linux-kernel - Re: Multiple potential races on vma->vm

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Date:	Wed, 23 Sep 2015 17:38:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: Multiple potential races on vma->vm_flags

On 09/23, Oleg Nesterov wrote:
>
> On 09/11, Kirill A. Shutemov wrote:
> >
> > This one is tricky. I *assume* the mm cannot be generally accessible after
> > mm_users drops to zero, but I'm not entirely sure about it.
> > procfs? ptrace?
>
> Well, all I can say is that proc/ptrace look fine afaics...
>
> This is off-topic, but how about the patch below? Different threads can
> expand different vma's at the same time under read_lock(mmap_sem), so
> vma_lock_anon_vma() can't help to serialize "locked_vm += grow".

perhaps vm_stat_account() should be moved too, but total_vm/etc is less
important.

Or I missed something?

Oleg.

> --- x/mm/mmap.c
> +++ x/mm/mmap.c
> @@ -2146,9 +2146,6 @@ static int acct_stack_growth(struct vm_a
>  	if (security_vm_enough_memory_mm(mm, grow))
>  		return -ENOMEM;
>  
> -	/* Ok, everything looks good - let it rip */
> -	if (vma->vm_flags & VM_LOCKED)
> -		mm->locked_vm += grow;
>  	vm_stat_account(mm, vma->vm_flags, vma->vm_file, grow);
>  	return 0;
>  }
> @@ -2210,6 +2207,8 @@ int expand_upwards(struct vm_area_struct
>  				 * against concurrent vma expansions.
>  				 */
>  				spin_lock(&vma->vm_mm->page_table_lock);
> +				if (vma->vm_flags & VM_LOCKED)
> +					mm->locked_vm += grow;
>  				anon_vma_interval_tree_pre_update_vma(vma);
>  				vma->vm_end = address;
>  				anon_vma_interval_tree_post_update_vma(vma);
> @@ -2281,6 +2280,8 @@ int expand_downwards(struct vm_area_stru
>  				 * against concurrent vma expansions.
>  				 */
>  				spin_lock(&vma->vm_mm->page_table_lock);
> +				if (vma->vm_flags & VM_LOCKED)
> +					mm->locked_vm += grow;
>  				anon_vma_interval_tree_pre_update_vma(vma);
>  				vma->vm_start = address;
>  				vma->vm_pgoff -= grow;

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