mm/page_vma_mapped: track if the page is mapped across page table boundary

Patch series "mm: Improve mlock tracking for large folios", v3.

The patchset includes several fixes and improvements related to mlock
tracking of large folios.

The main objective is to reduce the undercount of Mlocked memory in
/proc/meminfo and improve the accuracy of the statistics.

Patches 1-2:

These patches address a minor race condition in folio_referenced_one()
related to mlock_vma_folio().

Currently, mlock_vma_folio() is called on large folio without the page
table lock, which can result in a race condition with unmap (i.e. 
MADV_DONTNEED).  This can lead to partially mapped folios on the
unevictable LRU list.

While not a significant issue, I do not believe backporting is necessary.

Patch 3:

This patch adds mlocking logic similar to folio_referenced_one() to
try_to_unmap_one(), allowing for mlocking of large folios where possible.

Patch 4-5:

These patches modifies finish_fault() and faultaround to map in the entire
folio when possible, enabling efficient mlocking upon addition to the
rmap.

Patch 6:

This patch makes rmap mlock large folios if they are fully mapped,
addressing the primary source of mlock undercount for large folios.


This patch (of 6):

Add a PVMW_PGTABLE_CROSSSED flag that page_vma_mapped_walk() will set if
the page is mapped across page table boundary.  Unlike other PVMW_* flags,
this one is result of page_vma_mapped_walk() and not set by the caller.

folio_referenced_one() will use it to detect if it safe to mlock the
folio.

[akpm@linux-foundation.org: s/CROSSSED/CROSSED/]
Link: https://lkml.kernel.org/r/20250923110711.690639-1-kirill@shutemov.name
Link: https://lkml.kernel.org/r/20250923110711.690639-2-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index e8aff6d2deda..daa92a58585d 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -922,6 +922,11 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 /* Look for migration entries rather than present PTEs */
 #define PVMW_MIGRATION		(1 << 1)
 
+/* Result flags */
+
+/* The page is mapped across page table boundary */
+#define PVMW_PGTABLE_CROSSED	(1 << 16)
+
 struct page_vma_mapped_walk {
 	unsigned long pfn;
 	unsigned long nr_pages;
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index e981a1a292d2..c498a91b6706 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -309,6 +309,7 @@ next_pte:
 				}
 				pte_unmap(pvmw->pte);
 				pvmw->pte = NULL;
+				pvmw->flags |= PVMW_PGTABLE_CROSSED;
 				goto restart;
 			}
 			pvmw->pte++;