madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check

Commit 98b211d ("madvise: convert madvise_free_pte_range() to use a
folio") replaced the page_mapcount() with folio_mapcount() to check
whether the folio is shared by other mapping.

It's not correct for large folios. folio_mapcount() returns the total
mapcount of large folio which is not suitable to detect whether the folio
is shared.

Use folio_estimated_sharers() which returns a estimated number of shares.
That means it's not 100% correct. It should be OK for madvise case here.

User-visible effects is that the THP is skipped when user call madvise.
But the correct behavior is THP should be split and processed then.

NOTE: this change is a temporary fix to reduce the user-visible effects
before the long term fix from David is ready.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 98b211d ("madvise: convert madvise_free_pte_range() to use a folio")
Signed-off-by: Yin Fengwei <[email protected]>
Reviewed-by: Yu Zhao <[email protected]>
Reviewed-by: Ryan Roberts <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: fyin1

mm/madvise.c

@@ -680,7 +680,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 		if (folio_test_large(folio)) {
 			int err;
 
-			if (folio_mapcount(folio) != 1)
+			if (folio_estimated_sharers(folio) != 1)
 				break;
 			if (!folio_trylock(folio))
 				break;