netdev - Re: [PATCH v3 1/2] rtw88: pci: Rearrange the memory usage for skb in RX ISR

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Date:   Thu, 15 Aug 2019 13:25:46 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Jian-Hong Pan <jian-hong@...lessm.com>
Cc:     Yan-Hsuan Chuang <yhchuang@...ltek.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>,
        Larry Finger <Larry.Finger@...inger.net>,
        David Laight <David.Laight@...lab.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux@...lessm.com, Daniel Drake <drake@...lessm.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] rtw88: pci: Rearrange the memory usage for skb in
 RX ISR

Hi all,

I realize this already is merged, and it had some previous review
comments that led to the decisions in this patch, but I'd still like
to ask here, where I think I'm reaching the relevant parties:

On Wed, Jul 10, 2019 at 1:43 AM Jian-Hong Pan <jian-hong@...lessm.com> wrote:
...
> This patch allocates a new, data-sized skb first in RX ISR. After
> copying the data in, we pass it to the upper layers. However, if skb
> allocation fails, we effectively drop the frame. In both cases, the
> original, full size ring skb is reused.
>
> In addition, by fixing the kernel crash, the RX routine should now
> generally behave better under low memory conditions.
>
> Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=204053
> Signed-off-by: Jian-Hong Pan <jian-hong@...lessm.com>
> Cc: <stable@...r.kernel.org>
> ---
> v2:
>  - Allocate new data-sized skb and put data into it, then pass it to
>    mac80211. Reuse the original skb in RX ring by DMA sync.

Is it really wise to force an extra memcpy() for *every* delivery?
Isn't there some other strategy that could be used to properly handle
low-memory scenarios while still passing the original buffer up to
higher layers most of the time? Or is it really so bad to keep
re-allocating RTK_PCI_RX_BUF_SIZE (>8KB) of contiguous memory, to
re-fill the RX ring? And if that is so bad, can we reduce the
requirement for contiguous memory instead? (e.g., keep with smaller
buffers, and perform aggregation / scatter-gather only for frames that
are really larger?)

Anyway, that's mostly a long-term thought, as this patch is good for
fixing the important memory errors, even if it's not necessarily the
ideal solution.

Regards,
Brian