netdev - Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary condition

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Date:   Thu, 14 Jun 2018 16:49:36 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "kafai@...com" <kafai@...com>, Tariq Toukan <tariqt@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>
Subject: Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary
 condition



On 06/14/2018 02:04 PM, Saeed Mahameed wrote:

> I was looking at the code without my fix :)
> 
> with my fix:
> release = frags->page_offset + frag_info->frag_stride > PAGE_SIZE;
> 
> for XDP: frag_info->frag_stride is PAGE_SIZE, so release will always be
> true regardless of PAGE_SIZE.
> 
> So i guess i didn't quite understand your PowerPC concern.. can you
> elaborate ?
> 

So your maths with PAGE_SIZE=65536 and MTU 9000

frag_stride is about 9344

So if the last chunk of the page has 9100 bytes, we wont be able to use it, while really we should be able to use it.