Re: [PATCH] mm/huge_memory.c: introduce folio_split_unmapped

[Wei Yang]

On Fri, Nov 14, 2025 at 12:22:28PM +1100, Balbir Singh wrote:
[...]
>@@ -4079,6 +4091,36 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 	return ret;
> }
> 
>+/*
>+ * This function is a helper for splitting folios that have already been unmapped.
>+ * The use case is that the device or the CPU can refuse to migrate THP pages in
>+ * the middle of migration, due to allocation issues on either side
>+ *
>+ * The high level code is copied from __folio_split, since the pages are anonymous
>+ * and are already isolated from the LRU, the code has been simplified to not
>+ * burden __folio_split with unmapped sprinkled into the code.
>+ *
>+ * None of the split folios are unlocked
>+ */
>+int folio_split_unmapped(struct folio *folio, unsigned int new_order)
>+{
>+	int extra_pins, ret = 0;
>+
>+	VM_WARN_ON_FOLIO(folio_mapped(folio), folio);
>+	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>+	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
>+

Compared with original logic, we did check folio_split_supported() and
check whether new_order is supported for the file system.

Currently folio_split_unmapped() only pass 0 as new_order, which looks good.
But for a generic helper, it looks reasonable to do the check, IMHO.

>+	if (!can_split_folio(folio, 1, &extra_pins))
>+		return -EAGAIN;
>+
>+	local_irq_disable();
>+	ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL,
>+						NULL, false, NULL, SPLIT_TYPE_UNIFORM,
>+						0, extra_pins);
>+	local_irq_enable();
>+	return ret;
>+}
>+
> /*
>  * This function splits a large folio into smaller folios of order @new_order.
>  * @page can point to any page of the large folio to split. The split operation


-- 
Wei Yang
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