[PATCH v2 5/9] mm/swap: introduce swapin_entry for unified readahead policy

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Introduce swapin_entry which merges swapin_readahead and swapin_direct
making it the main entry for swapin pages, and use a unified swapin
policy.

This commit makes swapoff make use of this new helper and now swapping
off a 10G ZRAM (lzo-rle) is faster since readahead is skipped.

Before:
time swapoff /dev/zram0
real    0m12.337s
user    0m0.001s
sys     0m12.329s

After:
time swapoff /dev/zram0
real    0m9.728s
user    0m0.001s
sys     0m9.719s

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/memory.c     | 21 +++++++--------------
 mm/swap.h       | 16 ++++------------
 mm/swap_state.c | 49 +++++++++++++++++++++++++++++++++----------------
 mm/swapfile.c   |  7 ++-----
 4 files changed, 46 insertions(+), 47 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0165c8cad489..b56254a875f8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3801,6 +3801,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	rmap_t rmap_flags = RMAP_NONE;
 	bool exclusive = false;
 	swp_entry_t entry;
+	bool swapcached;
 	pte_t pte;
 	vm_fault_t ret = 0;
 
@@ -3864,21 +3865,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	swapcache = folio;
 
 	if (!folio) {
-		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
-		    __swap_count(entry) == 1) {
-			/* skip swapcache and readahead */
-			folio = swapin_direct(entry, GFP_HIGHUSER_MOVABLE, vmf);
-			if (folio)
-				page = &folio->page;
+		folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
+				     vmf, &swapcached);
+		if (folio) {
+			page = folio_file_page(folio, swp_offset(entry));
+			if (swapcached)
+				swapcache = folio;
 		} else {
-			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
-						vmf);
-			if (page)
-				folio = page_folio(page);
-			swapcache = folio;
-		}
-
-		if (!folio) {
 			/*
 			 * Back out if somebody else faulted in this pte
 			 * while we released the pte lock.
diff --git a/mm/swap.h b/mm/swap.h
index 83eab7b67e77..502a2801f817 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -54,10 +54,8 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_flags,
 		bool skip_if_exists);
 struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
 		struct mempolicy *mpol, pgoff_t ilx);
-struct page *swapin_readahead(swp_entry_t entry, gfp_t flag,
-			      struct vm_fault *vmf);
-struct folio *swapin_direct(swp_entry_t entry, gfp_t flag,
-			    struct vm_fault *vmf);
+struct folio *swapin_entry(swp_entry_t entry, gfp_t flag,
+			    struct vm_fault *vmf, bool *swapcached);
 
 static inline unsigned int folio_swap_flags(struct folio *folio)
 {
@@ -88,14 +86,8 @@ static inline struct folio *swap_cluster_readahead(swp_entry_t entry,
 	return NULL;
 }
 
-struct folio *swapin_direct(swp_entry_t entry, gfp_t flag,
-			struct vm_fault *vmf)
-{
-	return NULL;
-}
-
-static inline struct page *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
-			struct vm_fault *vmf)
+static inline struct folio *swapin_entry(swp_entry_t swp, gfp_t gfp_mask,
+			struct vm_fault *vmf, bool *swapcached)
 {
 	return NULL;
 }
diff --git a/mm/swap_state.c b/mm/swap_state.c
index d39c5369da21..66ff187aa5d3 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -316,6 +316,11 @@ void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
 	release_pages(pages, nr);
 }
 
+static inline bool swap_use_no_readahead(struct swap_info_struct *si, swp_entry_t entry)
+{
+	return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1;
+}
+
 static inline bool swap_use_vma_readahead(void)
 {
 	return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
@@ -870,8 +875,8 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
  * Returns the struct folio for entry and addr after the swap entry is read
  * in.
  */
-struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
-			    struct vm_fault *vmf)
+static struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
+				  struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct folio *folio;
@@ -908,33 +913,45 @@ struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
 }
 
 /**
- * swapin_readahead - swap in pages in hope we need them soon
+ * swapin_entry - swap in a page from swap entry
  * @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.
  *
  * Returns the struct page for entry and addr, after queueing swapin.
  *
- * It's a main entry function for swap readahead. By the configuration,
+ * It's a main entry function for swap in. By the configuration,
  * it will read ahead blocks by cluster-based(ie, physical disk based)
- * or vma-based(ie, virtual address based on faulty address) readahead.
+ * or vma-based(ie, virtual address based on faulty address) readahead,
+ * or skip the readahead (ie, ramdisk based swap device).
  */
-struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
-				struct vm_fault *vmf)
+struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
+			   struct vm_fault *vmf, bool *swapcached)
 {
 	struct mempolicy *mpol;
-	pgoff_t ilx;
 	struct folio *folio;
+	pgoff_t ilx;
+	bool cached;
 
-	mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
-	folio = swap_use_vma_readahead() ?
-		swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf) :
-		swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
-	mpol_cond_put(mpol);
+	if (swap_use_no_readahead(swp_swap_info(entry), entry)) {
+		folio = swapin_direct(entry, gfp_mask, vmf);
+		cached = false;
+	} else {
+		mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
+		if (swap_use_vma_readahead())
+			folio = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
+		else
+			folio = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
+		mpol_cond_put(mpol);
+		cached = true;
+	}
 
-	if (!folio)
-		return NULL;
-	return folio_file_page(folio, swp_offset(entry));
+	if (swapcached)
+		*swapcached = cached;
+
+	return folio;
 }
 
 #ifdef CONFIG_SYSFS
diff --git a/mm/swapfile.c b/mm/swapfile.c
index f7271504aa0a..ce4e6c10dce7 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1866,7 +1866,6 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 
 		folio = swap_cache_get_folio(entry, vma, addr);
 		if (!folio) {
-			struct page *page;
 			struct vm_fault vmf = {
 				.vma = vma,
 				.address = addr,
@@ -1874,10 +1873,8 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				.pmd = pmd,
 			};
 
-			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
-						&vmf);
-			if (page)
-				folio = page_folio(page);
+			folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
+					    &vmf, NULL);
 		}
 		if (!folio) {
 			/*
-- 
2.43.0