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RE: [PATCH] XFS: Free buffer pages array unconditionally

To: "Dave Chinner" <david@xxxxxxxxxxxxx>
Subject: RE: [PATCH] XFS: Free buffer pages array unconditionally
From: "Alex Elder" <aelder@xxxxxxx>
Date: Tue, 15 Dec 2009 16:51:31 -0600
Cc: <xfs@xxxxxxxxxxx>
In-reply-to: <20091215223658.GD4850@xxxxxxxxxxxxxxxx>
Thread-index: Acp910qtMIKS7Y3yTUmTI+qYhr5WSQAAURnw
Thread-topic: [PATCH] XFS: Free buffer pages array unconditionally
Dave Chinner wrote:
> On Tue, Dec 15, 2009 at 01:21:36PM -0600, Alex Elder wrote:
>> Dave Chinner wrote:
>>> The code in xfs_free_buf() only attempts to free the b_pages array if the
>>> buffer is a page cache backed or page allocated buffer. The extra log buffer
>>> that is used when the log wraps uses pages that are allocated to a different
>>> log buffer, but it still has a b_pages array allocated when those pages
>>> are associated to with the extra buffer in xfs_buf_associate_memory.

I get it now.  You're right, I was confusing the array
with the pages themselves.  And in hindsight the title
of your patch and the explanation is abundantly clear
on what's being freed...  Thank you for carefully
explaining it to me (again).

The patch looks good.

                                        -Alex

Reviewed-by: Alex Elder <aelder@xxxxxxx>
 
>>> Hence we need to always attempt to free the b_pages array when tearing
>>> down a buffer, not just on buffers that are explicitly marked as page 
>>> bearing
>>> buffers. This fixes a leak detected by the kernel memory leak code.
>> 
>> Three places call xfs_buf_get_pages();
>> - _xfs_buf_lookup_pages(), which sets the _XBF_PAGE_CACHE flag in the   
>> buffer after the call
>> - xfs_buf_associate_memory(), which sets no flag bit
>> - xfs_buf_get_noaddr(), which sets the _XBF_PAGES flag.
>> 
>> The only place that checks for _XBF_PAGES is xfs_buf_free().
>> 
>> Given that, I have two comments:
>> - You could just as easily have set the _XBF_PAGES flag in
>>   xfs_buf_associate_memory, thereby making that flag indicate
>>   consistently that the buffer has allocated pages
> 
> _XBF_PAGES is used to indicate that the buffer owns the
> pages and must free them when the buffer is freed.
> 
> _XBF_PAGE_CACHE indicates pages are allocated out of the buftarg
> page cache and need to be released when the buffer is freed.
> 
> In both cases, xfs_buf_free() has to free the pages
> associated with these buffers.
> 
> In the case of xfs_buf_associate_memory(), the memory attached to
> the buffer is owned by an external entity and hence we are not
> allowed to free it when we free the buffer. Therefore we can't set
> _XBF_PAGES on the buffer.
> 
>> - Or, since you are proposing unconditionally freeing the
>>   pages,
> 
> This change doesn't unconditionally free the pages attached
> to the buffer - it unconditionally frees the array used to
> index the pages on the buffer when the buffer is torn down.
> i.e:
> 
>    +------+         +------+
>    |  bp  |         |      |-------> page
>    |      | b_pages |      |-------> page
>    |      |-------->|      |.............
>    |      |         |      |-------> page
>    |      |         |      |-------> page
>    +------+         +------+
>                      ^^^^^^
>               This array is what we are
>               freeing unconditionally.
> 
> Cheers,
> 
> Dave.

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