[PATCH 0/4] zswap bugfix: memory leaks and other problem

[PATCH 0/4] zswap bugfix: memory leaks and other problem

[Weijie Yang]

This patch series fix a few bugs in zswap based on Linux-3.11-rc6.

Corresponding mail thread see: lkml.org/lkml/2013/8/18/59 .

These issues fixed are:
1. memory leaks when re-swapon
2. potential problem which store and reclaim functions is called recursively 
3. memory leaks when invalidate and reclaim occur simultaneously 
4. unnecessary page scanning

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

mm/zswap.c |   35 ++++++++++++++++++++++++-----------
 1 files changed, 24 insertions(+), 11 deletions(-)

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[PATCH 0/4] zswap bugfix: memory leaks and other problem

[Weijie Yang]

This patch series fix a few bugs in zswap based on Linux-3.11-rc6.

Corresponding mail thread see: lkml.org/lkml/2013/8/18/59 .

These issues fixed are:
 1. memory leaks when re-swapon
 2. potential problem which store and reclaim functions is called recursively
 3. memory leaks when invalidate and reclaim occur simultaneously
 4. unnecessary page scanning

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

mm/zswap.c |   35 ++++++++++++++++++++++++-----------
  1 files changed, 24 insertions(+), 11 deletions(-)

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Re: [PATCH 0/4] zswap bugfix: memory leaks and other problem

[Seth Jennings]

On Fri, Aug 23, 2013 at 07:26:01PM +0800, Weijie Yang wrote:
> This patch series fix a few bugs in zswap based on Linux-3.11-rc6.
> 
> Corresponding mail thread see: lkml.org/lkml/2013/8/18/59 .
> 
> These issues fixed are:
>  1. memory leaks when re-swapon
>  2. potential problem which store and reclaim functions is called recursively
>  3. memory leaks when invalidate and reclaim occur simultaneously
>  4. unnecessary page scanning

Thanks for the patches!

Patches 2-4 have whitespace corruption (line wrapping) probably caused
by your mail agent.  You might check Documentation/email-clients.txt on
how to prevent this.

Seth

> 
> Issues discussed in that mail thread NOT fixed as it happens rarely or
> not a big problem:
>  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.
> 
> mm/zswap.c |   35 ++++++++++++++++++++++++-----------
>   1 files changed, 24 insertions(+), 11 deletions(-)
> 

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