linux-kernel - Re: [PATCH 1/9] exec_path 1/9: introduce ->exec

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Date:	Fri, 5 Jun 2009 20:09:43 +0400
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Helsley <matthltc@...ibm.com>, xemul@...allels.com,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, dave@...ux.vnet.ibm.com,
	mingo@...e.hu, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/9] exec_path 1/9: introduce ->exec_path and switch
	/proc/*/exe

On Fri, Jun 05, 2009 at 08:49:56AM -0700, Linus Torvalds wrote:
> 
> 
> On Fri, 5 Jun 2009, Alexey Dobriyan wrote:
> > 
> > This mapping issue is what created VM_EXECUTABLE/MAP_EXECUTABLE in
> > the first place, I assume? Never mind it's cheap hack.
> > 
> > > and goes along with task->mm, and should be there, not in task_struct.
> > 
> > Because ->mm can be borrowed to unrelated task and user has to check
> > for it.
> 
> Not it can't.
> 
> You're confusing ->mm with ->active_mm.
> 
> The latter can be borrowed. The former can not.

Not permanently, but it can:

	static void aio_kick_handler(struct work_struct *work)
	{
	        struct kioctx *ctx = container_of(work, struct kioctx, wq.work);
	        mm_segment_t oldfs = get_fs();
	        struct mm_struct *mm;
	        int requeue;
	
	        set_fs(USER_DS);
	        use_mm(ctx->mm);
	        spin_lock_irq(&ctx->ctx_lock);
	        requeue =__aio_run_iocbs(ctx);
	        mm = ctx->mm;
	        spin_unlock_irq(&ctx->ctx_lock);
	        unuse_mm(mm);
	        set_fs(oldfs);
	        /*
	         * we're in a worker thread already, don't use
	         * queue_delayed_work,
	         */
	        if (requeue)
	                queue_delayed_work(aio_wq, &ctx->wq, 0);
	}

It's borrowed by kernel thread of course, not userspace task.

> The fact is, the executable is _inherently_ tied to the mm. It's what it 
> is mapped into. It makes no sense to tie it to anything else. It's simply 
> fundamentally not a "per-thread" thing.
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