[RFC PATCH 4/5] mm: userfaultfd: support continue for guest_memfd

[Nikita Kalyazin]

When userspace receives a page missing event, it is supposed to populate
the missing page in guest_memfd pagecache via the write syscall and
unblock the faulting process via UFFDIO_CONTINUE.

Signed-off-by: Nikita Kalyazin <kalya...@amazon.com>
---
 mm/userfaultfd.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index af3dfc3633db..aaff66a7f15b 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -19,6 +19,10 @@
 #include <asm/tlb.h>
 #include "internal.h"
 
+#ifdef CONFIG_KVM_PRIVATE_MEM
+bool kvm_gmem_vma_is_gmem(struct vm_area_struct *vma);
+#endif
+
 static __always_inline
 bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
 {
@@ -391,6 +395,16 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
        struct page *page;
        int ret;
 
+#ifdef CONFIG_KVM_PRIVATE_MEM
+       if (kvm_gmem_vma_is_gmem(dst_vma)) {
+               ret = 0;
+               folio = filemap_get_entry(inode->i_mapping, pgoff);
+               if (IS_ERR(folio))
+                       ret = PTR_ERR(folio);
+               else
+                       folio_lock(folio);
+       } else
+#endif
        ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
        /* Our caller expects us to return -EFAULT if we failed to find folio */
        if (ret == -ENOENT)
@@ -769,9 +783,16 @@ static __always_inline ssize_t mfill_atomic(struct 
userfaultfd_ctx *ctx,
                return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
                                             src_start, len, flags);
 
-       if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
+       if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma)
+#ifdef CONFIG_KVM_PRIVATE_MEM
+               && !kvm_gmem_vma_is_gmem(dst_vma)
+#endif
+       )
                goto out_unlock;
        if (!vma_is_shmem(dst_vma) &&
+#ifdef CONFIG_KVM_PRIVATE_MEM
+               !kvm_gmem_vma_is_gmem(dst_vma) &&
+#endif
            uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
                goto out_unlock;
 
-- 
2.47.1