linux-kernel - Re: [PATCH] genalloc: Make the avail variable an atomic64

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Message-ID: <1655113933.49689.1508978865628.JavaMail.zimbra@efficios.com>
Date:   Thu, 26 Oct 2017 00:47:45 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Stephen Bates <sbates@...thlin.com>
Cc:     Daniel Mentz <danielmentz@...gle.com>, logang@...tatee.com,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] genalloc: Make the avail variable an atomic64_t

----- On Oct 26, 2017, at 12:20 AM, Stephen Bates sbates@...thlin.com wrote:

>> I found that genalloc is very slow for large chunk sizes because
>> bitmap_find_next_zero_area has to grind through that entire bitmap.
>> Hence, I recommend avoiding genalloc for large chunk sizes.
> 
> Thanks for the feedback Daniel! We have been doing 16GiB without any noticeable
> issues.
> 
>> I'm thinking how this would behave on a 32 bit ARM platform
> 
> I don’t think people would be doing such big allocations on 32 bit (ARM
> systems). It would not make sense for them to be doing >4GB anyway.
> 
>>> --- a/lib/genalloc.c
>>> +++ b/lib/genalloc.c
>>> @@ -194,7 +194,7 @@ int gen_pool_add_virt(struct gen_pool *pool, unsigned long
>>> virt, phys_addr_t phy
>>>         chunk->phys_addr = phys;
>>>         chunk->start_addr = virt;
>>>         chunk->end_addr = virt + size - 1;
>>> -       atomic_set(&chunk->avail, size);
>>> +       atomic64_set(&chunk->avail, size);
> 
>> Isn't size defined as a size_t type which is 32 bit wide on ARM? How
>> can you ever set chunk->avail to anything larger than 2^32 - 1?
> 
> I did consider changing this type but it seems like there would never be a need
> to set this value to more than 4GiB on 32 bit systems.
> 
>>> @@ -464,7 +464,7 @@ size_t gen_pool_avail(struct gen_pool *pool)
>>>
>>>         rcu_read_lock();
>>>         list_for_each_entry_rcu(chunk, &pool->chunks, next_chunk)
>>> -               avail += atomic_read(&chunk->avail);
>>> +               avail += atomic64_read(&chunk->avail);
>>
>>avail is defined as size_t (32 bit). Aren't you going to overflow that variable?
> 
> Again, I don’t think people on 32 bit systems will be doing >4GB assignments so
> it would not be an issue.

We have atomic_long_t for that. Please use it instead. It will be
64-bit on 64-bit archs, and 32-bit on 32-bit archs, which seems to
fit your purpose here.

Thanks,

Mathieu


> 
> Stephen

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com