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

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Message-ID: <893BAB3D-6B3C-46BB-A16E-E64BF278D90E@raithlin.com>
Date:   Wed, 25 Oct 2017 22:20:20 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Daniel Mentz <danielmentz@...gle.com>,
        "logang@...tatee.com" <logang@...tatee.com>
CC:     "corbet@....net" <corbet@....net>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] genalloc: Make the avail variable an atomic64_t

> 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.

Stephen