[PATCH v2] z3fold: fix the potential encode bug in encod_handle

[PATCH v2] z3fold: fix the potential encode bug in encod_handle

[zhongjiang]

From: zhong jiang <zhongjiang@huawei.com>

At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
in encode_handle, it will lead to the the caller handle_to_buddy
return the error value.

The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
it will be consistent with handle_to_z3fold_header. At the same time,
change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
---
 mm/z3fold.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 8f9e89c..e8fc216 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -169,7 +169,7 @@ static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
 
 	handle = (unsigned long)zhdr;
 	if (bud != HEADLESS)
-		handle += (bud + zhdr->first_num) & BUDDY_MASK;
+		handle += (bud + zhdr->first_num) & PAGE_MASK;
 	return handle;
 }
 
@@ -183,7 +183,7 @@ static struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
 static enum buddy handle_to_buddy(unsigned long handle)
 {
 	struct z3fold_header *zhdr = handle_to_z3fold_header(handle);
-	return (handle - zhdr->first_num) & BUDDY_MASK;
+	return (handle - zhdr->first_num) & PAGE_MASK;
 }
 
 /*
-- 
1.8.3.1

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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[zhong jiang]

Hi,  Vitaly

About the following patch,  is it right?

Thanks
zhongjiang
On 2016/10/13 12:02, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
>
> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
> in encode_handle, it will lead to the the caller handle_to_buddy
> return the error value.
>
> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
> it will be consistent with handle_to_z3fold_header. At the same time,
> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.
>
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  mm/z3fold.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 8f9e89c..e8fc216 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -169,7 +169,7 @@ static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
>  
>  	handle = (unsigned long)zhdr;
>  	if (bud != HEADLESS)
> -		handle += (bud + zhdr->first_num) & BUDDY_MASK;
> +		handle += (bud + zhdr->first_num) & PAGE_MASK;
>  	return handle;
>  }
>  
> @@ -183,7 +183,7 @@ static struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
>  static enum buddy handle_to_buddy(unsigned long handle)
>  {
>  	struct z3fold_header *zhdr = handle_to_z3fold_header(handle);
> -	return (handle - zhdr->first_num) & BUDDY_MASK;
> +	return (handle - zhdr->first_num) & PAGE_MASK;
>  }
>  
>  /*


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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[Vitaly Wool]

Hi Zhong Jiang,

On Mon, Oct 17, 2016 at 3:58 AM, zhong jiang <zhongjiang@huawei.com> wrote:
> Hi,  Vitaly
>
> About the following patch,  is it right?
>
> Thanks
> zhongjiang
> On 2016/10/13 12:02, zhongjiang wrote:
>> From: zhong jiang <zhongjiang@huawei.com>
>>
>> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
>> in encode_handle, it will lead to the the caller handle_to_buddy
>> return the error value.
>>
>> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
>> it will be consistent with handle_to_z3fold_header. At the same time,
>> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.

are you seeing problems with the existing code? first_num should wrap around
BUDDY_MASK and this should be ok because it is way bigger than the number
of buddies.

~vitaly

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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[zhong jiang]

On 2016/10/17 20:03, Vitaly Wool wrote:
> Hi Zhong Jiang,
>
> On Mon, Oct 17, 2016 at 3:58 AM, zhong jiang <zhongjiang@huawei.com> wrote:
>> Hi,  Vitaly
>>
>> About the following patch,  is it right?
>>
>> Thanks
>> zhongjiang
>> On 2016/10/13 12:02, zhongjiang wrote:
>>> From: zhong jiang <zhongjiang@huawei.com>
>>>
>>> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
>>> in encode_handle, it will lead to the the caller handle_to_buddy
>>> return the error value.
>>>
>>> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
>>> it will be consistent with handle_to_z3fold_header. At the same time,
>>> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.
> are you seeing problems with the existing code? first_num should wrap around
> BUDDY_MASK and this should be ok because it is way bigger than the number
> of buddies.
>
> ~vitaly
>
> .
>
 first_num plus buddies can exceed the BUDDY_MASK. is it right?
 (first_num + buddies) & BUDDY_MASK may be a smaller value than first_num.

  but (handle - zhdr->first_num) & BUDDY_MASK will return incorrect value
  in handle_to_buddy.

  Thanks
  zhongjiang
 

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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[Dan Streetman]

On Mon, Oct 17, 2016 at 8:48 AM, zhong jiang <zhongjiang@huawei.com> wrote:
>
> On 2016/10/17 20:03, Vitaly Wool wrote:
> > Hi Zhong Jiang,
> >
> > On Mon, Oct 17, 2016 at 3:58 AM, zhong jiang <zhongjiang@huawei.com> wrote:
> >> Hi,  Vitaly
> >>
> >> About the following patch,  is it right?
> >>
> >> Thanks
> >> zhongjiang
> >> On 2016/10/13 12:02, zhongjiang wrote:
> >>> From: zhong jiang <zhongjiang@huawei.com>
> >>>
> >>> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
> >>> in encode_handle, it will lead to the the caller handle_to_buddy
> >>> return the error value.
> >>>
> >>> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
> >>> it will be consistent with handle_to_z3fold_header. At the same time,
> >>> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.
> > are you seeing problems with the existing code? first_num should wrap around
> > BUDDY_MASK and this should be ok because it is way bigger than the number
> > of buddies.
> >
> > ~vitaly
> >
> > .
> >
>  first_num plus buddies can exceed the BUDDY_MASK. is it right?

yes.

