Am 26.10.2017 um 18:47 schrieb Felix Kuehling:
On 2017-10-26 03:33 AM, Christian König wrote:
Am 21.10.2017 um 02:23 schrieb Felix Kuehling:
The kfd_process doesn't own a reference to the mm_struct, so it can
disappear without warning even while the kfd_process still exists.
In fact, the delayed kfd_process teardown is triggered by an MMU
notifier when the mm_struct is destroyed. Permanently holding a
reference to the mm_struct would prevent this from happening.
Therefore, avoid dereferencing the kfd_process.mm pointer and make
it opaque. Use get_task_mm to get a temporary reference to the mm
when it's needed.
Actually that patch is unnecessary.
Process tear down (and calling the MMU release callback) is triggered
when mm_struct->mm_users reaches zero.
The mm_struct is freed up when mm_struct->mm_count becomes zero.
So what you can do is grab a reference to the mm_struct with mmgrab()
and still be notified by process tear down.
Hmm, the mm_struct has two different reference counters. So if I grab
the right type of reference it would work. Either way, one of two
changes is needed:
* Take a permanent reference to the mm-struct, or
* Don't dereference the mm pointer because I'm not holding a reference
IMO, KFD doesn't need to hold a reference to the mm_struct permanently.
Ah! From the patch description I was assuming that you took a reference
(but the wrong one) and tried to fix a memory leak with that approach.
So I believe my change still makes sense. I should probably just remove
the comment "Permanently holding a reference to the mm_struct would
prevent ...". Like you say, that's not accurate.
Yeah, good idea.
But now reading the patch there is something else which I stumbled over:
+ /*
+ * This cast should be safe here because we grabbed a
+ * reference to the mm in kfd_process_notifier_release
+ */
+ WARN_ON(atomic_read(&((struct mm_struct *)p->mm)->mm_count) <= 0);
mmdrop(p->mm);
Well that isn't good coding style. You shouldn't obfuscate what pointer
it is by changing it to "void*", but rather set it to NULL as soon as
you know that it is stale.
Additional to that it is certainly not job of the driver to warn on a
run over mm_count.
Regards,
Christian.
Regards,
Felix
Regards,
Christian.
Signed-off-by: Felix Kuehling <[email protected]>
---
drivers/gpu/drm/amd/amdkfd/kfd_events.c | 19 +++++++++++++++----
drivers/gpu/drm/amd/amdkfd/kfd_priv.h | 7 ++++++-
drivers/gpu/drm/amd/amdkfd/kfd_process.c | 6 +++++-
3 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_events.c
b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
index 944abfa..61ce547 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_events.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_events.c
@@ -24,8 +24,8 @@
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/sched/signal.h>
+#include <linux/sched/mm.h>
#include <linux/uaccess.h>
-#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/memory.h>
#include "kfd_priv.h"
@@ -904,14 +904,24 @@ void kfd_signal_iommu_event(struct kfd_dev
*dev, unsigned int pasid,
* running so the lookup function returns a locked process.
*/
struct kfd_process *p = kfd_lookup_process_by_pasid(pasid);
+ struct mm_struct *mm;
if (!p)
return; /* Presumably process exited. */
+ /* Take a safe reference to the mm_struct, which may otherwise
+ * disappear even while the kfd_process is still referenced.
+ */
+ mm = get_task_mm(p->lead_thread);
+ if (!mm) {
+ mutex_unlock(&p->mutex);
+ return; /* Process is exiting */
+ }
+
memset(&memory_exception_data, 0, sizeof(memory_exception_data));
- down_read(&p->mm->mmap_sem);
- vma = find_vma(p->mm, address);
+ down_read(&mm->mmap_sem);
+ vma = find_vma(mm, address);
memory_exception_data.gpu_id = dev->id;
memory_exception_data.va = address;
@@ -937,7 +947,8 @@ void kfd_signal_iommu_event(struct kfd_dev *dev,
unsigned int pasid,
}
}
- up_read(&p->mm->mmap_sem);
+ up_read(&mm->mmap_sem);
+ mmput(mm);
mutex_lock(&p->event_mutex);
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
index 7d86ec9..1a483a7 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
@@ -494,7 +494,12 @@ struct kfd_process {
*/
struct hlist_node kfd_processes;
- struct mm_struct *mm;
+ /*
+ * Opaque pointer to mm_struct. We don't hold a reference to
+ * it so it should never be dereferenced from here. This is
+ * only used for looking up processes by their mm.
+ */
+ void *mm;
struct mutex mutex;
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
index 3ccb3b5..21d27e5 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
@@ -200,7 +200,11 @@ static void kfd_process_destroy_delayed(struct
rcu_head *rcu)
struct kfd_process *p;
p = container_of(rcu, struct kfd_process, rcu);
- WARN_ON(atomic_read(&p->mm->mm_count) <= 0);
+ /*
+ * This cast should be safe here because we grabbed a
+ * reference to the mm in kfd_process_notifier_release
+ */
+ WARN_ON(atomic_read(&((struct mm_struct *)p->mm)->mm_count) <= 0);
mmdrop(p->mm);
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