mm: extend rmap flags arguments for folio_add_new_anon_rmap

Patch series "mm: clarify folio_add_new_anon_rmap() and
__folio_add_anon_rmap()", v2.

This patchset is preparatory work for mTHP swapin.

folio_add_new_anon_rmap() assumes that new anon rmaps are always
exclusive.  However, this assumption doesn’t hold true for cases like
do_swap_page(), where a new anon might be added to the swapcache and is
not necessarily exclusive.

The patchset extends the rmap flags to allow folio_add_new_anon_rmap() to
handle both exclusive and non-exclusive new anon folios.  The
do_swap_page() function is updated to use this extended API with rmap
flags.  Consequently, all new anon folios now consistently use
folio_add_new_anon_rmap().  The special case for !folio_test_anon() in
__folio_add_anon_rmap() can be safely removed.

In conclusion, new anon folios always use folio_add_new_anon_rmap(),
regardless of exclusivity.  Old anon folios continue to use
__folio_add_anon_rmap() via folio_add_anon_rmap_pmd() and
folio_add_anon_rmap_ptes().


This patch (of 3):

In the case of a swap-in, a new anonymous folio is not necessarily
exclusive.  This patch updates the rmap flags to allow a new anonymous
folio to be treated as either exclusive or non-exclusive.  To maintain the
existing behavior, we always use EXCLUSIVE as the default setting.

[akpm@linux-foundation.org: cleanup and constifications per David and akpm]
[v-songbaohua@oppo.com: fix missing doc for flags of folio_add_new_anon_rmap()]
  Link: https://lkml.kernel.org/r/20240619210641.62542-1-21cnbao@gmail.com
[v-songbaohua@oppo.com: enhance doc for extend rmap flags arguments for folio_add_new_anon_rmap]
  Link: https://lkml.kernel.org/r/20240622030256.43775-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20240617231137.80726-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20240617231137.80726-2-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Tested-by: Shuai Yuan <yuanshuai@oppo.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 16 insertions, 9 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 69cbd7ac2a5c..c0c99f91ade1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1401,30 +1401,35 @@ void folio_add_anon_rmap_pmd(struct folio *folio, struct page *page,
  * @folio:	The folio to add the mapping to.
  * @vma:	the vm area in which the mapping is added
  * @address:	the user virtual address mapped
+ * @flags:	The rmap flags
  *
  * Like folio_add_anon_rmap_*() but must only be called on *new* folios.
  * This means the inc-and-test can be bypassed.
- * The folio does not have to be locked.
+ * The folio doesn't necessarily need to be locked while it's exclusive
+ * unless two threads map it concurrently. However, the folio must be
+ * locked if it's shared.
  *
- * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
- * is new, it's assumed to be mapped exclusively by a single process.
+ * If the folio is pmd-mappable, it is accounted as a THP.
  */
 void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
-		unsigned long address)
+		unsigned long address, rmap_t flags)
 {
-	int nr = folio_nr_pages(folio);
+	const int nr = folio_nr_pages(folio);
+	const bool exclusive = flags & RMAP_EXCLUSIVE;
 	int nr_pmdmapped = 0;
 
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
+	VM_WARN_ON_FOLIO(!exclusive && !folio_test_locked(folio), folio);
 	VM_BUG_ON_VMA(address < vma->vm_start ||
 			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
 	__folio_set_swapbacked(folio);
-	__folio_set_anon(folio, vma, address, true);
+	__folio_set_anon(folio, vma, address, exclusive);
 
 	if (likely(!folio_test_large(folio))) {
 		/* increment count (starts at -1) */
 		atomic_set(&folio->_mapcount, 0);
-		SetPageAnonExclusive(&folio->page);
+		if (exclusive)
+			SetPageAnonExclusive(&folio->page);
 	} else if (!folio_test_pmd_mappable(folio)) {
 		int i;
 
@@ -1433,7 +1438,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 
 			/* increment count (starts at -1) */
 			atomic_set(&page->_mapcount, 0);
-			SetPageAnonExclusive(page);
+			if (exclusive)
+				SetPageAnonExclusive(page);
 		}
 
 		/* increment count (starts at -1) */
@@ -1445,7 +1451,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 		/* increment count (starts at -1) */
 		atomic_set(&folio->_large_mapcount, 0);
 		atomic_set(&folio->_nr_pages_mapped, ENTIRELY_MAPPED);
-		SetPageAnonExclusive(&folio->page);
+		if (exclusive)
+			SetPageAnonExclusive(&folio->page);
 		nr_pmdmapped = nr;
 	}