linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Date:	Wed, 7 Apr 2010 08:30:52 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com, hannes@...xchg.org
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA



On Wed, 7 Apr 2010, Rik van Riel wrote:
>
>  - fix the locking issues spotted by Kosaki Motohiro

No, they're broken.

And Rik, please explain the locking rather than make even more of these 
kinds of random ad-hoc locking rules.

I've said this now _three_ times, but let me repeat once more:

 - the locking rules for that anon_vma_chain are very unclear. I _think_ 
   you mean for them to be "mmap_sem held for writing, _or_ mmap_sem held 
   for reading and page_table_lock held", but nowhere is that actually 
   documented.

Why is it so hard for you to just admit that? Especially after you 
yourself got it wrong.

> +		merge_vma = find_mergeable_anon_vma(vma);
> +		if (merge_vma) {
> +			int ret;
> +			spin_lock(&mm->page_table_lock);
> +			ret = anon_vma_clone(vma, merge_vma);
> +			if (!ret)
> +				vma->anon_vma = merge_vma->anon_vma;
> +			spin_unlock(&mm->page_table_lock);
> +			return ret;
> +		}

Rik, the above is obviously total crap.

anon_vma_clone() needs to allocate memory, and it does so with GFP_KERNEL. 
You can't do that with a spinlock held.

		Linus
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