shmem: support huge_fault to avoid pmd split

Transparent hugepage of tmpfs is useful to improve TLB miss, but
it will be split during cow memory fault.
This will happen if we mprotect and rewrite code segment (which is
private file map) to hotpatch a running process.

Users of huge= mount option prefer huge pages after cow.
We can avoid the splitting by adding a huge_fault function.

mm/shmem.c

@@ -2130,6 +2130,50 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
 	return ret;
 }
 
+static vm_fault_t shmem_huge_fault(struct vm_fault *vmf, enum page_entry_size pe_size)
+{
+	vm_fault_t ret = VM_FAULT_FALLBACK;
+	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	struct page *old_page, *new_page;
+	int gfp_flags = GFP_HIGHUSER_MOVABLE | __GFP_COMP;
+
+	/* read or shared fault will not split huge pmd */
+	if (!(vmf->flags & FAULT_FLAG_WRITE)
+			|| (vmf->vma->vm_flags & VM_SHARED))
+		return VM_FAULT_FALLBACK;
+	if (pe_size != PE_SIZE_PMD)
+		return VM_FAULT_FALLBACK;
+
+	if (pmd_none(*vmf->pmd)) {
+		if (shmem_fault(vmf) & VM_FAULT_ERROR)
+			goto out;
+		if (!PageTransHuge(vmf->page))
+			goto out;
+		old_page = vmf->page;
+	} else {
+		old_page = pmd_page(*vmf->pmd);
+		page_remove_rmap(old_page, vmf->vma, true);
+		pmdp_huge_clear_flush(vmf->vma, haddr, vmf->pmd);
+		add_mm_counter(vmf->vma->vm_mm, MM_SHMEMPAGES, -HPAGE_PMD_NR);
+	}
+
+	new_page = &vma_alloc_folio(gfp_flags, HPAGE_PMD_ORDER,
+			vmf->vma, haddr, true)->page;
+	if (!new_page)
+		goto out;
+	copy_user_huge_page(new_page, old_page, haddr, vmf->vma, HPAGE_PMD_NR);
+	__SetPageUptodate(new_page);
+
+	ret = do_set_pmd(vmf, new_page);
+
+out:
+	if (vmf->page) {
+		unlock_page(vmf->page);
+		put_page(vmf->page);
+	}
+	return ret;
+}
+
 unsigned long shmem_get_unmapped_area(struct file *file,
 				      unsigned long uaddr, unsigned long len,
 				      unsigned long pgoff, unsigned long flags)
@@ -3943,6 +3987,7 @@ static const struct super_operations shmem_ops = {
 
 static const struct vm_operations_struct shmem_vm_ops = {
 	.fault		= shmem_fault,
+	.huge_fault	= shmem_huge_fault,
 	.map_pages	= filemap_map_pages,
 #ifdef CONFIG_NUMA
 	.set_policy     = shmem_set_policy,