linux-kernel - Re: lib/genalloc

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Date:   Fri, 2 Nov 2018 22:56:34 +0200
From:   Alexey Skidanov <alexey.skidanov@...el.com>
To:     Daniel Mentz <danielmentz@...gle.com>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.co>,
        lkml <linux-kernel@...r.kernel.org>, labbott@...hat.com
Subject: Re: lib/genalloc



On 11/2/18 9:17 PM, Daniel Mentz wrote:
> On Thu, Nov 1, 2018 at 10:07 AM Alexey Skidanov
> <alexey.skidanov@...el.com> wrote:
>> On 11/1/18 18:48, Stephen  Bates wrote:
>>>>    I use gen_pool_first_fit_align() as pool allocation algorithm allocating
>>>>    buffers with requested alignment. But if a chunk base address is not
>>>>    aligned to the requested alignment(from some reason), the returned
>>>>    address is not aligned too.
>>>
>>> Alexey
>>>
>>> Can you try using gen_pool_first_fit_order_align()? Will that give you the alignment you need?
>>>
>>> Stephen
>>>
>>>
>> I think it will not help me. Let's assume that the chunk base address is
>> 0x2F400000 and I want to allocate 16MB aligned buffer. I get back the
>> 0x2F400000. I think it happens because of this string in the
>> gen_pool_alloc_algo():
>>
>> addr = chunk->start_addr + ((unsigned long)start_bit << order);
>>
>> and the gen_pool_first_fit_align() implementation that doesn't take into
>> account the "incorrect" chunk base alignment.
> 
> gen_pool_first_fit_align() has no information about the chunk base
> alignment. Hence, it can't take it into account.
> 
> How do you request the alignment in your code?
> 
> I agree with your analysis that gen_pool_first_fit_align() performs
> alignment only with respect to the start of the chunk not the memory
> address that gen_pool_alloc_algo() returns. I guess a solution would
> be to only add chunks that satisfy all your alignment requirements. In
> your case, you must only add chunks that are 16MB aligned.
> I am unsure whether this is by design, but I believe it's the way that
> the code currently works.
> 

Daniel,

I think the better solution is to use bitmap_find_next_zero_area_off()
that receives the bit offset (CMA allocator uses it to solve the same
issue). Of course, we need to pass the chunk base address to the
gen_pool_first_fit_align().

What do you think?

Thanks,
Alexey