linux-kernel - Re: [PATCH 5.19 319/365] swiotlb: panic if nslabs is too small

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Date:   Tue, 23 Aug 2022 11:51:16 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        stable <stable@...r.kernel.org>, Sasha Levin <sashal@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux.dev
Subject: Re: [PATCH 5.19 319/365] swiotlb: panic if nslabs is too small

On Tue, Aug 23, 2022 at 11:25 AM Dongli Zhang <dongli.zhang@...cle.com> wrote:
>
> Adding Robin, Yu and swiotlb list.

Thanks.

> There is an on-going discussion whether to revert this patch, because it breaks
> a corner case in MIPS

I wouldn't call it a corner case. Cavium Octeon is the major platform
we use to test Debian MIPS ports [1], and 4 out of 5 best-selling
Wi-Fi routers are MIPS-based [2].

[1] https://wiki.debian.org/MIPSPort
[2] https://www.amazon.com/bestsellers/pc/300189

> when many kernel CONFIGs are not enabled (related to PCI
> and device). As a result, MIPS pre-allocates only PAGE_SIZE buffer as swiotlb.
>
> https://lore.kernel.org/all/20220820012031.1285979-1-yuzhao@google.com/
>
> However, the core idea of the patch is to panic on purpose if the swiotlb is
> configured with <1MB memory, in order to sync with the remap failure handler in
> swiotlb_init_remap().
>
> Therefore, I am waiting for suggestion from Christoph whether (1) to revert this
> patch, or (2) enforce the restriction to disallow <1MB allocation.

There are other archs (arm, ppc, riscv, s390, etc.) that call
swiotlb_init(). Have you verified them all?