linux-kernel - Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

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Date:	Wed, 6 Nov 2013 13:09:59 -0800
From:	Nitin Gupta <ngupta@...are.org>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Olav Haugan <ohaugan@...eaurora.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

On Tue, Nov 5, 2013 at 5:56 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Tue, Nov 05, 2013 at 04:54:12PM -0800, Olav Haugan wrote:
>> zsmalloc encodes a handle using the page pfn and an object
>> index. On some hardware platforms the pfn could be 0 and this
>> causes the encoded handle to be 0 which is interpreted as an
>> allocation failure.
>
> What platforms specifically have this issue?
>
>>
>> To prevent this false error we ensure that the encoded handle
>> will not be 0 when allocation succeeds.
>>
>> Change-Id: Ifff930dcf254915b497aec5cb36f152a5e5365d6
>
> What is this?  What can anyone do with it?
>
>> Signed-off-by: Olav Haugan <ohaugan@...eaurora.org>
>> ---
>>  drivers/staging/zsmalloc/zsmalloc-main.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>> index 523b937..0e32c0f 100644
>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>> @@ -441,7 +441,7 @@ static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
>>       }
>>
>>       handle = page_to_pfn(page) << OBJ_INDEX_BITS;
>> -     handle |= (obj_idx & OBJ_INDEX_MASK);
>> +     handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);
>>
>>       return (void *)handle;
>>  }
>> @@ -451,7 +451,7 @@ static void obj_handle_to_location(unsigned long handle, struct page **page,
>>                               unsigned long *obj_idx)
>>  {
>>       *page = pfn_to_page(handle >> OBJ_INDEX_BITS);
>> -     *obj_idx = handle & OBJ_INDEX_MASK;
>> +     *obj_idx = (handle & OBJ_INDEX_MASK) - 1;
>>  }
>
> I need someone who knows how to test this code to ack it before I can
> take it...
>
> And I thought we were deleting zsmalloc anyway, why are you using this
> code?  Isn't it no longer needed anymore?
>

zsmalloc is used by zram. Other zstuff has switched to zbud since they
need to do shrinking which is much easier to implement with simpler
design of zbud. For zram, which is a block device, we don't do such
active shrinking, so uses zsmalloc which provides much better density.

Nitin
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