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

[John Garry]

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@xxxxxxxxxx/
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> >   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