linux-kernel - Re: [PATCH v1 4/4] swiotlb: panic if nslabs is too small

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Date:   Sun, 12 Jun 2022 23:49:46 -0700
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     iommu@...ts.linux-foundation.org, linux-doc@...r.kernel.org,
        x86@...nel.org
Cc:     corbet@....net, dave.hansen@...ux.intel.com, joe.jin@...cle.com,
        linux-kernel@...r.kernel.org, hch@...radead.org, mingo@...hat.com,
        bp@...en8.de, tglx@...utronix.de
Subject: Re: [PATCH v1 4/4] swiotlb: panic if nslabs is too small



On 6/11/22 1:25 AM, Dongli Zhang wrote:
> Panic on purpose if nslabs is too small, in order to sync with the remap
> retry logic.
> 
> In addition, print the number of bytes for tlb alloc failure.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
> ---
>  kernel/dma/swiotlb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index fd21f4162f4b..1758b724c7a8 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -242,6 +242,9 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
>  	if (swiotlb_force_disable)
>  		return;
>  
> +	if (nslabs < IO_TLB_MIN_SLABS)
> +		panic("%s: nslabs = %lu too small\n", __func__, nslabs);
> +
>  	/*
>  	 * By default allocate the bounce buffer memory from low memory, but
>  	 * allow to pick a location everywhere for hypervisors with guest
> @@ -254,7 +257,8 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
>  	else
>  		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
>  	if (!tlb) {
> -		pr_warn("%s: failed to allocate tlb structure\n", __func__);
> +		pr_warn("%s: Failed to allocate %zu bytes tlb structure\n",
> +			__func__, bytes);

Indeed I have a question on this pr_warn(). (I was going to send another patch
to retry with nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE)).

Why not retry with nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE), or panic here?

If the QEMU machine of my VM is i440fx, the boot is almost failed even here is
pr_warn. Why not sync with the remap failure handling?

1. retry with nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE))
2. and finally panic if nslabs is too small.

Thank you very much!

Dongli Zhang

>  		return;
>  	}
>  
>