mm: support order-1 folios in the page cache

Folios of order 1 have no space to store the deferred list.  This is not a
problem for the page cache as file-backed folios are never placed on the
deferred list.  All we need to do is prevent the core MM from touching the
deferred list for order 1 folios and remove the code which prevented us
from allocating order 1 folios.

Link: https://lore.kernel.org/linux-mm/90344ea7-4eec-47ee-5996-0c22f42d6a6a@google.com/
Link: https://lkml.kernel.org/r/20240226205534.1603748-3-zi.yan@sent.com
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: 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: 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/filemap.c

@@ -1912,8 +1912,6 @@ no_page:
 			gfp_t alloc_gfp = gfp;
 
 			err = -ENOMEM;
-			if (order == 1)
-				order = 0;
 			if (order > 0)
 				alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
 			folio = filemap_alloc_folio(alloc_gfp, order);

mm/huge_memory.c

@@ -790,8 +790,10 @@ struct deferred_split *get_deferred_split_queue(struct folio *folio)
 
 void folio_prep_large_rmappable(struct folio *folio)
 {
-	VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
-	INIT_LIST_HEAD(&folio->_deferred_list);
+	if (!folio || !folio_test_large(folio))
+		return;
+	if (folio_order(folio) > 1)
+		INIT_LIST_HEAD(&folio->_deferred_list);
 	folio_set_large_rmappable(folio);
 }
 
@@ -3114,7 +3116,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	spin_lock(&ds_queue->split_queue_lock);
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
-		if (!list_empty(&folio->_deferred_list)) {
+		if (folio_order(folio) > 1 &&
+		    !list_empty(&folio->_deferred_list)) {
 			ds_queue->split_queue_len--;
 			list_del(&folio->_deferred_list);
 		}
@@ -3165,6 +3168,9 @@ void folio_undo_large_rmappable(struct folio *folio)
 	struct deferred_split *ds_queue;
 	unsigned long flags;
 
+	if (folio_order(folio) <= 1)
+		return;
+
 	/*
 	 * At this point, there is no one trying to add the folio to
 	 * deferred_list. If folio is not in deferred_list, it's safe
@@ -3190,7 +3196,12 @@ void deferred_split_folio(struct folio *folio)
 #endif
 	unsigned long flags;
 
-	VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
+	/*
+	 * Order 1 folios have no space for a deferred list, but we also
+	 * won't waste much memory by not adding them to the deferred list.
+	 */
+	if (folio_order(folio) <= 1)
+		return;
 
 	/*
 	 * The try_to_unmap() in page reclaim path might reach here too,

mm/internal.h

@@ -420,8 +420,7 @@ static inline struct folio *page_rmappable_folio(struct page *page)
 {
 	struct folio *folio = (struct folio *)page;
 
-	if (folio && folio_order(folio) > 1)
-		folio_prep_large_rmappable(folio);
+	folio_prep_large_rmappable(folio);
 	return folio;
 }
 

mm/readahead.c

@@ -514,9 +514,6 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		/* Don't allocate pages past EOF */
 		while (index + (1UL << order) - 1 > limit)
 			order--;
-		/* THP machinery does not support order-1 */
-		if (order == 1)
-			order = 0;
 		err = ra_alloc_folio(ractl, index, mark, order, gfp);
 		if (err)
 			break;