[PATCH 1/4] filemap: avoid interfere with xas.xa_index

[Yin Fengwei]

Ryan noticed 1% performance regression for kernel build with
the ranged file map with ext4 file system. It was later identified
wrong xas.xa_index update in filemap_map_pages() when folio is
not large folio.

Matthew suggested to use XArray API instead of touch xas.xa_index
directly at [1].

[1] https://lore.kernel.org/linux-mm/ZBho6Q6Xq%2FYqRmBT@xxxxxxxxxxxxxxxxxxxx/

Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 30 ++++++------------------------
 1 file changed, 6 insertions(+), 24 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 40be33b5ee46..fdb3e0a339b3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3416,10 +3416,10 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
 	return false;
 }
 
-static struct folio *next_uptodate_page(struct folio *folio,
-				       struct address_space *mapping,
-				       struct xa_state *xas, pgoff_t end_pgoff)
+static struct folio *next_uptodate_folio(struct xa_state *xas,
+		struct address_space *mapping, pgoff_t end_pgoff)
 {
+	struct folio *folio = xas_next_entry(xas, end_pgoff);
 	unsigned long max_idx;
 
 	do {
@@ -3457,22 +3457,6 @@ static struct folio *next_uptodate_page(struct folio *folio,
 	return NULL;
 }
 
-static inline struct folio *first_map_page(struct address_space *mapping,
-					  struct xa_state *xas,
-					  pgoff_t end_pgoff)
-{
-	return next_uptodate_page(xas_find(xas, end_pgoff),
-				  mapping, xas, end_pgoff);
-}
-
-static inline struct folio *next_map_page(struct address_space *mapping,
-					 struct xa_state *xas,
-					 pgoff_t end_pgoff)
-{
-	return next_uptodate_page(xas_next_entry(xas, end_pgoff),
-				  mapping, xas, end_pgoff);
-}
-
 /*
  * Map page range [start_page, start_page + nr_pages) of folio.
  * start_page is gotten from start by folio_page(folio, start)
@@ -3543,12 +3527,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	unsigned long addr;
 	XA_STATE(xas, &mapping->i_pages, start_pgoff);
 	struct folio *folio;
-	unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
 	vm_fault_t ret = 0;
 	int nr_pages = 0;
 
 	rcu_read_lock();
-	folio = first_map_page(mapping, &xas, end_pgoff);
+	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
 	if (!folio)
 		goto out;
 
@@ -3570,15 +3553,14 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 
 		ret |= filemap_map_folio_range(vmf, folio,
 				xas.xa_index - folio->index, addr, nr_pages);
-		xas.xa_index += nr_pages;
 
 		folio_unlock(folio);
 		folio_put(folio);
-	} while ((folio = next_map_page(mapping, &xas, end_pgoff)) != NULL);
+		folio = next_uptodate_folio(&xas, mapping, end_pgoff);
+	} while (folio);
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
 out:
 	rcu_read_unlock();
-	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
 	return ret;
 }
 EXPORT_SYMBOL(filemap_map_pages);
-- 
2.30.2