mm: convert folio_estimated_sharers() to folio_likely_mapped_shared()

Callers of folio_estimated_sharers() only care about "mapped shared vs. 
mapped exclusively", not the exact estimate of sharers.  Let's consolidate
and unify the condition users are checking.  While at it clarify the
semantics and extend the discussion on the fuzziness.

Use the "likely mapped shared" terminology to better express what the
(adjusted) function actually checks.

Whether a partially-mappable folio is more likely to not be partially
mapped than partially mapped is debatable.  In the future, we might be
able to improve our estimate for partially-mappable folios, though.

Note that we will now consistently detect "mapped shared" only if the
first subpage is actually mapped multiple times.  When the first subpage
is not mapped, we will consistently detect it as "mapped exclusively". 
This change should currently only affect the usage in
madvise_free_pte_range() and queue_folios_pte_range() for large folios: if
the first page was already unmapped, we would have skipped the folio.

[david@redhat.com: folio_likely_mapped_shared() kerneldoc fixup]
  Link: https://lkml.kernel.org/r/dd0ad9f2-2d7a-45f3-9ba3-979488c7dd27@redhat.com
Link: https://lkml.kernel.org/r/20240227201548.857831-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Acked-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 63c97bb639d7..a31aa75d223d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2593,11 +2593,11 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
 	/*
 	 * Don't migrate file folios that are mapped in multiple processes
 	 * with execute permissions as they are probably shared libraries.
-	 * To check if the folio is shared, ideally we want to make sure
-	 * every page is mapped to the same process. Doing that is very
-	 * expensive, so check the estimated mapcount of the folio instead.
+	 *
+	 * See folio_likely_mapped_shared() on possible imprecision when we
+	 * cannot easily detect if a folio is shared.
 	 */
-	if (folio_estimated_sharers(folio) != 1 && folio_is_file_lru(folio) &&
+	if (folio_likely_mapped_shared(folio) && folio_is_file_lru(folio) &&
 	    (vma->vm_flags & VM_EXEC))
 		goto out;