mm: thp: enable THP allocation exclusively through khugepaged

khugepaged_enter_vma() ultimately invokes any attached BPF function with
the TVA_KHUGEPAGED flag set when determining whether or not to enable
khugepaged THP for a freshly faulted in VMA.

Currently, on fault, we invoke this in do_huge_pmd_anonymous_page(), as
invoked by create_huge_pmd() and only when we have already checked to
see if an allowable TVA_PAGEFAULT order is specified.

Since we might want to disallow THP on fault-in but allow it via
khugepaged, we move things around so we always attempt to enter
khugepaged upon fault.

This change is safe because:
- khugepaged operates at the MM level rather than per-VMA. The THP
  allocation might fail during page faults due to transient conditions
  (e.g., memory pressure), it is safe to add this MM to khugepaged for
  subsequent defragmentation.
- If __thp_vma_allowable_orders(TVA_PAGEFAULT) returns 0, then
  __thp_vma_allowable_orders(TVA_KHUGEPAGED) will also return 0.

While we could also extend prctl() to utilize this new policy, such a
change would require a uAPI modification to PR_SET_THP_DISABLE.

Signed-off-by: Yafang Shao <[email protected]>
Acked-by: Lance Yang <[email protected]>
Cc: Usama Arif <[email protected]>

Author: laoar

mm/huge_memory.c

@@ -1346,7 +1346,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 	ret = vmf_anon_prepare(vmf);
 	if (ret)
 		return ret;
-	khugepaged_enter_vma(vma);
 
 	if (!(vmf->flags & FAULT_FLAG_WRITE) &&
 			!mm_forbids_zeropage(vma->vm_mm) &&

mm/memory.c

@@ -6283,11 +6283,14 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
 	if (pud_trans_unstable(vmf.pud))
 		goto retry_pud;
 
-	if (pmd_none(*vmf.pmd) &&
-	    thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) {
-		ret = create_huge_pmd(&vmf);
-		if (!(ret & VM_FAULT_FALLBACK))
-			return ret;
+	if (pmd_none(*vmf.pmd)) {
+		if (vma_is_anonymous(vma))
+			khugepaged_enter_vma(vma);
+		if (thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) {
+			ret = create_huge_pmd(&vmf);
+			if (!(ret & VM_FAULT_FALLBACK))
+				return ret;
+		}
 	} else {
 		vmf.orig_pmd = pmdp_get_lockless(vmf.pmd);