[PATCH] mm: Support order-1 folios in the page cache

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

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/
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/huge_mm.h |  7 +++++--
 mm/filemap.c            |  2 --
 mm/huge_memory.c        | 23 ++++++++++++++++++-----
 mm/internal.h           |  4 +---
 mm/readahead.c          |  8 ++------
 5 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index fa0350b0812a..7b59ff685da3 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -140,7 +140,7 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
 unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 
-void folio_prep_large_rmappable(struct folio *folio);
+struct folio *folio_prep_large_rmappable(struct folio *folio);
 bool can_split_folio(struct folio *folio, int *pextra_pins);
 int split_huge_page_to_list(struct page *page, struct list_head *list);
 static inline int split_huge_page(struct page *page)
@@ -280,7 +280,10 @@ static inline bool hugepage_vma_check(struct vm_area_struct *vma,
 	return false;
 }
 
-static inline void folio_prep_large_rmappable(struct folio *folio) {}
+static inline struct folio *folio_prep_large_rmappable(struct folio *folio)
+{
+	return folio;
+}
 
 #define transparent_hugepage_flags 0UL
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 32eedf3afd45..61321e920e30 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1911,8 +1911,6 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 			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);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4f542444a91f..0df68a318922 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -610,11 +610,15 @@ struct deferred_split *get_deferred_split_queue(struct folio *folio)
 }
 #endif
 
-void folio_prep_large_rmappable(struct folio *folio)
+struct folio *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 folio;
+	if (folio_order(folio) > 1)
+		INIT_LIST_HEAD(&folio->_deferred_list);
 	folio_set_large_rmappable(folio);
+
+	return folio;
 }
 
 static inline bool is_transparent_hugepage(struct folio *folio)
@@ -2760,7 +2764,8 @@ 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);
 		}
@@ -2811,6 +2816,9 @@ 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
@@ -2836,7 +2844,12 @@ 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,
diff --git a/mm/internal.h b/mm/internal.h
index b61034bd50f5..11a9021614dd 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -419,9 +419,7 @@ 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);
-	return folio;
+	return folio_prep_large_rmappable(folio);
 }
 
 static inline void prep_compound_head(struct page *page, unsigned int order)
diff --git a/mm/readahead.c b/mm/readahead.c
index 6925e6959fd3..48cca8e8de17 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -513,14 +513,10 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		/* Align with smaller pages if needed */
 		if (index & ((1UL << order) - 1)) {
 			order = __ffs(index);
-			if (order == 1)
-				order = 0;
 		}
 		/* Don't allocate pages past EOF */
-		while (index + (1UL << order) - 1 > limit) {
-			if (--order == 1)
-				order = 0;
-		}
+		while (index + (1UL << order) - 1 > limit)
+			--order;
 		err = ra_alloc_folio(ractl, index, mark, order, gfp);
 		if (err)
 			break;
-- 
2.42.0