linux-kernel - Re: [PATCH] z3fold: limit first_num to the actual range of possible buddy indexes

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Date:   Tue, 18 Oct 2016 16:54:48 +0200
From:   Vitaly Wool <vitalywool@...il.com>
To:     Dan Streetman <ddstreet@...e.org>
Cc:     zhongjiang <zhongjiang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] z3fold: limit first_num to the actual range of possible
 buddy indexes

On Tue, Oct 18, 2016 at 3:57 PM, Dan Streetman <ddstreet@...e.org> wrote:
> On Tue, Oct 18, 2016 at 3:42 AM, zhongjiang <zhongjiang@...wei.com> wrote:
>> From: zhong jiang <zhongjiang@...wei.com>
>>
>> At present, Tying the first_num size to NCHUNKS_ORDER is confusing.
>> the number of chunks is completely unrelated to the number of buddies.
>>
>> The patch limit the first_num to actual range of possible buddy indexes.
>> and that is more reasonable and obvious without functional change.
>>
>> Suggested-by: Dan Streetman <ddstreet@...e.org>
>> Signed-off-by: zhong jiang <zhongjiang@...wei.com>
>
> Acked-by: Dan Streetman <ddstreet@...e.org>

Acked-by: Vitaly Wool <vitalywool@...il.com>

>> --->  mm/z3fold.c | 10 +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 8f9e89c..207e5dd 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -50,7 +50,7 @@
>>  #define ZHDR_SIZE_ALIGNED CHUNK_SIZE
>>  #define NCHUNKS                ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)
>>
>> -#define BUDDY_MASK     ((1 << NCHUNKS_ORDER) - 1)
>> +#define BUDDY_MASK     (0x3)
>>
>>  struct z3fold_pool;
>>  struct z3fold_ops {
>> @@ -109,7 +109,7 @@ struct z3fold_header {
>>         unsigned short middle_chunks;
>>         unsigned short last_chunks;
>>         unsigned short start_middle;
>> -       unsigned short first_num:NCHUNKS_ORDER;
>> +       unsigned short first_num:2;
>>  };
>>
>>  /*
>> @@ -179,7 +179,11 @@ static struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
>>         return (struct z3fold_header *)(handle & PAGE_MASK);
>>  }
>>
>> -/* Returns buddy number */
>> +/*
>> + * (handle & BUDDY_MASK) < zhdr->first_num is possible in encode_handle
>> + *  but that doesn't matter. because the masking will result in the
>> + *  correct buddy number.
>> + */
>>  static enum buddy handle_to_buddy(unsigned long handle)
>>  {
>>         struct z3fold_header *zhdr = handle_to_z3fold_header(handle);
>> --
>> 1.8.3.1
>>