[PATCH v3 4/4] mm: align file-backed mmap to exec folio order in thp_get_unmapped_area

[Usama Arif <usama.arif@linux.dev>]

thp_get_unmapped_area() is the get_unmapped_area callback for
filesystems like ext4, xfs, and btrfs. It attempts to align the virtual
address for PMD_SIZE THP mappings, but on arm64 with 64K base pages
PMD_SIZE is 512M, which is too large for typical shared library mappings,
so the alignment always fails and falls back to PAGE_SIZE.

This means shared libraries loaded by ld.so via mmap() get 64K-aligned
virtual addresses, preventing contpte mapping even when large folios are
allocated with properly aligned file offsets and physical addresses.

Add a fallback in thp_get_unmapped_area_vmflags() that uses
exec_folio_order() to determine alignment, matching the readahead
allocation strategy. This aligns mappings to the hardware TLB
coalescing size (e.g. 2M for contpte on arm64 64K pages, 64K for
contpte/HPA on arm64 4K/16K pages), capped to the mapping length via
rounddown_pow_of_two(len).

The fallback is naturally a no-op on architectures where
exec_folio_order() returns 0, and skips the retry when the alignment
would equal PMD_SIZE (already attempted above) incase another
architecture changes exec_folio_order() in the future.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/huge_memory.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b2a6060b3c20..ad97ac8406dc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1218,6 +1218,19 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
 	if (ret)
 		return ret;
 
+	if (filp && exec_folio_order()) {
+		unsigned long exec_folio_size = PAGE_SIZE << exec_folio_order();
+		unsigned long size = rounddown_pow_of_two(len);
+
+		size = min(size, exec_folio_size);
+		if (size > PAGE_SIZE && size != PMD_SIZE) {
+			ret = __thp_get_unmapped_area(filp, addr, len, off,
+						      flags, size, vm_flags);
+			if (ret)
+				return ret;
+		}
+	}
+
 	return mm_get_unmapped_area_vmflags(filp, addr, len, pgoff, flags,
 					    vm_flags);
 }
-- 
2.52.0