netdev - Re: [PATCH net-next v22 04/13] netdev: netdevice devmem allocator

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Message-ID: <CAHS8izP8T5Xj97M7efecBmCrG9z8E0PYTxWCYZ0ym0hv13-DKg@mail.gmail.com>
Date: Wed, 28 Aug 2024 00:20:23 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
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Subject: Re: [PATCH net-next v22 04/13] netdev: netdevice devmem allocator

On Tue, Aug 27, 2024 at 7:15 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sun, 25 Aug 2024 04:15:02 +0000 Mina Almasry wrote:
> > +void net_devmem_free_dmabuf(struct net_iov *niov)
> > +{
> > +     struct net_devmem_dmabuf_binding *binding = net_iov_binding(niov);
> > +     unsigned long dma_addr = net_devmem_get_dma_addr(niov);
> > +
> > +     if (gen_pool_has_addr(binding->chunk_pool, dma_addr, PAGE_SIZE))
> > +             gen_pool_free(binding->chunk_pool, dma_addr, PAGE_SIZE);
>
> Is the check necessary for correctness? Should it perhaps be a WARN
> under DEBUG_NET instead? The rest LGTM:
>

Not really necessary for correctness per se, but if we try to free a
dma_addr that is not in a gen_pool (due to some other bug in the
code), then gen_pool_free ends up BUG_ON, crashing the kernel.

Arguably gen_pool_free should not BUG_ON, but I think that's an old
API, and existing call sites have worked around the BUG_ON by doing a
gen_pool_has_addr check like I do here, for example kernel/dma/pool.c.
So I did not seek to change this established behavior.

I think WARN seems fine to me, but maybe not under DEBUG_NET. I don't
want production code crashing due to this error, if it's OK with you.

Unless I hear otherwise I'll add a WARN without debug here.

> Reviewed-by: Jakub Kicinski <kuba@...nel.org>
>

Thanks!

-- 
Thanks,
Mina