[RFC v2 09/14] mm: Support order-1 folios in the page cache

Pankaj Raghav (Samsung)
2024-02-13T09:37:56Z

From: "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 | 3 ---
5 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 5adb86af35fc..916a2a539517 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -263,7 +263,7 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
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)
@@ -410,7 +410,10 @@ static inline unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
return 0;
}

-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 7a6e15c47150..c8205a534532 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1922,8 +1922,6 @@ 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 < min_order)
order = min_order;
if (order > 0)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d897efc51025..6ec3417638a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -788,11 +788,15 @@ 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)
@@ -3095,7 +3099,8 @@ 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);
}
@@ -3146,6 +3151,9 @@ 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
@@ -3171,7 +3179,12 @@ 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 f309a010d50f..5174b5b0c344 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 a361fba18674..7d5f6a8792a8 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -560,9 +560,6 @@ void page_cache_ra_order(struct readahead_control *ractl,
/* Don't allocate pages past EOF */
while (order > min_order && index + (1UL << order) - 1 > limit)
order--;
- /* THP machinery does not support order-1 */
- if (order == 1)
- order = 0;

if (order < min_order)
order = min_order;
--
2.43.0