Re: [PATCH net-next v5 00/15] net: macb: implement context swapping

[Théo Lebrun]

Hello Paolo,

On Thu Jul 30, 2026 at 1:52 PM CEST, Paolo Abeni wrote:
> On 7/24/26 5:29 PM, Théo Lebrun wrote:
>> MACB has a pretty primitive approach to buffer management. They are all
>> stored in `struct macb *bp`. On operations that require buffer realloc
>> (set_ringparam & change_mtu at the moment), the only option is to close
>> the interface, change our global state and re-open the interface.
>> 
>> Two issues:
>> - It doesn't fly on memory pressured systems; we free our precious
>>   buffers and don't manage to reallocate fully, meaning our machine
>>   just lost its network access.
>> - Anecdotally, it is pretty slow because it implies a full PHY reinit.
>> 
>> Instead, we shall:
>>  - allocate a new context (including buffers) first
>>  - if it fails, early return without any impact to the interface
>>  - stop interface
>>  - update global state (bp, netdev, etc)
>>  - pass newly allocated buffer pointers to the hardware
>>  - start interface
>>  - free old context
>> 
>> This is what we implement here. Both .set_ringparam() and
>> .ndo_change_mtu() are covered by this series. In the future,
>> at least .set_channels() [0], XDP [1] and XSK [2] would benefit.
>> 
>> The change is super intrusive so conflicts will be major. Sorry!
>> 
>> --
>> 
>> During iteration V3, the LLM raised many race conditions because BH
>> features aren't disabled at close and might do a variety of bad things
>> (interact with NAPI while disabled or re-arm IRQs). This is NOT fixed
>> here. Here we focus on swap, not close. The series is way too long
>> already.
> Note that Sashiko gemini has more to say:
>
> https://sashiko.dev/#/patchset/20260724-macb-context-v5-0-569b1852bc7f%40bootlin.com
>
> At least the possible deadlock in patch 13/15 looks real to me.

Thanks. I've patched it by moving printk() calls out of the bp->lock
critical section. I'll appear in next revision.

>
> Please also note net-next commit c82ff94592fb

Thanks for the hint, makes sense. I had heard "rumors" but didn't know
it was official policy. I'll make a full pass before sending next
revision (tomorrow probably).

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com