[PATCH v2 6/9] mm/swap: handle swapcache lookup in swapin_entry

[Kairui Song]

From: Kairui Song <kasong@xxxxxxxxxxx>

Since all callers of swapin_entry need to check the swap cache first, we
can merge this common routine into swapin_entry, so it can be shared and
optimized later.

Also introduce a enum to better represent possible swap cache usage, and
add some comments about it, make the usage of swap cache easier to
understand.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/memory.c     | 45 ++++++++++++++++++++-------------------------
 mm/swap.h       | 20 ++++++++++++++++++--
 mm/swap_state.c | 22 ++++++++++++++--------
 mm/swapfile.c   | 21 +++++++++------------
 4 files changed, 61 insertions(+), 47 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index b56254a875f8..ab6e76c95632 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3795,13 +3795,13 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
 vm_fault_t do_swap_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
-	struct folio *swapcache, *folio = NULL;
+	struct folio *swapcache = NULL, *folio;
+	enum swap_cache_result cache_result;
 	struct page *page;
 	struct swap_info_struct *si = NULL;
 	rmap_t rmap_flags = RMAP_NONE;
 	bool exclusive = false;
 	swp_entry_t entry;
-	bool swapcached;
 	pte_t pte;
 	vm_fault_t ret = 0;
 
@@ -3859,31 +3859,26 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	if (unlikely(!si))
 		goto out;
 
-	folio = swap_cache_get_folio(entry, vma, vmf->address);
-	if (folio)
+	folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
+			     vmf, &cache_result);
+	if (folio) {
 		page = folio_file_page(folio, swp_offset(entry));
-	swapcache = folio;
-
-	if (!folio) {
-		folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
-				     vmf, &swapcached);
-		if (folio) {
-			page = folio_file_page(folio, swp_offset(entry));
-			if (swapcached)
-				swapcache = folio;
-		} else {
-			/*
-			 * Back out if somebody else faulted in this pte
-			 * while we released the pte lock.
-			 */
-			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
-					vmf->address, &vmf->ptl);
-			if (likely(vmf->pte &&
-				   pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
-				ret = VM_FAULT_OOM;
-			goto unlock;
-		}
+		if (cache_result != SWAP_CACHE_BYPASS)
+			swapcache = folio;
+	} else {
+		/*
+		 * Back out if somebody else faulted in this pte
+		 * while we released the pte lock.
+		 */
+		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
+				vmf->address, &vmf->ptl);
+		if (likely(vmf->pte &&
+			   pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
+			ret = VM_FAULT_OOM;
+		goto unlock;
+	}
 
+	if (cache_result != SWAP_CACHE_HIT) {
 		/* Had to read the page from swap area: Major fault */
 		ret = VM_FAULT_MAJOR;
 		count_vm_event(PGMAJFAULT);
diff --git a/mm/swap.h b/mm/swap.h
index 502a2801f817..1f4cdb324bf0 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -4,6 +4,22 @@
 
 struct mempolicy;
 
+/*
+ * Caller of swapin_entry may need to know the cache lookup result:
+ *
+ * SWAP_CACHE_HIT: cache hit, cached folio is retured.
+ * SWAP_CACHE_MISS: cache miss, folio is allocated, read from swap device
+ *                  and adde to swap cache, but still may return a cached
+ *                  folio if raced (check __read_swap_cache_async).
+ * SWAP_CACHE_BYPASS: cache miss, folio is new allocated and read
+ *                    from swap device bypassing the cache.
+ */
+enum swap_cache_result {
+	SWAP_CACHE_HIT,
+	SWAP_CACHE_MISS,
+	SWAP_CACHE_BYPASS,
+};
+
 #ifdef CONFIG_SWAP
 #include <linux/blk_types.h> /* for bio_end_io_t */
 
@@ -55,7 +71,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_flags,
 struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
 		struct mempolicy *mpol, pgoff_t ilx);
 struct folio *swapin_entry(swp_entry_t entry, gfp_t flag,
-			    struct vm_fault *vmf, bool *swapcached);
+			    struct vm_fault *vmf, enum swap_cache_result *result);
 
 static inline unsigned int folio_swap_flags(struct folio *folio)
 {
@@ -87,7 +103,7 @@ static inline struct folio *swap_cluster_readahead(swp_entry_t entry,
 }
 
 static inline struct folio *swapin_entry(swp_entry_t swp, gfp_t gfp_mask,
-			struct vm_fault *vmf, bool *swapcached)
+			struct vm_fault *vmf, enum swap_cache_result *result)
 {
 	return NULL;
 }
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 66ff187aa5d3..f6f1e6f5d782 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -917,8 +917,7 @@ static struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
  * @entry: swap entry of this memory
  * @gfp_mask: memory allocation flags
  * @vmf: fault information
- * @swapcached: pointer to a bool used as indicator if the
- *              page is swapped in through swapcache.
+ * @result: a return value to indicate swap cache usage.
  *
  * Returns the struct page for entry and addr, after queueing swapin.
  *
@@ -928,16 +927,22 @@ static struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
  * or skip the readahead (ie, ramdisk based swap device).
  */
 struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
-			   struct vm_fault *vmf, bool *swapcached)
+			   struct vm_fault *vmf, enum swap_cache_result *result)
 {
+	enum swap_cache_result cache_result;
 	struct mempolicy *mpol;
 	struct folio *folio;
 	pgoff_t ilx;
-	bool cached;
+
+	folio = swap_cache_get_folio(entry, vmf->vma, vmf->address);
+	if (folio) {
+		cache_result = SWAP_CACHE_HIT;
+		goto done;
+	}
 
 	if (swap_use_no_readahead(swp_swap_info(entry), entry)) {
 		folio = swapin_direct(entry, gfp_mask, vmf);
-		cached = false;
+		cache_result = SWAP_CACHE_BYPASS;
 	} else {
 		mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
 		if (swap_use_vma_readahead())
@@ -945,11 +950,12 @@ struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
 		else
 			folio = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
 		mpol_cond_put(mpol);
-		cached = true;
+		cache_result = SWAP_CACHE_MISS;
 	}
 
-	if (swapcached)
-		*swapcached = cached;
+done:
+	if (result)
+		*result = cache_result;
 
 	return folio;
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index ce4e6c10dce7..5aa44de11edc 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1845,6 +1845,13 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 		int ret;
 		pte_t ptent;
 
+		struct vm_fault vmf = {
+			.vma = vma,
+			.address = addr,
+			.real_address = addr,
+			.pmd = pmd,
+		};
+
 		if (!pte++) {
 			pte = pte_offset_map(pmd, addr);
 			if (!pte)
@@ -1864,18 +1871,8 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 		pte_unmap(pte);
 		pte = NULL;
 
-		folio = swap_cache_get_folio(entry, vma, addr);
-		if (!folio) {
-			struct vm_fault vmf = {
-				.vma = vma,
-				.address = addr,
-				.real_address = addr,
-				.pmd = pmd,
-			};
-
-			folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
-					    &vmf, NULL);
-		}
+		folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
+				     &vmf, NULL);
 		if (!folio) {
 			/*
 			 * The entry could have been freed, and will not
-- 
2.43.0