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

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Date:   Mon, 12 Sep 2016 21:10:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
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, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps

On Mon 12-09-16 08:01:06, Dave Hansen wrote:
> On 09/12/2016 05:54 AM, Michal Hocko wrote:
> >> > In order to fix this bug, we make 'file->version' indicate the end address
> >> > of current VMA
> > Doesn't this open doors to another weird cases. Say B would be partially
> > unmapped (tail of the VMA would get unmapped and reused for a new VMA.
> 
> In the end, this interface isn't about VMAs.  It's about addresses, and
> we need to make sure that the _addresses_ coming out of it are sane.  In
> the case that a VMA was partially unmapped, it doesn't make sense to
> show the "new" VMA because we already had some output covering the
> address of the "new" VMA from the old one.

OK, that is a fair point and it speaks for caching the vm_end rather
than vm_start+skip.

> > I am not sure we provide any guarantee when there are more read
> > syscalls. Hmm, even with a single read() we can get inconsistent results
> > from different threads without any user space synchronization.
> 
> Yeah, very true.  But, I think we _can_ at least provide the following
> guarantees (among others):
> 1. addresses don't go backwards
> 2. If there is something at a given vaddr during the entirety of the
>    life of the smaps walk, we will produce some output for it.

I guess we also want 
  3. no overlaps with previously printed values (assuming two subsequent
     reads without seek).

the patch tries to achieve the last part as well AFAICS but I guess this
is incomplete because at least /proc/<pid>/smaps will report counters
for the full vma range while the header (aka show_map_vma) will report
shorter (non-overlapping) range. I haven't checked other files which use
m_{start,next}

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.

-- 
Michal Hocko
SUSE Labs