linux-kernel - Re: [PATCH -next] mm,vmacache: also flush cache for VM

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Date:	Thu, 13 Mar 2014 15:59:41 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Davidlohr Bueso <davi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] mm,vmacache: also flush cache for VM_CLONE

Sorry for delay, I was distracted...

On 03/10, Davidlohr Bueso wrote:
>
> @@ -841,9 +841,6 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)
>  	if (mm->binfmt && !try_module_get(mm->binfmt->module))
>  		goto free_pt;
>
> -	/* initialize the new vmacache entries */
> -	vmacache_flush(tsk);
> -
>  	return mm;
>
>  free_pt:
> @@ -887,6 +884,9 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
>  	if (!oldmm)
>  		return 0;
>
> +	/* initialize the new vmacache entries */
> +	vmacache_flush(tsk);
> +
>  	if (clone_flags & CLONE_VM) {
>  		atomic_inc(&oldmm->mm_users);
>  		mm = oldmm;

Yes. But it seems that use_mm() and unuse_mm() should invalidate vmacache too.

Suppose that a kernel thread T does, say,

	use_mm(foreign_mm);
	get_user(...);
	unuse_mm();

This can trigger a fault and populate T->vmacache[]. If this code is called
again vmacache_find() can use the stale entries.

Or, assuming that only a kernel thread can do use_mm(), we can change
vmacache_valid() to also check !PF_KTHREAD.

Hmm. Another problem is that use_mm() doesn't take ->mmap_sem and thus
it can race with vmacache_flush_all()...

Finally. Shouldn't vmacache_update() check current->mm == mm as well?
What if access_remote_vm/get_user_pages trigger find_vma() ??? Unless
I missed something this is not theoretical at all and can lead to the
corrupted vmacache, no?

Oleg.

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