[20-2*sizeof(__kernel_ulong_t)-sizeof(__u32)];
> or after this ?

Before the padding and you should reduce the size of the padding by the
size of your new field.

> Also, I could not understand what this is for ?
> Do we need to update this since sture is changed ?

In general padding like that is so new fields can be added.  The
comment about libc5 makes me a wonder a bit, but I expect libc5 just
added the padding in it's copy of the structure that it exported to
userspace many many years ago so that new fields could be added.

Eric