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

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Date:   Wed, 25 Oct 2017 10:55:08 -0700
From:   Daniel Mentz <danielmentz@...gle.com>
To:     sbates@...thlin.com, logang@...tatee.com
Cc:     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.

I'm thinking how this would behave on a 32 bit ARM platform

On Wed, Oct 25, 2017 at 8:32 AM,  <sbates@...thlin.com> wrote:
> --- 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?

> @@ -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?