[PATCH 3/3] mm,swap: simplify VMA based swap readahead window calculation

[Huang Ying]

Replace PFNs with addresses in readahead window calculation.  This
simplified the logic and reduce the code line number.

No functionality change is expected.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Kairui Song <kasong@xxxxxxxxxxx>
Cc: Barry Song <v-songbaohua@xxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/swap_state.c | 66 +++++++++++++++++++------------------------------
 1 file changed, 25 insertions(+), 41 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index e1dac70198a6..d2adbd7b571b 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -740,54 +740,40 @@ void exit_swap_address_space(unsigned int type)
 	swapper_spaces[type] = NULL;
 }
 
-static unsigned short swap_vma_ra_win(struct vm_fault *vmf,
-				      unsigned short *offset,
-				      unsigned short *nr_pte)
+static int swap_vma_ra_win(struct vm_fault *vmf, unsigned long *start,
+			   unsigned long *end)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long ra_val;
-	unsigned long faddr, pfn, fpfn, lpfn, rpfn;
-	unsigned long start, end;
+	unsigned long faddr, prev_faddr, left, right;
 	unsigned int max_win, hits, prev_win, win;
 
-	max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
-			     SWAP_RA_ORDER_CEILING);
+	max_win = 1 << min(READ_ONCE(page_cluster), SWAP_RA_ORDER_CEILING);
 	if (max_win == 1)
 		return 1;
 
 	faddr = vmf->address;
-	fpfn = PFN_DOWN(faddr);
 	ra_val = GET_SWAP_RA_VAL(vma);
-	pfn = PFN_DOWN(SWAP_RA_ADDR(ra_val));
+	prev_faddr = SWAP_RA_ADDR(ra_val);
 	prev_win = SWAP_RA_WIN(ra_val);
 	hits = SWAP_RA_HITS(ra_val);
-	win = __swapin_nr_pages(pfn, fpfn, hits, max_win, prev_win);
-	atomic_long_set(&vma->swap_readahead_info,
-			SWAP_RA_VAL(faddr, win, 0));
+	win = __swapin_nr_pages(PFN_DOWN(prev_faddr), PFN_DOWN(faddr), hits,
+				max_win, prev_win);
+	atomic_long_set(&vma->swap_readahead_info, SWAP_RA_VAL(faddr, win, 0));
 	if (win == 1)
 		return 1;
 
-	if (fpfn == pfn + 1) {
-		lpfn = fpfn;
-		rpfn = fpfn + win;
-	} else if (pfn == fpfn + 1) {
-		lpfn = fpfn - win + 1;
-		rpfn = fpfn + 1;
-	} else {
-		unsigned int left = (win - 1) / 2;
-
-		lpfn = fpfn - left;
-		rpfn = fpfn + win - left;
-	}
-	if ((long)lpfn < 0)
-		lpfn = 0;
-	start = max3(lpfn, PFN_DOWN(vma->vm_start),
-		     PFN_DOWN(faddr & PMD_MASK));
-	end = min3(rpfn, PFN_DOWN(vma->vm_end),
-		   PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
-
-	*nr_pte = end - start;
-	*offset = fpfn - start;
+	if (faddr == prev_faddr + PAGE_SIZE)
+		left = faddr;
+	else if (prev_faddr == faddr + PAGE_SIZE)
+		left = faddr - (win << PAGE_SHIFT) + PAGE_SIZE;
+	else
+		left = faddr - (((win - 1) / 2) << PAGE_SHIFT);
+	right = left + (win << PAGE_SHIFT);
+	if ((long)left < 0)
+		left = 0;
+	*start = max3(left, vma->vm_start, faddr & PMD_MASK);
+	*end = min3(right, vma->vm_end, (faddr & PMD_MASK) + PMD_SIZE);
 
 	return win;
 }
@@ -815,22 +801,20 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 	struct swap_iocb *splug = NULL;
 	struct folio *folio;
 	pte_t *pte = NULL, pentry;
-	unsigned long addr;
+	int win;
+	unsigned long start, end, addr;
 	swp_entry_t entry;
 	pgoff_t ilx;
-	unsigned int i;
 	bool page_allocated;
-	unsigned short win, nr_pte, offset;
 
-	win = swap_vma_ra_win(vmf, &offset, &nr_pte);
+	win = swap_vma_ra_win(vmf, &start, &end);
 	if (win == 1)
 		goto skip;
 
-	addr = vmf->address - offset * PAGE_SIZE;
-	ilx = targ_ilx - offset;
+	ilx = targ_ilx - PFN_DOWN(vmf->address - start);
 
 	blk_start_plug(&plug);
-	for (i = 0; i < nr_pte; i++, ilx++, addr += PAGE_SIZE) {
+	for (addr = start; addr < end; ilx++, addr += PAGE_SIZE) {
 		if (!pte++) {
 			pte = pte_offset_map(vmf->pmd, addr);
 			if (!pte)
@@ -850,7 +834,7 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 			continue;
 		if (page_allocated) {
 			swap_read_folio(folio, false, &splug);
-			if (i != offset) {
+			if (addr != vmf->address) {
 				folio_set_readahead(folio);
 				count_vm_event(SWAP_RA);
 			}
-- 
2.39.2