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.

Reported-by Xinyu Zheng <[email protected]>

Closes https://lore.kernel.org/[email protected] open here

Fixes 7780d04046a2 ("mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock() fails")

Cc <[email protected]> # 6.5.x

Cc Hugh Dickins <[email protected]>

Signed-off-by SeongJae Park <[email protected]>

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

mm/damon/vaddr.c +2 -6

index 8c048f9b129e..7e834467b2d8 100644
@@ -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 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
 #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;

base-commit: 3169a901e935bc1f2d2eec0171abcf524b7747e4

Signature · 1 line

2.39.5