linux-kernel - [RFC] process /proc/PID/smaps vs /proc/PID/smaps

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Message-ID: <20200929020520.GC871730@jagdpanzerIV.localdomain>
Date:   Tue, 29 Sep 2020 11:05:20 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [RFC] process /proc/PID/smaps vs /proc/PID/smaps_rollup

Hello,

One of our unprivileged daemon process needs process PSS info. That
info is usually available in /proc/PID/smaps on per-vma basis, on
in /proc/PID/smaps_rollup as a bunch of accumulated per-vma values.
The latter one is much faster and simpler to get, but, unlike smaps,
smaps_rollup requires PTRACE_MODE_READ, which we don't want to
grant to our unprivileged daemon.

So the question is - can we get, somehow, accumulated PSS info from
a non-privileged process? (Iterating through all process' smaps
vma-s consumes quite a bit of CPU time). This is related to another
question - why do smaps and smaps_rollup have different permission
requirements?

	-ss