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

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Message-ID: <1bd6da9b-fa46-25e7-8921-cb56eb91e71b@gmail.com>
Date:   Tue, 19 Jun 2018 17:25:20 -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 06/19/2018 11:05 AM, Saeed Mahameed wrote:

> this is only true for XDP setup, for non XDP max stride_size can only
> be around ~3k and only for mtu > ~6k
> 
> For XDP setup you suggested:
> -               priv->frag_info[0].frag_size = eff_mtu;
> +               priv->frag_info[0].frag_size = PAGE_SIZE;
> 
> currently the condition is:
> 
> release = frags->page_offset + frag_info->frag_size > PAGE_SIZE;
> 
> so my solution and yours have the same problem you described above.
> 
> the problem is not with the initial values or with stride/farg size
> math, it just that in XDP we shouldn't reuse at ALL. I agree with you
> that we need to optimize and maybe for PAGE_SIZE > 8k we need to allow
> XDP setup to reuses. but for now there is a data corruption to handle.

Sure, we all agree there is a bug to fix.

The way you are fixing it is kind of illogical.

The NIC can use a frag if its _size_ is big enough to receive the frame.

The _stride_  is an abstraction created by the driver to report an estimation of the _truesize_,
or memory consumption, so that linux can better track overall memory usage.

For example, if MTU=1500, the size of the fragment is 1536 bytes, but since we can put only
2 fragments per 4KB page (on x86), we declare the _stride_ to be 2048 bytes.

Declaring that a final blob of a page, being 1600 bytes, not able to receive a frame because
_stride_ is 2048 is illogical and waste resources.