Date:   Fri, 7 Jan 2022 22:51:48 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Pintu Agarwal <pintu.ping@gmail.com>,
        Pintu Kumar <quic_pintu@quicinc.com>,
        open list <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>, ebiederm@xmission.com,
        sfr@canb.auug.org.au, legion@kernel.org, sashal@kernel.org,
        chris.hyser@oracle.com, ccross@google.com, pcc@google.com,
        dave@stgolabs.net, caoxiaofeng@yulong.com, david@redhat.com
Subject: Re: [PATCH] sysinfo: include availram field in sysinfo struct
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On Fri, Jan 07, 2022 at 01:04:51PM +0100, Christian Brauner wrote:
> > 
> > Wait. The userspace may pass old structure here, and in result we
> > return incorrect layout which won't match old one, no? Old binary
> > code has no clue about this header update.
> 
> Yes, that won't work as done.
> 

Yup. When I've been developing struct prctl_mm_map I reserved
PR_SET_MM_MAP_SIZE opcode so userspace would be able to query
current structure size and provide memory slab needed to fit
running structure.

As far as I see we can cut off some space from padding (at the
end of the structure) though one need to make a precise check
that there is no alignment holes appear on different architectures
other than x86. This area is pretty sensitive.