[PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

[PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

[Weijie Yang]

This patch series fix a few bugs in mm/zswap based on Linux-3.11.

v2 --> v3
	- keep GFP_KERNEL flag

v1 --> v2
	- free memory in zswap_frontswap_invalidate_area(in patch 1)
	- fix whitespace corruption (line wrapping)
	
Corresponding mail thread: https://lkml.org/lkml/2013/8/18/59

These issues fixed/optimized are:

 1. memory leaks when re-swapon
 
 2. memory leaks when invalidate and reclaim occur concurrently
 
 3. avoid unnecessary page scanning


Issues discussed in that mail thread NOT fixed as it happens rarely or
not a big problem or controversial:

 1. a "theoretical race condition" when reclaim page
When a handle alloced from zbud, zbud considers this handle is used
validly by upper(zswap) and can be a candidate for reclaim. But zswap has
to initialize it such as setting swapentry and adding it to rbtree.
so there is a race condition, such as:
 thread 0: obtain handle x from zbud_alloc
 thread 1: zbud_reclaim_page is called
 thread 1: callback zswap_writeback_entry to reclaim handle x
 thread 1: get swpentry from handle x (it is random value now)
 thread 1: bad thing may happen
 thread 0: initialize handle x with swapentry

2. frontswap_map bitmap not cleared after zswap reclaim
Frontswap uses frontswap_map bitmap to track page in "backend" implementation,
when zswap reclaim a page, the corresponding bitmap record is not cleared.

3. the potential that zswap store and reclaim functions called recursively


 mm/zswap.c |   28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

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Re: [PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

[Minchan Kim]

On Mon, Sep 23, 2013 at 04:19:36PM +0800, Weijie Yang wrote:
> This patch series fix a few bugs in mm/zswap based on Linux-3.11.
> 
> v2 --> v3
> 	- keep GFP_KERNEL flag

Why do you drop this?

It's plain BUG. I read Bob's reply but it couldn't justify to let the pain
remain. First of all, let's fix it and better idea could come later.

> 
> v1 --> v2
> 	- free memory in zswap_frontswap_invalidate_area(in patch 1)
> 	- fix whitespace corruption (line wrapping)
> 	
> Corresponding mail thread: https://lkml.org/lkml/2013/8/18/59
> 
> These issues fixed/optimized are:
> 
>  1. memory leaks when re-swapon
>  
>  2. memory leaks when invalidate and reclaim occur concurrently
>  
>  3. avoid unnecessary page scanning
> 
> 
> Issues discussed in that mail thread NOT fixed as it happens rarely or
> not a big problem or controversial:
> 
>  1. a "theoretical race condition" when reclaim page
> When a handle alloced from zbud, zbud considers this handle is used
> validly by upper(zswap) and can be a candidate for reclaim. But zswap has
> to initialize it such as setting swapentry and adding it to rbtree.
> so there is a race condition, such as:
>  thread 0: obtain handle x from zbud_alloc
>  thread 1: zbud_reclaim_page is called
>  thread 1: callback zswap_writeback_entry to reclaim handle x
>  thread 1: get swpentry from handle x (it is random value now)
>  thread 1: bad thing may happen
>  thread 0: initialize handle x with swapentry
> 
> 2. frontswap_map bitmap not cleared after zswap reclaim
> Frontswap uses frontswap_map bitmap to track page in "backend" implementation,
> when zswap reclaim a page, the corresponding bitmap record is not cleared.
> 
> 3. the potential that zswap store and reclaim functions called recursively
> 
> 
>  mm/zswap.c |   28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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-- 
Kind regards,
Minchan Kim

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Re: [PATCH v3 0/3] mm/zswap bugfix: memory leaks and other problems

[Weijie Yang]

On Tue, Sep 24, 2013 at 9:11 AM, Minchan Kim <minchan@kernel.org> wrote:
> On Mon, Sep 23, 2013 at 04:19:36PM +0800, Weijie Yang wrote:
>> This patch series fix a few bugs in mm/zswap based on Linux-3.11.
>>
>> v2 --> v3
>>       - keep GFP_KERNEL flag
>
> Why do you drop this?
>
> It's plain BUG. I read Bob's reply but it couldn't justify to let the pain
> remain. First of all, let's fix it and better idea could come later.

Hi, Minchan. Thanks for review

What I thought is that better idea could come sooner if I keep this
flag rather than fixing it

Yes, you are right. Fixing a bug has a higher priority than waiting
for better idea.

I will add it again in the next version patch set.

>>
>> v1 --> v2
>>       - free memory in zswap_frontswap_invalidate_area(in patch 1)
>>       - fix whitespace corruption (line wrapping)
>>
>> Corresponding mail thread: https://lkml.org/lkml/2013/8/18/59
>>
>> These issues fixed/optimized are:
>>
>>  1. memory leaks when re-swapon
>>
>>  2. memory leaks when invalidate and reclaim occur concurrently
>>
>>  3. avoid unnecessary page scanning
>>
>>
>> Issues discussed in that mail thread NOT fixed as it happens rarely or
>> not a big problem or controversial:
>>
>>  1. a "theoretical race condition" when reclaim page
>> When a handle alloced from zbud, zbud considers this handle is used
>> validly by upper(zswap) and can be a candidate for reclaim. But zswap has
>> to initialize it such as setting swapentry and adding it to rbtree.
>> so there is a race condition, such as:
>>  thread 0: obtain handle x from zbud_alloc
>>  thread 1: zbud_reclaim_page is called
>>  thread 1: callback zswap_writeback_entry to reclaim handle x
>>  thread 1: get swpentry from handle x (it is random value now)
>>  thread 1: bad thing may happen
>>  thread 0: initialize handle x with swapentry
>>
>> 2. frontswap_map bitmap not cleared after zswap reclaim
>> Frontswap uses frontswap_map bitmap to track page in "backend" implementation,
>> when zswap reclaim a page, the corresponding bitmap record is not cleared.
>>
>> 3. the potential that zswap store and reclaim functions called recursively
>>
>>
>>  mm/zswap.c |   28 ++++++++++++++++++++--------
>>  1 file changed, 20 insertions(+), 8 deletions(-)
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
> --
> Kind regards,
> Minchan Kim

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