Re: [PATCH v3 11/15] mm, swap: use the swap table for the swap cache and switch API

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Sep 11, 2025 at 10:54:19AM +0800, Lance Yang wrote:
>
>
> On 2025/9/11 10:48, Kairui Song wrote:
> > On Thu, Sep 11, 2025 at 10:34 AM Lance Yang <lance.yang@linux.dev> wrote:
> > >
> > > On Thu, Sep 11, 2025 at 10:27 AM Lance Yang <lance.yang@linux.dev> wrote:
> > > >
> > > > Hi Kairui,
> > > >
> > > > I'm hitting a build error with allnoconfig:
> > > >
> > > > In file included from mm/shmem.c:44:
> > > > mm/swap.h: In function ‘folio_index’:
> > > > mm/swap.h:462:24: error: implicit declaration of function
> > > > ‘swp_offset’; did you mean ‘pmd_offset’?
> > > > [-Wimplicit-function-declaration]
> > > > 462 | return swp_offset(folio->swap);
> > > >
> > > > It looks like a header might be missing in mm/swap.h. Please let me know
> > > > if you need any more information.
> > >
> > > Confirmed that just adding #include <linux/swapops.h> into mm/swap.h fixes it.
> > >
> > > diff --git a/mm/swap.h b/mm/swap.h
> > > index ad339547ee8c..271e8c560fcc 100644
> > > --- a/mm/swap.h
> > > +++ b/mm/swap.h
> > > @@ -3,6 +3,7 @@
> > >   #define _MM_SWAP_H
> > >
> > >   #include <linux/atomic.h> /* for atomic_long_t */
> > > +#include <linux/swapops.h>
> > >   struct mempolicy;
> > >   struct swap_iocb;
> > >
> > > Cheers,
> > > Lance
> > >
> >
> > Hi Lance,
> >
> > You are testing V2 not V3 right? The build error is in V2 and I can
> > confirm that. But In V3 I've added "ifdef CONFIG_SWAP" for the
> > swp_offset usage in swap.h. I've just tested allnoconfig and it
> > works fine on aarch64 and x86_64.
> >
> > V2: https://lore.kernel.org/linux-mm/20250905191357.78298-12-ryncsn@gmail.com/
>
>
> Ah, I was testing V2. My apologies for the noise!
>

OK so my bug is different then :)

I think the issue is that you're not properly checking for:

#ifdef CONFIG_MMU
...
#endif

This is v3 (I see the #ifdef CONFIG_SWAP) btw.

So swapops.h is wrapped by #ifdef CONFIG_MMU which is why the declaration is
missing.

The below fixpatch fixes things for me.

Cheers, Lorenzo

----8<----
From 5a3969a438af9c33422a45fe813a44068d784b2f Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Date: Fri, 12 Sep 2025 10:28:40 +0100
Subject: [PATCH] fix

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 arch/sparc/include/asm/pgtable_64.h |  3 +++
 mm/swap.h                           | 37 +++++++++++++++++------------
 2 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index b8000ce4b59f..b06f55915653 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -1050,6 +1050,9 @@ int page_in_phys_avail(unsigned long paddr);

 int remap_pfn_range(struct vm_area_struct *, unsigned long, unsigned long,
 		   unsigned long, pgprot_t);
+void remap_pfn_range_prepare(struct vm_area_desc *desc, unsigned long pfn);
+int remap_pfn_range_complete(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t pgprot);

 void adi_restore_tags(struct mm_struct *mm, struct vm_area_struct *vma,
 		     unsigned long addr, pte_t pte);
diff --git a/mm/swap.h b/mm/swap.h
index caff4fe30fc5..18651687fcd1 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -53,7 +53,7 @@ enum swap_cluster_flags {
 	CLUSTER_FLAG_MAX,
 };

-#ifdef CONFIG_SWAP
+#if defined(CONFIG_SWAP) && defined(CONFIG_MMU)
 #include <linux/swapops.h> /* for swp_offset */
 #include <linux/blk_types.h> /* for bio_end_io_t */

@@ -317,7 +317,26 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
 	return i;
 }

+/**
+ * folio_index - File index of a folio.
+ * @folio: The folio.
+ *
+ * For a folio which is either in the page cache or the swap cache,
+ * return its index within the address_space it belongs to.  If you know
+ * the folio is definitely in the page cache, you can look at the folio's
+ * index directly.
+ *
+ * Return: The index (offset in units of pages) of a folio in its file.
+ */
+static inline pgoff_t folio_index(struct folio *folio)
+{
+	if (unlikely(folio_test_swapcache(folio)))
+		return swp_offset(folio->swap);
+	return folio->index;
+}
+
 #else /* CONFIG_SWAP */
+
 struct swap_iocb;
 static inline struct swap_cluster_info *swap_cluster_lock(
 	struct swap_info_struct *si, pgoff_t offset, bool irq)
@@ -442,24 +461,12 @@ static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
 {
 	return 0;
 }
-#endif /* CONFIG_SWAP */

-/**
- * folio_index - File index of a folio.
- * @folio: The folio.
- *
- * For a folio which is either in the page cache or the swap cache,
- * return its index within the address_space it belongs to.  If you know
- * the folio is definitely in the page cache, you can look at the folio's
- * index directly.
- *
- * Return: The index (offset in units of pages) of a folio in its file.
- */
 static inline pgoff_t folio_index(struct folio *folio)
 {
-	if (unlikely(folio_test_swapcache(folio)))
-		return swp_offset(folio->swap);
 	return folio->index;
 }

+#endif
+
 #endif /* _MM_SWAP_H */
--
2.51.0