linux-kernel - Re: lib/genalloc

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Date:   Fri, 2 Nov 2018 14:16:35 -0700
From:   Daniel Mentz <danielmentz@...gle.com>
To:     alexey.skidanov@...el.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 Fri, Nov 2, 2018 at 1:55 PM Alexey Skidanov
<alexey.skidanov@...el.com> wrote:
>
>
>
> 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?

Yeah, I guess you could extend genpool_algo_t to include the
information you need i.e. the offset and then provide a modified
version of gen_pool_first_fit_align() that does take your offset into
account. I wouldn't change gen_pool_first_fit_align(), though, because
existing users might depend on the current behavior.