[RFC v2 PATCH 03/17] mm: Introduce try_vma_alloc_movable_folio()

[Ryan Roberts <ryan.roberts@arm.com>]

Opportunistically attempt to allocate high-order folios in highmem,
optionally zeroed. Retry with lower orders all the way to order-0, until
success. Although, of note, order-1 allocations are skipped since a
large folio must be at least order-2 to work with the THP machinery. The
user must check what they got with folio_order().

This will be used to oportunistically allocate large folios for
anonymous memory with a sensible fallback under memory pressure.

For attempts to allocate non-0 orders, we set __GFP_NORETRY to prevent
high latency due to reclaim, instead preferring to just try for a lower
order. The same approach is used by the readahead code when allocating
large folios.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 mm/memory.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 9d5e8be49f3b..ca32f59acef2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2989,6 +2989,39 @@ static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf)
 	return 0;
 }

+static inline struct folio *vma_alloc_movable_folio(struct vm_area_struct *vma,
+				unsigned long vaddr, int order, bool zeroed)
+{
+	gfp_t gfp = order > 0 ? __GFP_NORETRY | __GFP_NOWARN : 0;
+
+	if (zeroed)
+		return vma_alloc_zeroed_movable_folio(vma, vaddr, gfp, order);
+	else
+		return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp, order, vma,
+								vaddr, false);
+}
+
+/*
+ * Opportunistically attempt to allocate high-order folios, retrying with lower
+ * orders all the way to order-0, until success. order-1 allocations are skipped
+ * since a folio must be at least order-2 to work with the THP machinery. The
+ * user must check what they got with folio_order(). vaddr can be any virtual
+ * address that will be mapped by the allocated folio.
+ */
+static struct folio *try_vma_alloc_movable_folio(struct vm_area_struct *vma,
+				unsigned long vaddr, int order, bool zeroed)
+{
+	struct folio *folio;
+
+	for (; order > 1; order--) {
+		folio = vma_alloc_movable_folio(vma, vaddr, order, zeroed);
+		if (folio)
+			return folio;
+	}
+
+	return vma_alloc_movable_folio(vma, vaddr, 0, zeroed);
+}
+
 /*
  * Handle write page faults for pages that can be reused in the current vma
  *
--
2.25.1