Re: [PATCH v5 7/8] mm: thp: split huge page to any lower order pages

Zi Yan
2024-03-07T14:58:45Z

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On 26 Feb 2024, at 15:55, Zi Yan wrote:

> From: Zi Yan <ziy@nvidia.com>
>
> To split a THP to any lower order pages, we need to reform THPs on
> subpages at given order and add page refcount based on the new page
> order. Also we need to reinitialize page_deferred_list after removing
> the page from the split_queue, otherwise a subsequent split will
> see list corruption when checking the page_deferred_list again.
>
> Note: Anonymous 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. File-backed order-1 folios are
> fine, since they do not use _deferred_list.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> include/linux/huge_mm.h | 21 +++++---
> mm/huge_memory.c | 110 +++++++++++++++++++++++++++++++---------
> 2 files changed, 99 insertions(+), 32 deletions(-)

Hi Andrew,

Can you fold the patch below into this patch 7? It is based on the discussion
with Dan Carpenter at https://lore.kernel.org/linux-mm/7dda9283-b437-4cf8-ab0d-83c330deb9c0@moroto.mountain/.
It prevents invalid new_order input from causing unexpected outcome of
split_huge_page_to_list_to_order(). Especially in patch 8, new_order can come
from debugfs without any restriction. I will send another fixup to
patch 8 to check new_order from debugfs.

Thanks.

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a81a09236c16..57fca7bffd20 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3052,6 +3052,9 @@ 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);

+ if (new_order >= folio_order(folio))
+ return -EINVAL;
+
/* Cannot split anonymous THP to order-1 */
if (new_order == 1 && folio_test_anon(folio)) {
VM_WARN_ONCE(1, "Cannot split to order-1 folio");

--
Best Regards,
Yan, Zi

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