netdev - Re: [PATCH V3,net-next, 3/4] net: mana: Enable RX path to handle various MTU sizes

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Date:   Mon, 17 Apr 2023 10:52:29 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Paul Rosswurm <paulros@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "leon@...nel.org" <leon@...nel.org>,
        Long Li <longli@...rosoft.com>,
        "ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        Ajay Sharma <sharmaajay@...rosoft.com>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3,net-next, 3/4] net: mana: Enable RX path to handle
 various MTU sizes

On Sat, 15 Apr 2023 14:25:29 +0000 Haiyang Zhang wrote:
> > Allocating frag larger than a page is not safe.  
> 
>  I saw other drivers doing this - use napi_alloc_frag for size bigger than a page.
> And it returns compound page. Why it's not safe? Should we use other allocator
> when need compound pages?

I believe so. There was a thread about this within the last year.
Someone was trying to fix the page frag allocator to not fall back
to order 0 pages in case of failure if requested size is > PAGE_SIZE.
But there was push back and folks were saying that it's simply not 
a case supported by the frag allocator. 🤷️

> > Frag allocator falls back to allocating single pages, doesn't it?  
> 
> Actually I checked it. Compound page is still returned for size smaller than PAGE_SIZE,
> so I used single page allocation for that.

https://elixir.bootlin.com/linux/v6.3-rc6/source/mm/page_alloc.c#L5723

Jumbo frames should really be supported as scatter transfers, 
if possible.