[PATCH] [fixlet] mm/filemap: add comment explaining design decision

[Usama Arif]

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
 mm/filemap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index bfb891d9da1f..0a3facf452b3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3319,6 +3319,14 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 
 	/* Use the readahead code, even if readahead is disabled */
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) {
+		/*
+		 * Preserve PMD-sized readahead where it already fits in
+		 * the page cache. Otherwise cap the new fallback path at
+		 * 2MB: this is the common PMD-sized hugepage size, and it
+		 * avoids memory pressure from very large forced readahead
+		 * when mapping_max_folio_order() is high (for example,
+		 * 128MB with 64K base pages on arm64).
+		 */
 		if (HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
 			force_thp_readahead = true;
 			thp_order = HPAGE_PMD_ORDER;
-- 
2.53.0-Meta