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Subject[PATCH 26/36] HMM: fork copy migrated memory into system memory for child process.
Date
From: Jérôme Glisse <jglisse@redhat.com>

When forking if process being fork had any memory migrated to some
device memory, we need to make a system copy for the child process.
Latter patches can revisit this and use the same COW semantic for
device memory.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
mm/hmm.c | 38 +++++++++++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 1208f64..143c6ab 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -487,7 +487,37 @@ int hmm_mm_fork(struct mm_struct *src_mm,
unsigned long start,
unsigned long end)
{
- return -ENOMEM;
+ unsigned long npages = (end - start) >> PAGE_SHIFT;
+ struct hmm_event event;
+ dma_addr_t *dst;
+ struct hmm *hmm;
+ pte_t *new_pte;
+ int ret;
+
+ hmm = hmm_ref(src_mm->hmm);
+ if (!hmm)
+ return -EINVAL;
+
+
+ dst = kzalloc(npages * sizeof(*dst), GFP_KERNEL);
+ if (!dst) {
+ hmm_unref(hmm);
+ return -ENOMEM;
+ }
+ new_pte = kzalloc(npages * sizeof(*new_pte), GFP_KERNEL);
+ if (!new_pte) {
+ kfree(dst);
+ hmm_unref(hmm);
+ return -ENOMEM;
+ }
+
+ hmm_event_init(&event, hmm, start, end, HMM_FORK);
+ ret = hmm_migrate_back(hmm, &event, dst_mm, dst_vma, new_pte,
+ dst, start, end);
+ hmm_unref(hmm);
+ kfree(new_pte);
+ kfree(dst);
+ return ret;
}
EXPORT_SYMBOL(hmm_mm_fork);

@@ -662,6 +692,12 @@ static void hmm_mirror_update_pte(struct hmm_mirror *mirror,
}

if (hmm_pte_test_valid_dev(hmm_pte)) {
+ /*
+ * On fork device memory is duplicated so no need to write
+ * protect it.
+ */
+ if (event->etype == HMM_FORK)
+ return;
*hmm_pte &= event->pte_mask;
if (!hmm_pte_test_valid_dev(hmm_pte))
hmm_pt_iter_directory_unref(iter, mirror->pt.llevel);
--
1.9.3


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