linux-kernel - [PATCH] iommu/iova: Improve 32-bit free space estimate

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Date:   Thu,  3 Mar 2022 14:40:08 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     joro@...tes.org, will@...nel.org
Cc:     iommu@...ts.linux-foundation.org, yf.wang@...iatek.com,
        miles.chen@...iatek.com, wsd_upstream@...iatek.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH] iommu/iova: Improve 32-bit free space estimate

For various reasons based on the allocator behaviour and typical
use-cases at the time, when the max32_alloc_size optimisation was
introduced it seemed reasonable to couple the reset of the tracked
size to the update of cached32_node upon freeing a relevant IOVA.
However, since subsequent optimisations focused on helping genuine
32-bit devices make best use of even more limited address spaces, it
is now a lot more likely for cached32_node to be anywhere in a "full"
32-bit address space, and as such more likely for space to become
available from IOVAs below that node being freed.

At this point, the short-cut in __cached_rbnode_delete_update() really
doesn't hold up any more, and we need to fix the logic to reliably
provide the expected behaviour. We still want cached32_node to only move
upwards, but we should reset the allocation size if *any* 32-bit space
has become available.

Reported-by: Yunfei Wang <yf.wang@...iatek.com>
Signed-off-by: Robin Murphy <robin.murphy@....com>
---
 drivers/iommu/iova.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index b28c9435b898..170e0f33040e 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -95,10 +95,11 @@ __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
 	cached_iova = to_iova(iovad->cached32_node);
 	if (free == cached_iova ||
 	    (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);
+
+	if (free->pfn_lo < iovad->dma_32bit_pfn)
 		iovad->max32_alloc_size = iovad->dma_32bit_pfn;
-	}

 	cached_iova = to_iova(iovad->cached_node);
 	if (free->pfn_lo >= cached_iova->pfn_lo)
-- 
2.28.0.dirty