+ mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success.patch added to mm-hotfixes-unstable branch

+ mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/vaddr: do not repeat pte_offset_map_lock() until success
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: SeongJae Park <sj@kernel.org>
Subject: mm/damon/vaddr: do not repeat pte_offset_map_lock() until success
Date: Mon, 29 Sep 2025 17:44:09 -0700

DAMON's virtual address space operation set implementation (vaddr) calls
pte_offset_map_lock() inside the page table walk callback function.  This
is for reading and writing page table accessed bits.  If
pte_offset_map_lock() fails, it retries by returning the page table walk
callback function with ACTION_AGAIN.

pte_offset_map_lock() can continuously fail if the target is a pmd
migration entry, though.  Hence it could cause an infinite page table walk
if the migration cannot be done until the page table walk is finished. 
This indeed caused a soft lockup when CPU hotplugging and DAMON were
running in parallel.

Avoid the infinite loop by simply not retrying the page table walk.  DAMON
is promising only a best-effort accuracy, so missing access to such pages
is no problem.

Link: https://lkml.kernel.org/r/20250930004410.55228-1-sj@kernel.org
Fixes: 7780d04046a2 ("mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock() fails")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Xinyu Zheng <zhengxinyu6@huawei.com>
Closes: https://lore.kernel.org/20250918030029.2652607-1-zhengxinyu6@huawei.com
Acked-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>	[6.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/damon/vaddr.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/mm/damon/vaddr.c~mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success
+++ a/mm/damon/vaddr.c
@@ -328,10 +328,8 @@ static int damon_mkold_pmd_entry(pmd_t *
 	}
 
 	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
-	if (!pte) {
-		walk->action = ACTION_AGAIN;
+	if (!pte)
 		return 0;
-	}
 	if (!pte_present(ptep_get(pte)))
 		goto out;
 	damon_ptep_mkold(pte, walk->vma, addr);
@@ -481,10 +479,8 @@ regular_page:
 #endif	/* CONFIG_TRANSPARENT_HUGEPAGE */
 
 	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
-	if (!pte) {
-		walk->action = ACTION_AGAIN;
+	if (!pte)
 		return 0;
-	}
 	ptent = ptep_get(pte);
 	if (!pte_present(ptent))
 		goto out;
_

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-vaddr-do-not-repeat-pte_offset_map_lock-until-success.patch