[PATCH RFC v3 12/35] mm: Call arch_swap_prepare_to_restore() before arch_swap_restore()

[Alexandru Elisei <alexandru.elisei@arm.com>]

arm64 uses arch_swap_restore() to restore saved tags before the page is
swapped in and it's called in atomic context (with the ptl lock held).

Introduce arch_swap_prepare_to_restore() that will allow an architecture to
perform extra work during swap in and outside of a critical section.
This will be used by arm64 to allocate a buffer in memory where to
temporarily save tags if tag storage is not available for the page being
swapped in.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---
 include/linux/pgtable.h | 7 +++++++
 mm/memory.c             | 4 ++++
 mm/shmem.c              | 9 +++++++++
 mm/swapfile.c           | 5 +++++
 4 files changed, 25 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 98f81ca08cbe..2d0f04042f62 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -959,6 +959,13 @@ static inline void arch_swap_invalidate_area(int type)
 }
 #endif
 
+#ifndef __HAVE_ARCH_SWAP_PREPARE_TO_RESTORE
+static inline vm_fault_t arch_swap_prepare_to_restore(swp_entry_t entry, struct folio *folio)
+{
+	return 0;
+}
+#endif
+
 #ifndef __HAVE_ARCH_SWAP_RESTORE
 static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
 {
diff --git a/mm/memory.c b/mm/memory.c
index 7e1f4849463a..8a421e168b57 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3975,6 +3975,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 
 	folio_throttle_swaprate(folio, GFP_KERNEL);
 
+	ret = arch_swap_prepare_to_restore(entry, folio);
+	if (ret)
+		goto out_page;
+
 	/*
 	 * Back out if somebody else already faulted in this pte.
 	 */
diff --git a/mm/shmem.c b/mm/shmem.c
index 14427e9982f9..621fabc3b8c6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1855,6 +1855,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	struct swap_info_struct *si;
 	struct folio *folio = NULL;
 	swp_entry_t swap;
+	vm_fault_t ret;
 	int error;
 
 	VM_BUG_ON(!*foliop || !xa_is_value(*foliop));
@@ -1903,6 +1904,14 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	}
 	folio_wait_writeback(folio);
 
+	ret = arch_swap_prepare_to_restore(swap, folio);
+	if (ret) {
+		if (fault_type)
+			*fault_type = ret;
+		error = -EINVAL;
+		goto unlock;
+	}
+
 	/*
 	 * Some architectures may have to restore extra metadata to the
 	 * folio after reading from swap.
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 556ff7347d5f..49425598f778 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1785,6 +1785,11 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
 		goto setpte;
 	}
 
+	if (arch_swap_prepare_to_restore(entry, folio)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	/*
 	 * Some architectures may have to restore extra metadata to the page
 	 * when reading from swap. This metadata may be indexed by swap entry
-- 
2.43.0