From: SeongJae Park Date: Tue, 30 Sep 2025 00:44:09 +0000 (-0700) Subject: mm/damon/vaddr: do not repeat pte_offset_map_lock() until success X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=b93af2cc8e036754c0d9970d9ddc47f43cc94b9f;p=users%2Fhch%2Fmisc.git 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 Reported-by: Xinyu Zheng Closes: https://lore.kernel.org/20250918030029.2652607-1-zhengxinyu6@huawei.com Acked-by: Hugh Dickins Cc: [6.5+] Signed-off-by: Andrew Morton --- diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c index 8c048f9b129e..7e834467b2d8 100644 --- 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;