Re: [PATCH v4 5/7] mm: thp: split huge page to any lower order pages (except order-1).

[Zi Yan <ziy@nvidia.com>]

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On 13 Feb 2024, at 17:15, Zi Yan wrote:

> On 13 Feb 2024, at 17:05, Luis Chamberlain wrote:
>
>> On Tue, Feb 13, 2024 at 04:55:18PM -0500, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>> Order-1 folio is not supported because _deferred_list, which is used by
>>> partially mapped folios, is stored in subpage 2 and an order-1 folio only
>>> has subpage 0 and 1.
>>
>> The LBS patches has the patch from Matthew which enables and allowed us
>> to successfully test order 1. So this restriction could be dropped if
>> that gets merged.
>
> OK. But it only applies to file-backed folios IIUC. Anonymous folios still
> cannot be split to order-1.

Something like this would lift the restriction:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0564b007cbd1..7ee7f1906617 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3045,9 +3045,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
        VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
        VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);

-       /* Cannot split THP to order-1 (no order-1 THPs) */
-       if (new_order == 1) {
-               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
+       /* Cannot split anonymous folios to order-1 (no order-1 anon folios) */
+       if (new_order == 1 && folio_test_anon(folio)) {
+               VM_WARN_ONCE(1, "Cannot split to order-1 anonymous folio");
                return -EINVAL;
        }

--
Best Regards,
Yan, Zi

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