linux-kernel - Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps

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Message-ID: <57D8277E.80505@intel.com>
Date:   Tue, 13 Sep 2016 09:21:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Oleg Nesterov <oleg@...hat.com>, Michal Hocko <mhocko@...nel.org>
Cc:     Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        pbonzini@...hat.com, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, gleb@...nel.org, mtosatti@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        stefanha@...hat.com, yuhuang@...hat.com, linux-mm@...ck.org,
        ross.zwisler@...ux.intel.com
Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps

On 09/13/2016 07:59 AM, Oleg Nesterov wrote:
> On 09/12, Michal Hocko wrote:
>> > Considering how this all can be tricky and how partial reads can be
>> > confusing and even misleading I am really wondering whether we
>> > should simply document that only full reads will provide a sensible
>> > results.
> I agree. I don't even understand why this was considered as a bug.
> Obviously, m_stop() which drops mmap_sep should not be called, or
> all the threads should be stopped, if you want to trust the result.

There was a mapping at a given address.  That mapping did not change, it
was not split, its attributes did not change.  But, it didn't show up
when reading smaps.  Folks _actually_ noticed this in a test suite
looking for that address range in smaps.

IOW, we had goofy kernel behavior, and it broke a reasonable test
program.  The test program just used fgets() to read into a fixed-length
buffer, which is a completely normal thing to do.

To get "sensible results", doesn't userspace have to somehow know in
advance how many bytes of data a given VMA will generate in smaps output?