linux-kernel - Re: [PATCH v4 2/6] iova: Allow rcache range upper limit to be flexible

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Date:   Mon, 2 Aug 2021 16:01:53 +0100
From:   Will Deacon <will@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     joro@...tes.org, robin.murphy@....com, baolu.lu@...ux.intel.com,
        iommu@...ts.linux-foundation.org, linuxarm@...wei.com,
        thierry.reding@...il.com, airlied@...ux.ie, daniel@...ll.ch,
        jonathanh@...dia.com, sakari.ailus@...ux.intel.com,
        bingbu.cao@...el.com, tian.shu.qiu@...el.com, mchehab@...nel.org,
        gregkh@...uxfoundation.org, digetx@...il.com, mst@...hat.com,
        jasowang@...hat.com, linux-kernel@...r.kernel.org,
        chenxiang66@...ilicon.com
Subject: Re: [PATCH v4 2/6] iova: Allow rcache range upper limit to be
 flexible

On Wed, Jul 14, 2021 at 06:36:39PM +0800, John Garry wrote:
> Some LLDs may request DMA mappings whose IOVA length exceeds that of the
> current rcache upper limit.

What's an LLD?

> This means that allocations for those IOVAs will never be cached, and
> always must be allocated and freed from the RB tree per DMA mapping cycle.
> This has a significant effect on performance, more so since commit
> 4e89dce72521 ("iommu/iova: Retry from last rb tree node if iova search
> fails"), as discussed at [0].
> 
> As a first step towards allowing the rcache range upper limit be
> configured, hold this value in the IOVA rcache structure, and allocate
> the rcaches separately.
> 
> [0] https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@huawei.com/
> 
> Signed-off-by: John Garry <john.garry@...wei.com>
> ---
>  drivers/iommu/dma-iommu.c |  2 +-
>  drivers/iommu/iova.c      | 23 +++++++++++++++++------
>  include/linux/iova.h      |  4 ++--
>  3 files changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 98ba927aee1a..4772278aa5da 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -434,7 +434,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
>  	 * rounding up anything cacheable to make sure that can't happen. The
>  	 * order of the unadjusted size will still match upon freeing.
>  	 */
> -	if (iova_len < (1 << (IOVA_RANGE_CACHE_MAX_SIZE - 1)))
> +	if (iova_len < (1 << (iovad->rcache_max_size - 1)))
>  		iova_len = roundup_pow_of_two(iova_len);
>  
>  	dma_limit = min_not_zero(dma_limit, dev->bus_dma_limit);
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index b6cf5f16123b..07ce73fdd8c1 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -15,6 +15,8 @@
>  /* The anchor node sits above the top of the usable address space */
>  #define IOVA_ANCHOR	~0UL
>  
> +#define IOVA_RANGE_CACHE_MAX_SIZE 6    /* log of max cached IOVA range size (in pages) */

Is that the same as an 'order'? i.e. IOVA_RANGE_CACHE_MAX_ORDER?

> +
>  static bool iova_rcache_insert(struct iova_domain *iovad,
>  			       unsigned long pfn,
>  			       unsigned long size);
> @@ -881,7 +883,14 @@ static void init_iova_rcaches(struct iova_domain *iovad)
>  	unsigned int cpu;
>  	int i;
>  
> -	for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) {
> +	iovad->rcache_max_size = IOVA_RANGE_CACHE_MAX_SIZE;
> +
> +	iovad->rcaches = kcalloc(iovad->rcache_max_size,
> +				 sizeof(*iovad->rcaches), GFP_KERNEL);
> +	if (!iovad->rcaches)
> +		return;

Returning quietly here doesn't seem like the right thing to do. At least, I
don't think the rest of the functions here are checking rcaches against
NULL.

Will