linux-kernel - Re: [Intel-gfx] [PATCH v4] lib/scatterlist: Avoid potential scatterlist entry overflow

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Date:   Mon, 16 Jan 2017 10:05:58 +0000
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Tvrtko Ursulin <tursulin@...ulin.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Intel-gfx@...ts.freedesktop.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Subject: Re: [Intel-gfx] [PATCH v4] lib/scatterlist: Avoid potential
 scatterlist entry overflow


On 13/01/2017 22:23, Andy Shevchenko wrote:
>>>> @@ -402,9 +403,16 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
>>>>
>>>>         /* compute number of contiguous chunks */
>>>>         chunks = 1;
>>>> -       for (i = 1; i < n_pages; ++i)
>>>> -               if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) +
>>>> 1)
>>>> +       seg_len = PAGE_SIZE;
>>>> +       for (i = 1; i < n_pages; ++i) {
>>>> +               if (seg_len >= max_segment ||
>>>> +                   page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) +
>>>> 1) {
>>>>                         ++chunks;
>>>> +                       seg_len = PAGE_SIZE;
>>>> +               } else {
>>>> +                       seg_len += PAGE_SIZE;
>>>> +               }
>>>> +       }
>>>
>>>
>>> Wouldn't be following looks more readable?
>>>
>>> seg_len = 0;
>>> // Are compilers so stupid doing calculation per iteration in
>>> for-conditional?
>>> // for (i = 0; i + 1 < n_pages; i++) ?
>>
>>
>> I didn't get what you meant here?
>
> Why do we start from 1? I see here two micro (?) optimizations:
> 1) starting from 1 on believe that compiler dumb enough to every time
> do a calculation in condition;

The existing code starts from 1 because the pfn condition looks up page 
i - 1. I don't feel there is a need to change that as well.

> 2) ++i instead of i++, but this is just matter of style, it's not a c++.

Note that I haven't changed the existing code in this respect. I am 
happy to change it though.

>>> for (i = 1; i < n_pages; ++i) {
>>>   seg_len += PAGE_SIZE;
>>>   if (seg_len >= max_segment || page_to_pfn(pages[i]) !=
>>> page_to_pfn(pages[i - 1]) + 1) {
>>>     ++chunks;
>>>     seg_len = PAGE_SIZE;
>>>   }
>>> }
>>
>>
>> Tried it in my unit tester but it doesn't work for all scenarios, guess
>> there is a subtle bug somewhere. I don't find it that unreadable so would
>> prefer to leave it since it works.
>
> Last seems has to be
> seg_len = 0;

Oh right, of course. Your suggestion generates a tiny bit smaller binary 
so I am happy to change that as well. I'll resend the patch hopefully today.

Regards,

Tvrtko