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SubjectRe: [PATCH v4 2/6] iova: Allow rcache range upper limit to be flexible
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On 02/08/2021 16:01, Will Deacon wrote:
> 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?
>

low-level driver

maybe I'll stick with simply "drivers"

>> 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@huawei.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?

Yeah, that may be better. I was just using the same name as before.

>
>> +
>> 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.
>

For sure, but that is what other code which can fail here already does,
like:

static void init_iova_rcaches(struct iova_domain *iovad)
{
...

for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) {
...

rcache->cpu_rcaches = __alloc_percpu(sizeof(*cpu_rcache),
cache_line_size());
if (WARN_ON(!rcache->cpu_rcaches))
continue;
}

and that is not safe either.

This issue was raised a while ago. I don't mind trying to fix it - a
slightly painful part is that it touches a few subsystems.

Thanks,
John

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