Re: [RFC PATCH 0/1] vmxnet3: Adjust maximum Rx ring buffer size

Florian Fainelli
2025-01-08T16:53:18Z

On 1/6/25 16:57, Jakub Kicinski wrote:
> On Mon, 6 Jan 2025 15:51:10 -0800 Florian Fainelli wrote:
>> On 1/6/25 15:47, 'Jakub Kicinski' via BCM-KERNEL-FEEDBACK-LIST,PDL wrote:
>>> On Sun, 5 Jan 2025 21:30:35 +0000 Aaron Tomlin wrote:
>>>> I managed to trigger the MAX_PAGE_ORDER warning in the context of function
>>>> __alloc_pages_noprof() with /usr/sbin/ethtool --set-ring rx 4096 rx-mini
>>>> 2048 [devname]' using the maximum supported Ring 0 and Rx ring buffer size.
>>>> Admittedly this was under the stock Linux kernel-4.18.0-477.27.1.el8_8
>>>> whereby CONFIG_CMA is not enabled. I think it does not make sense to
>>>> attempt a large memory allocation request for physically contiguous memory,
>>>> to hold the Rx Data ring that could exceed the maximum page-order supported
>>>> by the system.
>>>
>>> I think CMA should be a bit orthogonal to the warning.
>>>
>>> Off the top of my head the usual way to solve the warning is to add
>>> __GFP_NOWARN to the allocations which trigger it. And then handle
>>> the error gracefully.
>>
>> That IMHO should really be the default for any driver that calls
>> __netdev_alloc_skb() under the hood, we should not really have to
>> specify __GFP_NOWARN, rather if people want it, they should specify it.
>
> True, although TBH I don't fully understand why this flag exists
> in the first place. Is it just supposed to be catching programming
> errors, or is it due to potential DoS implications of users triggering
> large allocations?
>

There is some value IMHO in printing when allocations fail, where they
came from, their gfp_t flags and page order so you can track high order
offenders in hot paths (one of our Wi-Fi driver was notorious for doing
that and having verbose out of memory dumps by default definitively
helped). Once you fix those however, hogging the system while dumping
lines and lines of information onto a slow console tends to be worse
than the recovery from out of memory itself. One could argue that
triggering an OOM plus dumping information can result in a DoS, so that
should be frowned upon...
--
Florian