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

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Message-ID: <a244d7f9-762e-6f26-a537-0524765c6815@linux.intel.com>
Date:   Tue, 13 Sep 2016 11:01:09 +0800
From:   Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     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 09/13/2016 03:10 AM, Michal Hocko wrote:
> 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}

You are right. Will fix both /proc/PID/smaps and /proc/PID/maps in
the next version.

>
> 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.

Make sense. Will document the guarantee in
Documentation/filesystems/proc.txt

Thank you, Dave and Michal, for figuring out the right direction. :)