Commit b93af2cc authored by SeongJae Park's avatar SeongJae Park Committed by Andrew Morton
Browse files

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: 7780d040 ("mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock() fails")
Signed-off-by: default avatarSeongJae Park <sj@kernel.org>
Reported-by: default avatarXinyu Zheng <zhengxinyu6@huawei.com>
Closes: https://lore.kernel.org/20250918030029.2652607-1-zhengxinyu6@huawei.com


Acked-by: default avatarHugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>	[6.5+]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 9658d698

mm/damon/vaddr.c

+2 −6
Original line number Diff line number Diff line
@@ -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;