folio_split() splits a large folio in the same way as buddy allocator
splits a large free page for allocation. The purpose is to minimize the
number of folios after the split. For example, if user wants to free the
3rd subpage in a order-9 folio, folio_split() will split the order-9 folio
as:
O-0, O-0, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-8 if it is anon,
since anon folio does not support order-1 yet.
-----------------------------------------------------------------
| | | | | | | | |
|O-0|O-0|O-0|O-0| O-2 |...| O-7 | O-8 |
| | | | | | | | |
-----------------------------------------------------------------
O-1, O-0, O-0, O-2, O-3, O-4, O-5, O-6, O-7, O-9 if it is pagecache
---------------------------------------------------------------
| | | | | | | |
| O-1 |O-0|O-0| O-2 |...| O-7 | O-8 |
| | | | | | | |
---------------------------------------------------------------
It generates fewer folios (i.e., 11 or 10) than existing page split
approach, which splits the order-9 to 512 order-0 folios. It also reduces
the number of new xa_node needed during a pagecache folio split from
8 to 1, potentially decreasing the folio split failure rate due to memory
constraints.
folio_split() and existing split_huge_page_to_list_to_order() share
the folio unmapping and remapping code in __folio_split() and the common
backend split code in __split_unmapped_folio() using
uniform_split variable to distinguish their operations.
uniform_split_supported() and non_uniform_split_supported() are added
to factor out check code and will be used outside __folio_split() in the
following commit.
Signed-off-by: Zi Yan <z...@nvidia.com>
---
mm/huge_memory.c | 137 ++++++++++++++++++++++++++++++++++-------------
1 file changed, 100 insertions(+), 37 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 21ebe2dec5a4..400dfe8a6e60 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3853,12 +3853,68 @@ static int __split_unmapped_folio(struct folio *folio,
int new_order,
return ret;
}
+static bool non_uniform_split_supported(struct folio *folio, unsigned int
new_order,
+ bool warns)
+{
+ /* order-1 is not supported for anonymous THP. */
+ if (folio_test_anon(folio) && new_order == 1) {
+ VM_WARN_ONCE(warns, "Cannot split to order-1 folio");
+ return false;
+ }
+
+ /*
+ * No split if the file system does not support large folio.
+ * Note that we might still have THPs in such mappings due to
+ * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
+ * does not actually support large folios properly.
+ */
+ if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
+ !mapping_large_folio_support(folio->mapping)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split file folio to non-0 order");
+ return false;
+ }
+
+ /* Only swapping a whole PMD-mapped folio is supported */
+ if (folio_test_swapcache(folio)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split swapcache folio to non-0 order");
+ return false;
+ }
+
+ return true;
+}
+
+/* See comments in non_uniform_split_supported() */
+static bool uniform_split_supported(struct folio *folio, unsigned int
new_order,
+ bool warns)
+{
+ if (folio_test_anon(folio) && new_order == 1) {
+ VM_WARN_ONCE(warns, "Cannot split to order-1 folio");
+ return false;
+ }
+
+ if (new_order) {
+ if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
+ !mapping_large_folio_support(folio->mapping)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split file folio to non-0 order");
+ return false;
+ }
+ if (folio_test_swapcache(folio)) {
+ VM_WARN_ONCE(warns,
+ "Cannot split swapcache folio to non-0 order");
+ return false;
+ }
+ }
+ return true;
+}
+
static int __folio_split(struct folio *folio, unsigned int new_order,
- struct page *page, struct list_head *list)
+ struct page *page, struct list_head *list, bool uniform_split)
{
struct deferred_split *ds_queue = get_deferred_split_queue(folio);
- /* reset xarray order to new order after split */
- XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
+ XA_STATE(xas, &folio->mapping->i_pages, folio->index);
bool is_anon = folio_test_anon(folio);
struct address_space *mapping = NULL;
struct anon_vma *anon_vma = NULL;
@@ -3873,29 +3929,11 @@ static int __folio_split(struct folio *folio, unsigned
int new_order,
if (new_order >= folio_order(folio))
return -EINVAL;
- if (is_anon) {
- /* order-1 is not supported for anonymous THP. */
- if (new_order == 1) {
- VM_WARN_ONCE(1, "Cannot split to order-1 folio");
- return -EINVAL;
- }
- } else if (new_order) {
- /*
- * No split if the file system does not support large folio.
