Re: [PATCH] iommu/iova: Optimise attempts to allocate iova from 32bit address range

From: Robin Murphy
Date: Thu Aug 09 2018 - 12:24:53 EST


On 07/08/18 09:54, Ganapatrao Kulkarni wrote:
As an optimisation for PCI devices, there is always first attempt
been made to allocate iova from SAC address range. This will lead
to unnecessary attempts/function calls, when there are no free ranges
available.

This patch optimises by adding flag to track previous failed attempts
and avoids further attempts until replenish happens.

Agh, what I overlooked is that this still suffers from the original problem, wherein a large allocation which fails due to fragmentation then blocks all subsequent smaller allocations, even if they may have succeeded.

For a minimal change, though, what I think we could do is instead of just having a flag, track the size of the last 32-bit allocation which failed. If we're happy to assume that nobody's likely to mix aligned and unaligned allocations within the same domain, then that should be sufficiently robust whilst being no more complicated than this version, i.e. (modulo thinking up a better name for it):


Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@xxxxxxxxxx>
---
This patch is based on comments from Robin Murphy <robin.murphy@xxxxxxx>
for patch [1]

[1] https://lkml.org/lkml/2018/4/19/780

drivers/iommu/iova.c | 11 ++++++++++-
include/linux/iova.h | 1 +
2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 83fe262..d97bb5a 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -56,6 +56,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
iovad->granule = granule;
iovad->start_pfn = start_pfn;
iovad->dma_32bit_pfn = 1UL << (32 - iova_shift(iovad));
+ iovad->free_32bit_pfns = true;

iovad->max_32bit_free = iovad->dma_32bit_pfn;

iovad->flush_cb = NULL;
iovad->fq = NULL;
iovad->anchor.pfn_lo = iovad->anchor.pfn_hi = IOVA_ANCHOR;
@@ -139,8 +140,10 @@ __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
cached_iova = rb_entry(iovad->cached32_node, struct iova, node);
if (free->pfn_hi < iovad->dma_32bit_pfn &&
- free->pfn_lo >= cached_iova->pfn_lo)
+ free->pfn_lo >= cached_iova->pfn_lo) {
iovad->cached32_node = rb_next(&free->node);
+ iovad->free_32bit_pfns = true;

iovad->max_32bit_free = iovad->dma_32bit_pfn;

+ }
cached_iova = rb_entry(iovad->cached_node, struct iova, node);
if (free->pfn_lo >= cached_iova->pfn_lo)
@@ -290,6 +293,10 @@ alloc_iova(struct iova_domain *iovad, unsigned long size,
struct iova *new_iova;
int ret;
+ if (limit_pfn <= iovad->dma_32bit_pfn &&
+ !iovad->free_32bit_pfns)

size >= iovad->max_32bit_free)

+ return NULL;
+
new_iova = alloc_iova_mem();
if (!new_iova)
return NULL;
@@ -299,6 +306,8 @@ alloc_iova(struct iova_domain *iovad, unsigned long size,
if (ret) {
free_iova_mem(new_iova);
+ if (limit_pfn <= iovad->dma_32bit_pfn)
+ iovad->free_32bit_pfns = false;

iovad->max_32bit_free = size;

What do you think?

Robin.

return NULL;
}
diff --git a/include/linux/iova.h b/include/linux/iova.h
index 928442d..3810ba9 100644
--- a/include/linux/iova.h
+++ b/include/linux/iova.h
@@ -96,6 +96,7 @@ struct iova_domain {
flush-queues */
atomic_t fq_timer_on; /* 1 when timer is active, 0
when not */
+ bool free_32bit_pfns;
};
static inline unsigned long iova_size(struct iova *iova)