Re: [PATCH 1/1] mm,swapops: Fix compile error on set_pmd_migration_entry.

[Barry Song <21cnbao@gmail.com>]

On Mon, Dec 29, 2025 at 10:37 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Sat, 27 Dec 2025 17:58:40 +0800 wale zhang <wale.zhang.ftd@gmail.com> wrote:
>
> > > No. You still don’t understand my point. The BUILD_BUG means this
> > > code will never have a chance to be compiled under any possible
> > > kernel configuration. That makes it dead code. If there is any
> > > kernel .config in which it can be built, then we should fix it.
> > >
> > > This set_pmd_migration_entry() has no chance of ever being built.
> >
> > Hello Barry,
> >
>
> Please share the .config which was used to trigger this build error.
>
> (It is strange to have this should-never-be-compiled code.  Why not just
> delete it and let the compiler/linker report the error.)

Right. We should entirely drop it in mainline. If Wale does want an
O0 build, then the only call to set_pmd_migration_entry() that is not
under

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION

is migrate_vma_collect_huge_pmd() in mm/migrate_device.c.

Then what we really need is as follows:

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 8cfc966eae48..fdd4375d5d90 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -339,17 +339,6 @@ static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
 }
 
 #else  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
-static inline int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
-		struct page *page)
-{
-	BUILD_BUG();
-}
-
-static inline void remove_migration_pmd(struct page_vma_mapped_walk *pvmw,
-		struct page *new)
-{
-	BUILD_BUG();
-}
 
 static inline void pmd_migration_entry_wait(struct mm_struct *m, pmd_t *p) { }
 
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 23379663b1e1..74d23c91626a 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -195,8 +195,9 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
 		return migrate_vma_collect_skip(start, end, walk);
 	}
 
-	if (thp_migration_supported() &&
-		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
+
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+	if ((migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
 		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
 		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
 
@@ -226,6 +227,7 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
 		spin_unlock(ptl);
 		return 0;
 	}
+#endif
 
 fallback:
 	spin_unlock(ptl);

Thanks
Barry