linux-kernel - Re: [net-next v1 08/16] memory-provider: dmabuf devmem memory provider

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Date:   Tue, 12 Dec 2023 11:08:34 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Shailend Chand <shailend@...gle.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        bpf@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jeroen de Borst <jeroendb@...gle.com>,
        Praveen Kaligineedi <pkaligineedi@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Ahern <dsahern@...nel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Harshitha Ramamurthy <hramamurthy@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kaiyuan Zhang <kaiyuanz@...gle.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [net-next v1 08/16] memory-provider: dmabuf devmem memory
 provider

On Tue, Dec 12, 2023 at 06:58:17AM -0800, Mina Almasry wrote:

> Jason, we set the LSB on page_pool_iov pointers before casting it to
> struct page pointers. The resulting pointers are not useable as page
> pointers at all.

I understand that, the second ask is about maintainability of the mm
by using correct types.

> > Perhaps you can simply avoid this by arranging for this driver to also
> > exclusively use some special type to indicate the dual nature of the
> > pointer and leave the other drivers as using the struct page version.
> 
> This is certainly possible, but it requires us to rename all the page
> pointers in the page_pool to the new type, and requires the driver
> adding devmem TCP support to rename all the page* pointer instances to
> the new type. It's possible but it introduces lots of code churn. Is
> the LSB + cast not a reasonable compromise here? I feel like the trick
> of setting the least significant bit on a pointer to indicate it's
> something else has a fair amount of precedent in the kernel.

Linus himself has complained about exactly this before, and written a cleanup:

https://lore.kernel.org/linux-mm/20221108194139.57604-1-torvalds@linux-foundation.org/

If you mangle a pointer *so it is no longer a pointer* then give it a
proper opaque type so the compiler can check everything statically and
require that the necessary converters are called in all cases.

You call it churn, I call it future maintainability. :(

No objection to using the LSB, just properly type a LSB mangled
pointer so everyone knows what is going on and don't call it MM's
struct page *.

I would say this is important here because it is a large driver facing
API surface.

Jason