linux-kernel - Re: [PATCH] frontswap: enable call to invalidate area on swapoff

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 10 Oct 2013 09:29:07 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Seth Jennings <spartacus06@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-mm@...ck.org, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, Shaohua Li <shli@...ionio.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] frontswap: enable call to invalidate area on swapoff


On 10/09/2013 10:40 PM, Seth Jennings wrote:
> On Tue, Oct 08, 2013 at 01:08:53PM -0700, Andrew Morton wrote:
>> On Tue, 08 Oct 2013 10:13:20 +0200 Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
>>
>>> On pon, 2013-10-07 at 15:03 -0700, Andrew Morton wrote:
>>>> On Mon, 07 Oct 2013 17:25:41 +0200 Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
>>>>
>>>>> During swapoff the frontswap_map was NULL-ified before calling
>>>>> frontswap_invalidate_area(). However the frontswap_invalidate_area()
>>>>> exits early if frontswap_map is NULL. Invalidate was never called during
>>>>> swapoff.
>>>>>
>>>>> This patch moves frontswap_map_set() in swapoff just after calling
>>>>> frontswap_invalidate_area() so outside of locks
>>>>> (swap_lock and swap_info_struct->lock). This shouldn't be a problem as
>>>>> during swapon the frontswap_map_set() is called also outside of any
>>>>> locks.
>>>>>
>>>>
>>>> Ahem.  So there's a bunch of code in __frontswap_invalidate_area()
>>>> which hasn't ever been executed and nobody noticed it.  So perhaps that
>>>> code isn't actually needed?
>>>>
>>>> More seriously, this patch looks like it enables code which hasn't been
>>>> used or tested before.  How well tested was this?
>>>>
>>>> Are there any runtime-visible effects from this change?
>>>
>>> I tested zswap on x86 and x86-64 and there was no difference. This is
>>> good as there shouldn't be visible anything because swapoff is unusing
>>> all pages anyway:
>>> 	try_to_unuse(type, false, 0); /* force all pages to be unused */
>>>
>>> I haven't tested other frontswap users.
>>
>> So is that code in __frontswap_invalidate_area() unneeded?
> 
> Yes, to expand on what Bob said, __frontswap_invalidate_area() is still
> needed to let any frontswap backend free per-swaptype resources.
> 
> __frontswap_invalidate_area() is _not_ for freeing structures associated
> with individual swapped out pages since all of the pages should be
> brought back into memory by try_to_unuse() before
> __frontswap_invalidate_area() is called.
> 
> The reason we never noticed this for zswap is that zswap has no
> dynamically allocated per-type resources.  In the expected case,
> where all of the pages have been drained from zswap,
> zswap_frontswap_invalidate_area() is a no-op.
> 

Not exactly, see the bug fix "mm/zswap: bugfix: memory leak when
re-swapon" from Weijie.
Zswap needs invalidate_area() also.

Thanks,
-Bob

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/