mm/damon/vaddr: do not repeat pte_offset_map_lock() until success

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>

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 8c048f9b129e6b76c70b3f0d11d4578502ca464b..7e834467b2d8121ab9fae3f66185defd82747e54 100644 (file)
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -328,10 +328,8 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
        }
 
        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;