linux-kernel - Re: [PATCH 1/2] mm/khugepaged: do synchronous writeback for MADV

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Message-ID: <b12340fa-358a-4799-a040-636994beef36@lucifer.local>
Date: Mon, 10 Nov 2025 21:16:23 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Garg, Shivank" <shivankg@....com>
Cc: Zi Yan <ziy@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Zach O'Keefe <zokeefe@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Branden Moore <Branden.Moore@....com>
Subject: Re: [PATCH 1/2] mm/khugepaged: do synchronous writeback for
 MADV_COLLAPSE

OK ignore the past mail, I have managed to repro this locally and have a
fix.

Turns out the swap code is doing something quite insane... I will send
fix-patches to the series shortly.

Meanwhile I attach fix-patch! :)

Cheers, Lorenzo

----8<----
>From c705fd85a806f53017df31e6b072c4bfa839e3a2 Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Mon, 10 Nov 2025 21:11:52 +0000
Subject: [PATCH] fix

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 include/linux/leafops.h |  4 ++--
 mm/swapfile.c           | 12 ++++++++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/linux/leafops.h b/include/linux/leafops.h
index a464a7e08c76..c4cd36760ea0 100644
--- a/include/linux/leafops.h
+++ b/include/linux/leafops.h
@@ -56,7 +56,7 @@ static inline softleaf_t softleaf_from_pte(pte_t pte)
 {
 	softleaf_t arch_entry;

-	if (pte_present(pte))
+	if (pte_present(pte) || pte_none(pte))
 		return softleaf_mk_none();

 	pte = pte_swp_clear_flags(pte);
@@ -95,7 +95,7 @@ static inline softleaf_t softleaf_from_pmd(pmd_t pmd)
 {
 	softleaf_t arch_entry;

-	if (pmd_present(pmd))
+	if (pmd_present(pmd) || pmd_none(pmd))
 		return softleaf_mk_none();

 	if (pmd_swp_soft_dirty(pmd))
diff --git a/mm/swapfile.c b/mm/swapfile.c
index fd23d9f7ae10..f0dcf261f652 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3202,9 +3202,17 @@ static int claim_swapfile(struct swap_info_struct *si, struct inode *inode)
  */
 unsigned long generic_max_swapfile_size(void)
 {
-	const softleaf_t entry = swp_entry(0, ~0UL);
+	softleaf_t entry = swp_entry(0, ~0UL);
+	const pte_t pte = softleaf_to_pte(entry);

-	return swp_offset(softleaf_from_pte(softleaf_to_pte(entry))) + 1;
+	/*
+	* Since the PTE can be an invalid swap entry (i.e. the none PTE), we do
+	* this manually.
+	*/
+	entry = __pte_to_swp_entry(pte);
+	entry = swp_entry(__swp_type(entry), __swp_offset(entry));
+
+	return swp_offset(entry) + 1;
 }

 /* Can be overridden by an architecture for additional checks. */
--
2.51.0