mm/huge_memory: only split PMD mapping when necessary in unmap_folio()

Patch series "Split a folio to any lower order folios", v5.

File folio supports any order and multi-size THP is upstreamed[1], so both
file and anonymous folios can be >0 order.  Currently, split_huge_page()
only splits a huge page to order-0 pages, but splitting to orders higher
than 0 might better utilize large folios, if done properly.  In addition,
Large Block Sizes in XFS support would benefit from it during truncate[2].
This patchset adds support for splitting a large folio to any lower order
folios.

In addition to this implementation of split_huge_page_to_list_to_order(),
a possible optimization could be splitting a large folio to arbitrary
smaller folios instead of a single order.  As both Hugh and Ryan pointed
out [3,5] that split to a single order might not be optimal, an order-9
folio might be better split into 1 order-8, 1 order-7, ..., 1 order-1, and
2 order-0 folios, depending on subsequent folio operations.  Leave this as
future work.

[1] https://lore.kernel.org/all/20231207161211.2374093-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-mm/20240226094936.2677493-1-kernel@pankajraghav.com/
[3] https://lore.kernel.org/linux-mm/9dd96da-efa2-5123-20d4-4992136ef3ad@google.com/
[4] https://lore.kernel.org/linux-mm/cbb1d6a0-66dd-47d0-8733-f836fe050374@arm.com/
[5] https://lore.kernel.org/linux-mm/20240213215520.1048625-1-zi.yan@sent.com/


This patch (of 8):

As multi-size THP support is added, not all THPs are PMD-mapped, thus
during a huge page split, there is no need to always split PMD mapping in
unmap_folio().  Make it conditional.

Link: https://lkml.kernel.org/r/20240226205534.1603748-1-zi.yan@sent.com
Link: https://lkml.kernel.org/r/20240226205534.1603748-2-zi.yan@sent.comSigned-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Koutny <mkoutny@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2727,11 +2727,14 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
 
 static void unmap_folio(struct folio *folio)
 {
-	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
-		TTU_SYNC | TTU_BATCH_FLUSH;
+	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SYNC |
+		TTU_BATCH_FLUSH;
 
 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
 
+	if (folio_test_pmd_mappable(folio))
+		ttu_flags |= TTU_SPLIT_HUGE_PMD;
+
 	/*
 	 * Anon pages need migration entries to preserve them, but file
 	 * pages can simply be left unmapped, then faulted back on demand.