[PATCH 3/3] iommu/iova: Try harder to allocate from rcache magazine

From: Robin Murphy
Date: Tue Sep 19 2017 - 09:49:10 EST


When devices with different DMA masks are using the same domain, or for
PCI devices where we usually try a speculative 32-bit allocation first,
there is a fair possibility that the top PFN of the rcache stack at any
given time may be unsuitable for the lower limit, prompting a fallback
to allocating anew from the rbtree. Consequently, we may end up
artifically increasing pressure on the 32-bit IOVA space as unused IOVAs
accumulate lower down in the rcache stacks, while callers with 32-bit
masks also impose unnecessary rbtree overhead.

In such cases, let's try a bit harder to satisfy the allocation locally
first - scanning the whole stack should still be relatively inexpensive,
and even rotating an entry up from the very bottom probably has less
overall impact than going to the rbtree.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
drivers/iommu/iova.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 8f8b436afd81..a7af8273fa98 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -826,12 +826,25 @@ static bool iova_magazine_empty(struct iova_magazine *mag)
static unsigned long iova_magazine_pop(struct iova_magazine *mag,
unsigned long limit_pfn)
{
+ int i;
+ unsigned long pfn;
+
BUG_ON(iova_magazine_empty(mag));

- if (mag->pfns[mag->size - 1] > limit_pfn)
- return 0;
+ /*
+ * If we can pull a suitable pfn from anywhere in the stack, that's
+ * still probably preferable to falling back to the rbtree.
+ */
+ for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
+ if (i == 0)
+ return 0;

- return mag->pfns[--mag->size];
+ pfn = mag->pfns[i];
+ mag->size--;
+ for (; i < mag->size; i++)
+ mag->pfns[i] = mag->pfns[i + 1];
+
+ return pfn;
}

static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
--
2.13.4.dirty