linux-kernel - Re: [Fwd: [-mm] Add an owner to the mm

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Message-ID: <6599ad830804170934i6df9fe42kd979492babe03be@mail.gmail.com>
Date:	Thu, 17 Apr 2008 09:34:50 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Oleg Nesterov" <oleg@...sign.ru>
Cc:	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>, serue@...ibm.com,
	penberg@...helsinki.fi, linux-kernel@...r.kernel.org
Subject: Re: [Fwd: [-mm] Add an owner to the mm_struct (v9)]

On Thu, Apr 17, 2008 at 4:30 AM, Oleg Nesterov <oleg@...sign.ru> wrote:
>  >
>  > I had this loop earlier (inspired from zap_threads()), is this loop more
>  > efficient than what we have?
>
>  All sub-threads have the same ->mm. Once we see that c->mm != mm, we don't
>  need to waste CPU iterating over the all other threads in the thread group.

Technically they don't have to have the same mm, right? You can use
CLONE_THREAD without CLONE_VM when creating a new subthread.

So the complete loop is required for correctness - but it might make
sense to include your version of the loop first, since that will be
faster whenever there are heavily-threaded apps on the system, and
will give the right answer 99.9% of the time (i.e. except when the
user is doing something really weird with clone flags).

>  chance you have the "for dummies" explanation what mm->owner is?
>  I mean, I can't understand how it is possible that 2 CLONE_VM tasks
>  are not equal wrt "ownering".

The idea is to be able to get from an mm to a task, where that task is
representative of the tasks using the mm. Uses include:

- virtual address space cgroup - when we extend an mm, we don't always
have a task pointer available currently.

- swap cgroup - when swapping from an mm, find a task whose swap
cgroup we can charge the swap page to

- revokeat() - apparently needs this for locating tasks based on file mappings

> When the old owner dies, we choose a
>  random thread with the same mm. But we do nothing when the last user
>  of ->mm dies. What is the point? (please feel free to ignore my q

When the last user of the mm dies, the mm is freed, so there's no need
for mm->owner to be valid any longer.

Paul
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