>
>  (first_num + buddies) & BUDDY_MASK may be a smaller value than first_num.

yes, but that doesn't matter; the value stored in the handle is never
accessed directly.

>
>   but (handle - zhdr->first_num) & BUDDY_MASK will return incorrect value
>   in handle_to_buddy.

the subtraction and masking will result in the correct buddy number,
even if (handle & BUDDY_MASK) < zhdr->first_num.

However, I agree it's nonobvious, and tying the first_num size to
NCHUNKS_ORDER is confusing - the number of chunks is completely
unrelated to the number of buddies.

Possibly a better way to handle first_num is to limit it to the order
of enum buddy to the actual range of possible buddy indexes, which is
0x3, i.e.:

#define BUDDY_MASK      (0x3)

and

       unsigned short first_num:2;

with that and a small bit of explanation in the encode_handle or
handle_to_buddy comments, it should be clear how the first_num and
buddy numbering work, including that overflow/underflow are ok (due to
the masking)...

>
>   Thanks
>   zhongjiang
>
>

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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[zhong jiang]

On 2016/10/17 23:30, Dan Streetman wrote:
> On Mon, Oct 17, 2016 at 8:48 AM, zhong jiang <zhongjiang@huawei.com> wrote:
>> On 2016/10/17 20:03, Vitaly Wool wrote:
>>> Hi Zhong Jiang,
>>>
>>> On Mon, Oct 17, 2016 at 3:58 AM, zhong jiang <zhongjiang@huawei.com> wrote:
>>>> Hi,  Vitaly
>>>>
>>>> About the following patch,  is it right?
>>>>
>>>> Thanks
>>>> zhongjiang
>>>> On 2016/10/13 12:02, zhongjiang wrote:
>>>>> From: zhong jiang <zhongjiang@huawei.com>
>>>>>
>>>>> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
>>>>> in encode_handle, it will lead to the the caller handle_to_buddy
>>>>> return the error value.
>>>>>
>>>>> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
>>>>> it will be consistent with handle_to_z3fold_header. At the same time,
>>>>> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.
>>> are you seeing problems with the existing code? first_num should wrap around
>>> BUDDY_MASK and this should be ok because it is way bigger than the number
>>> of buddies.
>>>
>>> ~vitaly
>>>
>>> .
>>>
>>  first_num plus buddies can exceed the BUDDY_MASK. is it right?
> yes.
>
>>  (first_num + buddies) & BUDDY_MASK may be a smaller value than first_num.
> yes, but that doesn't matter; the value stored in the handle is never
> accessed directly.
>
>>   but (handle - zhdr->first_num) & BUDDY_MASK will return incorrect value
>>   in handle_to_buddy.
> the subtraction and masking will result in the correct buddy number,
> even if (handle & BUDDY_MASK) < zhdr->first_num.
 yes, I see. it is hard to read.
> However, I agree it's nonobvious, and tying the first_num size to
> NCHUNKS_ORDER is confusing - the number of chunks is completely
> unrelated to the number of buddies.
 yes. indeed.
> Possibly a better way to handle first_num is to limit it to the order
> of enum buddy to the actual range of possible buddy indexes, which is
> 0x3, i.e.:
>
> #define BUDDY_MASK      (0x3)
>
> and
>
>        unsigned short first_num:2;
>
> with that and a small bit of explanation in the encode_handle or
> handle_to_buddy comments, it should be clear how the first_num and
> buddy numbering work, including that overflow/underflow are ok (due to
> the masking)...
 yes, It is better and clearer. Thanks for your relpy and advice. I will
 resend the patch.
>>   Thanks
>>   zhongjiang
>>
>>
> .
>


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Re: [PATCH v2] z3fold: fix the potential encode bug in encod_handle

[zhong jiang]

On 2016/10/17 20:03, Vitaly Wool wrote:
> Hi Zhong Jiang,
>
> On Mon, Oct 17, 2016 at 3:58 AM, zhong jiang <zhongjiang@huawei.com> wrote:
>> Hi,  Vitaly
>>
>> About the following patch,  is it right?
>>
>> Thanks
>> zhongjiang
>> On 2016/10/13 12:02, zhongjiang wrote:
>>> From: zhong jiang <zhongjiang@huawei.com>
>>>
>>> At present, zhdr->first_num plus bud can exceed the BUDDY_MASK
>>> in encode_handle, it will lead to the the caller handle_to_buddy
>>> return the error value.
>>>
>>> The patch fix the issue by changing the BUDDY_MASK to PAGE_MASK,
>>> it will be consistent with handle_to_z3fold_header. At the same time,
>>> change the BUDDY_MASK to PAGE_MASK in handle_to_buddy is better.
> are you seeing problems with the existing code? first_num should wrap around
> BUDDY_MASK and this should be ok because it is way bigger than the number
> of buddies.
>
> ~vitaly
>
> .
>
 I am curious about the z3fold implement, Thus, I am reviewing the code.

 Thanks
 zhongjiang

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