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From:   vjitta@codeaurora.org
To:     joro@8bytes.org, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Cc:     vinmenon@codeaurora.org, kernel-team@android.com,
        vjitta@codeaurora.org
Subject: [PATCH] iommu/iova: Retry from last rb tree node if iova search fails
Date:   Thu,  7 May 2020 01:31:57 +0530
Message-Id: <1588795317-20879-1-git-send-email-vjitta@codeaurora.org>
Sender: linux-kernel-owner@vger.kernel.org
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From: Vijayanand Jitta <vjitta@codeaurora.org>

When ever a new iova alloc request comes iova is always searched
from the cached node and the nodes which are previous to cached
node. So, even if there is free iova space available in the nodes
which are next to the cached node iova allocation can still fail
because of this approach.

Consider the following sequence of iova alloc and frees on
1GB of iova space

1) alloc - 500MB
2) alloc - 12MB
3) alloc - 499MB
4) free -  12MB which was allocated in step 2
5) alloc - 13MB

After the above sequence we will have 12MB of free iova space and
cached node will be pointing to the iova pfn of last alloc of 13MB
which will be the lowest iova pfn of that iova space. Now if we get an
alloc request of 2MB we just search from cached node and then look
for lower iova pfn's for free iova and as they aren't any, iova alloc
fails though there is 12MB of free iova space.

To avoid such iova search failures do a retry from the last rb tree node
when iova search fails, this will search the entire tree and get an iova
if its available

Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
---
 drivers/iommu/iova.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 0e6a953..2985222 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -186,6 +186,7 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
 	unsigned long flags;
 	unsigned long new_pfn;
 	unsigned long align_mask = ~0UL;
+	bool retry = false;
 
 	if (size_aligned)
 		align_mask <<= fls_long(size - 1);
@@ -198,6 +199,8 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
 
 	curr = __get_cached_rbnode(iovad, limit_pfn);
 	curr_iova = rb_entry(curr, struct iova, node);
+
+retry_search:
 	do {
 		limit_pfn = min(limit_pfn, curr_iova->pfn_lo);
 		new_pfn = (limit_pfn - size) & align_mask;
@@ -207,6 +210,14 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
 	} while (curr && new_pfn <= curr_iova->pfn_hi);
 
 	if (limit_pfn < size || new_pfn < iovad->start_pfn) {
+		if (!retry) {
+			curr = rb_last(&iovad->rbroot);
+			curr_iova = rb_entry(curr, struct iova, node);
+			limit_pfn = curr_iova->pfn_lo;
+			retry = true;
+			goto retry_search;
+		}
+
 		iovad->max32_alloc_size = size;
 		goto iova32_full;
 	}
-- 
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