[PATCH v12 27/29] HMM: fork copy migrated memory into system memory for child process.

[Jérôme Glisse]

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 <jgli...@redhat.com>
---
 mm/hmm.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 435e376..4dcd98f 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -483,7 +483,37 @@ 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 = kcalloc(npages, sizeof(*dst), GFP_KERNEL);
+       if (!dst) {
+               hmm_unref(hmm);
+               return -ENOMEM;
+       }
+       new_pte = kcalloc(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);
 
@@ -665,6 +695,12 @@ 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);
-- 
2.4.3