linux-kernel - Re: [RFC patch] vm: clear swap entry before copying pte

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Message-ID: <alpine.LSU.2.00.1207301907010.3953@eggly.anvils>
Date:	Mon, 30 Jul 2012 19:34:10 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [RFC patch] vm: clear swap entry before copying pte

On Fri, 27 Jul 2012, Hillf Danton wrote:
> 
> If swap entry is cleared, we can see the reason that copying pte is
> interrupted. If due to page table lock held long enough, no need to
> increase swap count.

I can't see a bug to be fixed here.

How would it break out of the loop above without freshly setting entry
(given that mmap_sem is held with down_write, so the entries cannot be
munmap'ped by another thread)?  How would it matter if it could (given
that add_swap_count_continuation already allows for races; and if there
were a problem, the call just made could be equally at fault)?

Nor do I understand your description.

But I can see that the lack of reinitialization of entry.val here
does raise doubt and confusion.  A better tidyup would be to remove
the initialization of swp_entry_t entry from its onstack declaration,
and do it at the again label instead.

If you send a patch to do that instead, I could probably ack it -
but expect I shall want to change your description.

Hugh

> 
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
> 
> --- a/mm/memory.c	Fri Jul 27 21:33:32 2012
> +++ b/mm/memory.c	Fri Jul 27 21:35:24 2012
> @@ -971,6 +971,7 @@ again:
>  		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
>  			return -ENOMEM;
>  		progress = 0;
> +		entry.val = 0;
>  	}
>  	if (addr != end)
>  		goto again;
> --
--
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