[PATCH RFC 07/13] mm/rmap: remove CONFIG_PAGE_MAPCOUNT

["David Hildenbrand (Arm)" <david@kernel.org>]

page->mapcount is still updated but essentially unused. So let's
remove CONFIG_PAGE_MAPCOUNT. Given that CONFIG_NO_PAGE_MAPCOUNT is the
only remaining variant, that Kconfig can go as well.

We can replace some instances of "orig_nr_pages" by the "nr_pages" as
the latter is no longer modified.

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 Documentation/mm/transhuge.rst |  3 ---
 include/linux/rmap.h           | 11 +----------
 mm/Kconfig                     | 17 -----------------
 mm/rmap.c                      | 36 ++++++------------------------------
 4 files changed, 7 insertions(+), 60 deletions(-)

diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index f200c1ac19cb..eb5ac076e4c6 100644
--- a/Documentation/mm/transhuge.rst
+++ b/Documentation/mm/transhuge.rst
@@ -129,9 +129,6 @@ pages:
     corresponding mapcount), and the current status ("maybe mapped shared" vs.
     "mapped exclusively").
 
-    With CONFIG_PAGE_MAPCOUNT, we also increment/decrement
-    page->_mapcount.
-
 split_huge_page internally has to distribute the refcounts in the head
 page to the tail pages before clearing all PG_head/tail bits from the page
 structures. It can be done easily for refcounts taken by page table
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index e5569f5fdaec..4894e43e5f52 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -493,8 +493,6 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
 		struct page *page, int nr_pages, struct vm_area_struct *dst_vma,
 		enum pgtable_level level)
 {
-	const int orig_nr_pages = nr_pages;
-
 	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
 
 	switch (level) {
@@ -504,12 +502,7 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
 			break;
 		}
 
-		if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
-			do {
-				atomic_inc(&page->_mapcount);
-			} while (page++, --nr_pages > 0);
-		}
-		folio_add_large_mapcount(folio, orig_nr_pages, dst_vma);
+		folio_add_large_mapcount(folio, nr_pages, dst_vma);
 		break;
 	case PGTABLE_LEVEL_PMD:
 	case PGTABLE_LEVEL_PUD:
@@ -608,8 +601,6 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
 		do {
 			if (PageAnonExclusive(page))
 				ClearPageAnonExclusive(page);
-			if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
-				atomic_inc(&page->_mapcount);
 		} while (page++, --nr_pages > 0);
 		folio_add_large_mapcount(folio, orig_nr_pages, dst_vma);
 		break;
diff --git a/mm/Kconfig b/mm/Kconfig
index bd283958d675..576db4fdf16e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -948,25 +948,8 @@ config READ_ONLY_THP_FOR_FS
 	  support of file THPs will be developed in the next few release
 	  cycles.
 
-config NO_PAGE_MAPCOUNT
-	bool "No per-page mapcount (EXPERIMENTAL)"
-	help
-	  Do not maintain per-page mapcounts for pages part of larger
-	  allocations, such as transparent huge pages.
-
-	  When this config option is enabled, some interfaces that relied on
-	  this information will rely on less-precise per-allocation information
-	  instead: for example, using the average per-page mapcount in such
-	  a large allocation instead of the per-page mapcount.
-
-	  EXPERIMENTAL because the impact of some changes is still unclear.
-
 endif # TRANSPARENT_HUGEPAGE
 
-# simple helper to make the code a bit easier to read
-config PAGE_MAPCOUNT
-	def_bool !NO_PAGE_MAPCOUNT
-
 #
 # The architecture supports pgtable leaves that is larger than PAGE_SIZE
 #
diff --git a/mm/rmap.c b/mm/rmap.c
index df42c38fe387..27488183448b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1354,7 +1354,6 @@ static __always_inline void __folio_add_rmap(struct folio *folio,
 		enum pgtable_level level)
 {
 	int nr = 0, nr_pmdmapped = 0, mapcount;
-	const int orig_nr_pages = nr_pages;
 
 	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
 
@@ -1365,14 +1364,8 @@ static __always_inline void __folio_add_rmap(struct folio *folio,
 			break;
 		}
 
-		if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
-			do {
-				atomic_inc(&page->_mapcount);
-			} while (page++, --nr_pages > 0);
-		}
-
-		mapcount = folio_add_return_large_mapcount(folio, orig_nr_pages, vma);
-		if (mapcount == orig_nr_pages)
+		mapcount = folio_add_return_large_mapcount(folio, nr_pages, vma);
+		if (mapcount == nr_pages)
 			nr = folio_large_nr_pages(folio);
 		break;
 	case PGTABLE_LEVEL_PMD:
@@ -1518,15 +1511,6 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
 		VM_WARN_ON_FOLIO(folio_test_large(folio) &&
 				 folio_entire_mapcount(folio) > 1 &&
 				 PageAnonExclusive(cur_page), folio);
-		if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT))
-			continue;
-
-		/*
-		 * While PTE-mapping a THP we have a PMD and a PTE
-		 * mapping.
-		 */
-		VM_WARN_ON_FOLIO(atomic_read(&cur_page->_mapcount) > 0 &&
-				 PageAnonExclusive(cur_page), folio);
 	}
 
 	/*
@@ -1628,14 +1612,12 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 		int i;
 
 		nr = folio_large_nr_pages(folio);
-		for (i = 0; i < nr; i++) {
-			struct page *page = folio_page(folio, i);
+		if (exclusive) {
+			for (i = 0; i < nr; i++) {
+				struct page *page = folio_page(folio, i);
 
-			if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
-				/* increment count (starts at -1) */
-				atomic_set(&page->_mapcount, 0);
-			if (exclusive)
 				SetPageAnonExclusive(page);
+			}
 		}
 
 		folio_set_large_mapcount(folio, nr, vma);
@@ -1769,12 +1751,6 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		if (!mapcount)
 			nr = folio_large_nr_pages(folio);
 
-		if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
-			do {
-				atomic_dec(&page->_mapcount);
-			} while (page++, --nr_pages > 0);
-		}
-
 		partially_mapped = __folio_certainly_partially_mapped(folio, mapcount);
 		break;
 	case PGTABLE_LEVEL_PMD:

-- 
2.43.0