linux-kernel - Re: [PATCH] [for mmotm-1113] mm: Simplify try_to_unmap

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Date:	Wed, 18 Nov 2009 15:18:03 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH]  [for mmotm-1113] mm: Simplify try_to_unmap_one()

On Tue, 17 Nov 2009 17:39:27 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> +out_mlock:
> +	pte_unmap_unlock(pte, ptl);
> +
> +	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> +		if (vma->vm_flags & VM_LOCKED) {
> +			mlock_vma_page(page);
> +			ret = SWAP_MLOCK;
>  		}
> +		up_read(&vma->vm_mm->mmap_sem);

It's somewhat unobvious why we're using a trylock here.  Ranking versus
lock_page(), perhaps?

In general I think a trylock should have an associated comment which explains

a) why it is being used at this site and

b) what happens when the trylock fails - why this isn't a
   bug, how the kernel recovers from the inconsistency, what its
   overall effect is, etc.

<wonders why we need to take mmap_sem here at all>
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