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SubjectRe: [PATCH] genalloc: Make the avail variable an atomic64_t
----- On Oct 26, 2017, at 12:20 AM, Stephen Bates sbates@raithlin.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
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