All of lore.kernel.org
 help / color / mirror / Atom feed
From: Dave Hansen <dave.hansen@intel.com>
To: Michal Hocko <mhocko@kernel.org>,
	Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: pbonzini@redhat.com, akpm@linux-foundation.org,
	dan.j.williams@intel.com, gleb@kernel.org, mtosatti@redhat.com,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	stefanha@redhat.com, yuhuang@redhat.com, linux-mm@kvack.org,
	ross.zwisler@linux.intel.com, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps
Date: Mon, 12 Sep 2016 08:01:06 -0700	[thread overview]
Message-ID: <57D6C332.4000409@intel.com> (raw)
In-Reply-To: <20160912125447.GM14524@dhcp22.suse.cz>

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.

> 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.

> So in other words isn't this fixing a bug by introducing a slightly
> different one while we are not really guaranteeing anything strong here?

Well, the (original) bug here _is_ pretty crummy.  It's not printing a
VMA, and that VMA was never touched.  It's just collateral damage from
the previous guy who got destroyed.

WARNING: multiple messages have this Message-ID (diff)
From: Dave Hansen <dave.hansen@intel.com>
To: Michal Hocko <mhocko@kernel.org>,
	Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: pbonzini@redhat.com, akpm@linux-foundation.org,
	dan.j.williams@intel.com, gleb@kernel.org, mtosatti@redhat.com,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	stefanha@redhat.com, yuhuang@redhat.com, linux-mm@kvack.org,
	ross.zwisler@linux.intel.com, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps
Date: Mon, 12 Sep 2016 08:01:06 -0700	[thread overview]
Message-ID: <57D6C332.4000409@intel.com> (raw)
In-Reply-To: <20160912125447.GM14524@dhcp22.suse.cz>

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.

> 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.

> So in other words isn't this fixing a bug by introducing a slightly
> different one while we are not really guaranteeing anything strong here?

Well, the (original) bug here _is_ pretty crummy.  It's not printing a
VMA, and that VMA was never touched.  It's just collateral damage from
the previous guy who got destroyed.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

  reply	other threads:[~2016-09-12 15:02 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-12  3:12 [PATCH v2] mm, proc: Fix region lost in /proc/self/smaps Xiao Guangrong
2016-09-12  3:12 ` Xiao Guangrong
2016-09-12 12:54 ` Michal Hocko
2016-09-12 12:54   ` Michal Hocko
2016-09-12 15:01   ` Dave Hansen [this message]
2016-09-12 15:01     ` Dave Hansen
2016-09-12 19:10     ` Michal Hocko
2016-09-12 19:10       ` Michal Hocko
2016-09-13  3:01       ` Xiao Guangrong
2016-09-13  3:01         ` Xiao Guangrong
2016-09-13 14:59       ` Oleg Nesterov
2016-09-13 14:59         ` Oleg Nesterov
2016-09-13 16:21         ` Dave Hansen
2016-09-13 16:21           ` Dave Hansen
2016-09-14 15:38           ` Oleg Nesterov
2016-09-14 15:38             ` Oleg Nesterov
2016-09-19  7:21             ` Xiao Guangrong
2016-09-19  7:21               ` Xiao Guangrong

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=57D6C332.4000409@intel.com \
    --to=dave.hansen@intel.com \
    --cc=akpm@linux-foundation.org \
    --cc=dan.j.williams@intel.com \
    --cc=gleb@kernel.org \
    --cc=guangrong.xiao@linux.intel.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@kernel.org \
    --cc=mtosatti@redhat.com \
    --cc=oleg@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=ross.zwisler@linux.intel.com \
    --cc=stefanha@redhat.com \
    --cc=yuhuang@redhat.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.