Re: [PATCH] mm: pgtable: Unlock pml without branches when !start_pte

[Andrew Morton]

On Mon, 10 Feb 2025 18:09:48 +0800 I Hsin Cheng <richard120310@xxxxxxxxx> wrote:

> When !start_pte is true, the branch for "start_pte" in "out_ptl" label
> section is surely false, and "ptl != pml" must be true since "ptl" is
> NULL in this case.
> 
> It means both branches in "out_ptl" are redundant, only one thing to be
> done is to unlock "pml", make it directly unlock "pml" and return in
> this case.

Hopefully the compiler will skip the `if (start_pte)' test.

Generally, we try to avoid multiple function return points.  We could do

--- a/mm/pt_reclaim.c~mm-pgtable-unlock-pml-without-branches-when-start_pte
+++ a/mm/pt_reclaim.c
@@ -43,7 +43,7 @@ void try_to_free_pte(struct mm_struct *m
 	pml = pmd_lock(mm, pmd);
 	start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pmdval, &ptl);
 	if (!start_pte)
-		goto out_ptl;
+		goto out_unlock;
 	if (ptl != pml)
 		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
 
@@ -67,5 +67,6 @@ out_ptl:
 	if (start_pte)
 		pte_unmap_unlock(start_pte, ptl);
 	if (ptl != pml)
+out_unlock:
 		spin_unlock(pml);
 }
_

but that's really ugly.