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Subject[PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup
hmm_devmem_find() requires rcu_read_lock_held() but there's nothing
which actually uses the RCU protection. The only caller is
hmm_devmem_pages_create() which already grabs the mutex and does
superflous rcu_read_lock/unlock() around the function.

This doesn't add anything and just adds to confusion. Remove the RCU
protection and open-code the radix tree lookup. If this needs to
become more sophisticated in the future, let's add them back when

Signed-off-by: Tejun Heo <>
Cc: Jérôme Glisse <>
Cc: Linus Torvalds <>
Hello, Jérôme.

This came up while auditing percpu_ref users for missing explicit RCU
grace periods. HMM doesn't seem to depend on RCU protection at all,
so I thought it'd be better to remove it for now. It's only compile


mm/hmm.c | 12 ++----------
1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 320545b98..d4627c5 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data)

-static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
- WARN_ON_ONCE(!rcu_read_lock_held());
- return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
resource_size_t key, align_start, align_size, align_end;
@@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
struct hmm_devmem *dup;

- rcu_read_lock();
- dup = hmm_devmem_find(key);
- rcu_read_unlock();
+ dup = radix_tree_lookup(&hmm_devmem_radix,
if (dup) {
dev_err(device, "%s: collides with mapping for %s\n",
__func__, dev_name(dup->device));
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