- * Note that we might still have THPs in such mappings due to
- * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
- * does not actually support large folios properly.
- */
- if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
- !mapping_large_folio_support(folio->mapping)) {
- VM_WARN_ONCE(1,
- "Cannot split file folio to non-0 order");
- return -EINVAL;
- }
- }
+ if (uniform_split && !uniform_split_supported(folio, new_order, true))
+ return -EINVAL;
- /* Only swapping a whole PMD-mapped folio is supported */
- if (folio_test_swapcache(folio) && new_order)
+ if (!uniform_split &&
+ !non_uniform_split_supported(folio, new_order, true))
return -EINVAL;
is_hzp = is_huge_zero_folio(folio);
@@ -3952,10 +3990,13 @@ static int __folio_split(struct folio *folio, unsigned
int new_order,
goto out;
}
- xas_split_alloc(&xas, folio, folio_order(folio), gfp);
- if (xas_error(&xas)) {
- ret = xas_error(&xas);
- goto out;
+ if (uniform_split) {
+ xas_set_order(&xas, folio->index, new_order);
+ xas_split_alloc(&xas, folio, folio_order(folio), gfp);
+ if (xas_error(&xas)) {
+ ret = xas_error(&xas);
+ goto out;
+ }
}
anon_vma = NULL;
@@ -4020,7 +4061,6 @@ static int __folio_split(struct folio *folio, unsigned
int new_order,
if (mapping) {
int nr = folio_nr_pages(folio);
- xas_split(&xas, folio, folio_order(folio));
if (folio_test_pmd_mappable(folio) &&
new_order < HPAGE_PMD_ORDER) {
if (folio_test_swapbacked(folio)) {
@@ -4034,12 +4074,8 @@ static int __folio_split(struct folio *folio, unsigned
int new_order,
}
}
- if (is_anon) {
- mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
- mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order
- new_order));
- }
- __split_huge_page(page, list, end, new_order);
- ret = 0;
+ ret = __split_unmapped_folio(page_folio(page), new_order,
+ page, list, end, &xas, mapping, uniform_split);
} else {
spin_unlock(&ds_queue->split_queue_lock);
fail:
@@ -4117,7 +4153,34 @@ int split_huge_page_to_list_to_order(struct page *page,
struct list_head *list,
{
struct folio *folio = page_folio(page);
- return __folio_split(folio, new_order, page, list);
+ return __folio_split(folio, new_order, page, list, true);
+}
+
+/*
+ * folio_split: split a folio at @page to a @new_order folio
+ * @folio: folio to split
+ * @new_order: the order of the new folio
+ * @page: a page within the new folio
+ *
+ * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
+ * split but not to @new_order, the caller needs to check)
+ *
+ * It has the same prerequisites and returns as
+ * split_huge_page_to_list_to_order().
+ *
+ * Split a folio at offset_in_new_order to a new_order folio, leave the
+ * remaining subpages of the original folio as large as possible. For example,
+ * split an order-9 folio at its third order-3 subpages to an order-3 folio.
+ * There are 2^6=64 order-3 subpages in an order-9 folio and the result will be
+ * a set of folios with different order and the new folio is in bracket:
+ * [order-4, {order-3}, order-3, order-5, order-6, order-7, order-8].
+ *
+ * After split, folio is left locked for caller.
+ */
+int folio_split(struct folio *folio, unsigned int new_order,
+ struct page *page, struct list_head *list)
+{
+ return __folio_split(folio, new_order, page, list, false);
}
int min_order_for_split(struct folio *folio)
--
2.47.2
