mm: introduce a pageflag for partially mapped folios

Currently folio->_deferred_list is used to keep track of partially_mapped
folios that are going to be split under memory pressure.  In the next
patch, all THPs that are faulted in and collapsed by khugepaged are also
going to be tracked using _deferred_list.

This patch introduces a pageflag to be able to distinguish between
partially mapped folios and others in the deferred_list at split time in
deferred_split_scan.  Its needed as __folio_remove_rmap decrements
_mapcount, _large_mapcount and _entire_mapcount, hence it won't be
possible to distinguish between partially mapped folios and others in
deferred_split_scan.

Eventhough it introduces an extra flag to track if the folio is partially
mapped, there is no functional change intended with this patch and the
flag is not useful in this patch itself, it will become useful in the next
patch when _deferred_list has non partially mapped folios.

Link: https://lkml.kernel.org/r/20240830100438.3623486-5-usamaarif642@gmail.com
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Cc: Alexander Zhu <alexlzhu@fb.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Shuang Zhai <zhais@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Shuang Zhai <szhai2@cs.rochester.edu>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 14 insertions, 3 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 4902e2f7e896..e3896ebf0f94 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -333,7 +333,7 @@ static inline int split_huge_page(struct page *page)
 {
 	return split_huge_page_to_list_to_order(page, NULL, 0);
 }
-void deferred_split_folio(struct folio *folio);
+void deferred_split_folio(struct folio *folio, bool partially_mapped);
 
 void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long address, bool freeze, struct folio *folio);
@@ -507,7 +507,7 @@ static inline int split_huge_page(struct page *page)
 {
 	return 0;
 }
-static inline void deferred_split_folio(struct folio *folio) {}
+static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
 #define split_huge_pmd(__vma, __pmd, __address)	\
 	do { } while (0)
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 2175ebceb41c..1b3a76710487 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -186,6 +186,7 @@ enum pageflags {
 	/* At least one page in this folio has the hwpoison flag set */
 	PG_has_hwpoisoned = PG_active,
 	PG_large_rmappable = PG_workingset, /* anon or file-backed */
+	PG_partially_mapped = PG_reclaim, /* was identified to be partially mapped */
 };
 
 #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
@@ -859,8 +860,18 @@ static inline void ClearPageCompound(struct page *page)
 	ClearPageHead(page);
 }
 FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
+FOLIO_TEST_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
+/*
+ * PG_partially_mapped is protected by deferred_split split_queue_lock,
+ * so its safe to use non-atomic set/clear.
+ */
+__FOLIO_SET_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
+__FOLIO_CLEAR_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
 #else
 FOLIO_FLAG_FALSE(large_rmappable)
+FOLIO_TEST_FLAG_FALSE(partially_mapped)
+__FOLIO_SET_FLAG_NOOP(partially_mapped)
+__FOLIO_CLEAR_FLAG_NOOP(partially_mapped)
 #endif
 
 #define PG_head_mask ((1UL << PG_head))
@@ -1171,7 +1182,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 #define PAGE_FLAGS_SECOND						\
 	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
-	 1UL << PG_large_rmappable)
+	 1UL << PG_large_rmappable	| 1UL << PG_partially_mapped)
 
 #define PAGE_FLAGS_PRIVATE				\
 	(1UL << PG_private | 1UL << PG_private_2)