Re: [PATCH v7 2/8] mm/huge_memory: add two new (not yet used) functions for folio_split()

[Zi Yan <ziy@nvidia.com>]

On 11 Feb 2025, at 10:50, Zi Yan wrote:

> This is a preparation patch, both added functions are not used yet.
>
> The added __split_unmapped_folio() is able to split a folio with
> its mapping removed in two manners: 1) uniform split (the existing way),
> and 2) buddy allocator like split.
>
> The added __split_folio_to_order() can split a folio into any lower order.
> For uniform split, __split_unmapped_folio() calls it once to split
> the given folio to the new order. For buddy allocator split,
> __split_unmapped_folio() calls it (folio_order - new_order) times
> and each time splits the folio containing the given page to one lower
> order.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  mm/huge_memory.c | 349 ++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 348 insertions(+), 1 deletion(-)
>

Hi Andrew,

Can you fold the patch below into this one? It addresses the error
reported by syzbot at: https://lore.kernel.org/all/67af65cb.050a0220.21dd3.004a.GAE@google.com/ and the concern raised
by David Hildenbrand at: https://lore.kernel.org/linux-mm/db77d017-4a1e-4a47-9064-e335cb0313af@redhat.com/.

Let me know if you prefer a new version of the whole series.

Thanks.



From a6bd83dfbb1143f1614ede4817cccb1e8cc6290d Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Fri, 14 Feb 2025 16:18:24 -0500
Subject: [PATCH] mm/huge_memory: do not drop the original folio during
 truncate.

The caller expects to handle the original folio itself.

also make __split_unmapped_folio() never fail, per discussion with David
Hildenbrand.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2eda2a9ec8fc..87cb62c81bf3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3292,16 +3292,12 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
  * It splits @folio into @new_order folios and copies the @folio metadata to
  * all the resulting folios.
  */
-static int __split_folio_to_order(struct folio *folio, int new_order)
+static void __split_folio_to_order(struct folio *folio, int new_order)
 {
-	int curr_order = folio_order(folio);
 	long nr_pages = folio_nr_pages(folio);
 	long new_nr_pages = 1 << new_order;
 	long index;

-	if (curr_order <= new_order)
-		return -EINVAL;
-
 	/*
 	 * Skip the first new_nr_pages, since the new folio from them have all
 	 * the flags from the original folio.
@@ -3396,8 +3392,6 @@ static int __split_folio_to_order(struct folio *folio, int new_order)

 	if (!new_order)
 		ClearPageCompound(&folio->page);
-
-	return 0;
 }

 /*
@@ -3491,7 +3485,6 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 		int old_order = folio_order(folio);
 		struct folio *release;
 		struct folio *end_folio = folio_next(folio);
-		int status;

 		/* order-1 anonymous folio is not supported */
 		if (folio_test_anon(folio) && split_order == 1)
@@ -3524,12 +3517,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 		split_page_owner(&folio->page, old_order, split_order);
 		pgalloc_tag_split(folio, old_order, split_order);

-		status = __split_folio_to_order(folio, split_order);
-
-		if (status < 0) {
-			stop_split = true;
-			ret = -EINVAL;
-		}
+		__split_folio_to_order(folio, split_order);

 after_split:
 		/*
@@ -3567,8 +3555,10 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 				     folio_test_swapcache(origin_folio)) ?
 					     folio_nr_pages(release) : 0));

-			if (release != origin_folio)
-				lru_add_page_tail(origin_folio, &release->page,
+			if (release == origin_folio)
+				continue;
+			
+			lru_add_page_tail(origin_folio, &release->page,
 						lruvec, list);

 			/* Some pages can be beyond EOF: drop them from page cache */
-- 
2.47.2



Best Regards,
Yan, Zi