[PATCH RFC 2/2] mm: Use pte markers for swap errors

[Peter Xu]

PTE markers are ideal mechanism for things like SWP_SWAPIN_ERROR.  Using a
whole swap entry type for this purpose can be an overkill, especially if we
already have PTE markers.  Define a new bit for swapin error and replace it
with pte markers.  Then we can safely drop SWP_SWAPIN_ERROR and give one
device slot back to swap.

We used to have SWP_SWAPIN_ERROR taking the page pfn as part of the swap
entry, but it's never used.  Neither do I see how it can be useful because
normally the swapin failure should not be caused by a bad page but bad swap
device.  Drop it alongside.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 include/linux/swap.h    |  6 +-----
 include/linux/swapops.h | 26 ++++++++++++++------------
 mm/memory.c             |  6 ++++--
 mm/shmem.c              |  2 +-
 mm/swapfile.c           |  2 +-
 5 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 211aeca9bfa7..fec6647a289a 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -55,10 +55,6 @@ static inline int current_is_kswapd(void)
  * actions on faults.
  */
 
-#define SWP_SWAPIN_ERROR_NUM 1
-#define SWP_SWAPIN_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
-			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
-			     SWP_PTE_MARKER_NUM)
 /*
  * PTE markers are used to persist information onto PTEs that otherwise
  * should be a none pte.  As its name "PTE" hints, it should only be
@@ -121,7 +117,7 @@ static inline int current_is_kswapd(void)
 #define MAX_SWAPFILES \
 	((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
 	SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - \
-	SWP_PTE_MARKER_NUM - SWP_SWAPIN_ERROR_NUM)
+	SWP_PTE_MARKER_NUM)
 
 /*
  * Magic header for a swap area. The first part of the union is
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 41a6e7b23415..dd087b7287b0 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -162,16 +162,6 @@ static inline void *swp_to_radix_entry(swp_entry_t entry)
 	return xa_mk_value(entry.val);
 }
 
-static inline swp_entry_t make_swapin_error_entry(struct page *page)
-{
-	return swp_entry(SWP_SWAPIN_ERROR, page_to_pfn(page));
-}
-
-static inline int is_swapin_error_entry(swp_entry_t entry)
-{
-	return swp_type(entry) == SWP_SWAPIN_ERROR;
-}
-
 #if IS_ENABLED(CONFIG_DEVICE_PRIVATE)
 static inline swp_entry_t make_readable_device_private_entry(pgoff_t offset)
 {
@@ -409,8 +399,9 @@ static inline bool is_migration_entry_dirty(swp_entry_t entry)
 
 typedef unsigned long pte_marker;
 
-#define  PTE_MARKER_UFFD_WP  BIT(0)
-#define  PTE_MARKER_MASK     (PTE_MARKER_UFFD_WP)
+#define  PTE_MARKER_UFFD_WP			BIT(0)
+#define  PTE_MARKER_SWAP_ERROR			BIT(1)
+#define  PTE_MARKER_MASK			(BIT(2) - 1)
 
 static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
 {
@@ -437,6 +428,17 @@ static inline pte_t make_pte_marker(pte_marker marker)
 	return swp_entry_to_pte(make_pte_marker_entry(marker));
 }
 
+static inline swp_entry_t make_swapin_error_entry(void)
+{
+	return make_pte_marker_entry(PTE_MARKER_SWAP_ERROR);
+}
+
+static inline int is_swapin_error_entry(swp_entry_t entry)
+{
+	return is_pte_marker_entry(entry) &&
+	    (pte_marker_get(entry) & PTE_MARKER_SWAP_ERROR);
+}
+
 /*
  * This is a special version to check pte_none() just to cover the case when
  * the pte is a pte marker.  It existed because in many cases the pte marker
diff --git a/mm/memory.c b/mm/memory.c
index 71114f05a68c..cf602c3854c3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3705,6 +3705,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
 	if (WARN_ON_ONCE(!marker))
 		return VM_FAULT_SIGBUS;
 
+	/* Higher priority than uffd-wp when data corrupted */
+	if (marker & PTE_MARKER_SWAP_ERROR)
+		return VM_FAULT_SIGBUS;
+
 	if (pte_marker_entry_uffd_wp(entry))
 		return pte_marker_handle_uffd_wp(vmf);
 
@@ -3764,8 +3768,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			put_page(vmf->page);
 		} else if (is_hwpoison_entry(entry)) {
 			ret = VM_FAULT_HWPOISON;
-		} else if (is_swapin_error_entry(entry)) {
-			ret = VM_FAULT_SIGBUS;
 		} else if (is_pte_marker_entry(entry)) {
 			ret = handle_pte_marker(vmf);
 		} else {
diff --git a/mm/shmem.c b/mm/shmem.c
index c5f7a6668416..47dc595e98a3 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1682,7 +1682,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 	swp_entry_t swapin_error;
 	void *old;
 
-	swapin_error = make_swapin_error_entry(&folio->page);
+	swapin_error = make_swapin_error_entry();
 	old = xa_cmpxchg_irq(&mapping->i_pages, index,
 			     swp_to_radix_entry(swap),
 			     swp_to_radix_entry(swapin_error), 0);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5fc1237a9f21..13f2569a6719 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1781,7 +1781,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
 		pte_t pteval;
 
 		dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
-		pteval = swp_entry_to_pte(make_swapin_error_entry(page));
+		pteval = swp_entry_to_pte(make_swapin_error_entry());
 		set_pte_at(vma->vm_mm, addr, pte, pteval);
 		swap_free(entry);
 		ret = 0;
-- 
2.37.3