[PATCH v3 17/20] fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE (CONFIG_NO_PAGE_MAPCOUNT)

[David Hildenbrand <david@redhat.com>]

Let's implement an alternative when per-page mapcounts in large folios are
no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.

PM_MMAP_EXCLUSIVE will now be set if folio_likely_mapped_shared() is
true -- when the folio is considered "mapped shared", including when
it once was "mapped shared" but no longer is, as documented.

This might result in and under-indication of "exclusively mapped", which
is considered better than over-indicating it: under-estimating the USS
(Unique Set Size) is better than over-estimating it.

As an alternative, we could simply remove that flag with
CONFIG_NO_PAGE_MAPCOUNT completely, but there might be value to it. So,
let's keep it like that and document the behavior.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/pagemap.rst | 11 +++++++++++
 fs/proc/task_mmu.c                       | 11 +++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index d6647daca9122..afce291649dd6 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -38,6 +38,17 @@ There are four components to pagemap:
    precisely which pages are mapped (or in swap) and comparing mapped
    pages between processes.
 
+   Traditionally, bit 56 indicates that a page is mapped exactly once and bit
+   56 is clear when a page is mapped multiple times, even when mapped in the
+   same process multiple times. In some kernel configurations, the semantics
+   for pages part of a larger allocation (e.g., THP) can differ: bit 56 is set
+   if all pages part of the corresponding large allocation are *certainly*
+   mapped in the same process, even if the page is mapped multiple times in that
+   process. Bit 56 is clear when any page page of the larger allocation
+   is *maybe* mapped in a different process. In some cases, a large allocation
+   might be treated as "maybe mapped by multiple processes" even though this
+   is no longer the case.
+
    Efficient users of this interface will use ``/proc/pid/maps`` to
    determine which areas of memory are actually mapped and llseek to
    skip over unmapped regions.
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 1162f0e72df2e..f937c2df7b3f4 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1652,6 +1652,13 @@ static int add_to_pagemap(pagemap_entry_t *pme, struct pagemapread *pm)
 	return 0;
 }
 
+static bool __folio_page_mapped_exclusively(struct folio *folio, struct page *page)
+{
+	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+		return folio_precise_page_mapcount(folio, page) == 1;
+	return !folio_maybe_mapped_shared(folio);
+}
+
 static int pagemap_pte_hole(unsigned long start, unsigned long end,
 			    __always_unused int depth, struct mm_walk *walk)
 {
@@ -1742,7 +1749,7 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 		if (!folio_test_anon(folio))
 			flags |= PM_FILE;
 		if ((flags & PM_PRESENT) &&
-		    folio_precise_page_mapcount(folio, page) == 1)
+		    __folio_page_mapped_exclusively(folio, page))
 			flags |= PM_MMAP_EXCLUSIVE;
 	}
 	if (vma->vm_flags & VM_SOFTDIRTY)
@@ -1817,7 +1824,7 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
 			pagemap_entry_t pme;
 
 			if (folio && (flags & PM_PRESENT) &&
-			    folio_precise_page_mapcount(folio, page + idx) == 1)
+			    __folio_page_mapped_exclusively(folio, page))
 				cur_flags |= PM_MMAP_EXCLUSIVE;
 
 			pme = make_pme(frame, cur_flags);
-- 
2.48.1