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

[Bob Liu]

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@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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

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