linux-kernel - Re: [PATCH v2] net: ethernet: mtk_eth_soc: fix misuse of mem alloc interface netdev[napi]_alloc

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Date:   Fri, 3 Jun 2022 12:55:16 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Chen Lin <chen45464546@....com>, Felix Fietkau <nbd@....name>,
        john@...ozen.org, sean.wang@...iatek.com, Mark-MC.Lee@...iatek.com,
        David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mediatek@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH v2] net: ethernet: mtk_eth_soc: fix misuse of mem alloc
 interface netdev[napi]_alloc_frag

On Fri, 3 Jun 2022 12:11:43 -0700 Eric Dumazet wrote:
> Yes, we only have to review callers and change the documentation and
> implementation.
> 
> The confusion/overhead/generalization came with :
> 
> commit 7ba7aeabbaba484347cc98fbe9045769ca0d118d
> Author: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Date:   Fri Jun 7 21:20:34 2019 +0200
> 
>     net: Don't disable interrupts in napi_alloc_frag()
> 
>     netdev_alloc_frag() can be used from any context and is used by NAPI
>     and non-NAPI drivers. Non-NAPI drivers use it in interrupt context
>     and NAPI drivers use it during initial allocation (->ndo_open() or
>     ->ndo_change_mtu()). Some NAPI drivers share the same function for the  
>     initial allocation and the allocation in their NAPI callback.
> 
>     The interrupts are disabled in order to ensure locked access from every
>     context to `netdev_alloc_cache'.
> 
>     Let netdev_alloc_frag() check if interrupts are disabled. If they are,
>     use `netdev_alloc_cache' otherwise disable BH and invoke
>     __napi_alloc_frag() for the allocation. The IRQ check is cheaper
>     compared to disabling & enabling interrupts and memory allocation with
>     disabled interrupts does not work on -RT.

Hm, should have looked at the code. Were you thinking of adding a
helper which would replace both netdev_ and napi_ variants and DTRT
internally?

An option for getting GFP_KERNEL in there would be having an rtnl frag
cache. Users who need frags on the reconfig path should be under rtnl,
they can call rtnl_alloc_frag(), which can use GFP_KERNEL internally.
Otherwise the GFP_KERNEL frag cache would need to be protected by
another mutex, I presume. Pre-allocating memory before using the napi
cache seems hard.