From nobody Thu Oct 2 00:46:00 2025 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 31E994A0A; Tue, 30 Sep 2025 00:44:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759193057; cv=none; b=lzXNNvAK+agDgr2PrYpOH6hnMPK1d0aFpR+ReEABUpc69Lszdy0Hue3mGB7YN0Mi77RF+jPHib/0Z/CzdkIzIgzNvbjOKl93MWRQEEsacqUCcWBgS15yFCC7mJggnzBotRMWHUwxePqdUVeQhX+PFIjko9q9hm4Q+PKN4E+KrVM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759193057; c=relaxed/simple; bh=UG8UCbAm0S/mQH2S995hl54PecX4r3H8TVBvdoOGWy0=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=G0wi2AtFoX5qM37kTlX3UWOadwGGzJfc895zv90mt9DItZDexmRazDrnKu8eMw5PMdvlqVMP6w5JyM/o5rnXTYKrpSnLzHFIQ4bf2iT0HTrfCW0BBTOVANyWVEECMm4URpyZoSR5sQlzQ21zGh909/XthvBAWPOe8MsY+yvczhc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WFgJtw5A; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WFgJtw5A" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7868BC4CEF4; Tue, 30 Sep 2025 00:44:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1759193056; bh=UG8UCbAm0S/mQH2S995hl54PecX4r3H8TVBvdoOGWy0=; h=From:To:Cc:Subject:Date:From; b=WFgJtw5ACgIRAyKx3TKKMwbhW2WgSueANdrKKbBKdBndFiq7YY5atdMu2O22g8QN1 yu+15Uc2wTr8dJIc8VNZlP5mGQf6Q5zfJO2YWMgfzwZdgYrHg9absTE3yyFSk6iExz j5bRLRcB1/XBqJOaDqwy81kYqxdKW/M0YRCDY2QYVRFAYw02zcyMPigm6BLoyHaVb8 BZ29MuEK9lvrWETT4xLxojgvwUWqgJr9r0V13dn3yWY0BOns/PvvyFbJXk0lLkO23U wz0aXjFsmt7nrXiMJCwxlnoBUvWj6bgS4REbPqTpJ67KcRA9C2Z8cCLnSBfRno8VdM zeAbiqY3mfOdA== From: SeongJae Park To: Andrew Morton Cc: SeongJae Park , "# 6 . 5 . x" , Hugh Dickins , damon@lists.linux.dev, kernel-team@meta.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Xinyu Zheng Subject: [PATCH] mm/damon/vaddr: do not repeat pte_offset_map_lock() until success Date: Mon, 29 Sep 2025 17:44:09 -0700 Message-Id: <20250930004410.55228-1-sj@kernel.org> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 Closes: https://lore.kernel.org/20250918030029.2652607-1-zhengxinyu6@huawei= .com Fixes: 7780d04046a2 ("mm/pagewalkers: ACTION_AGAIN if pte_offset_map_lock()= fails") Cc: # 6.5.x Cc: Hugh Dickins Signed-off-by: SeongJae Park Acked-by: Hugh Dickins --- mm/damon/vaddr.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) 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, } =20 pte =3D pte_offset_map_lock(walk->mm, pmd, addr, &ptl); - if (!pte) { - walk->action =3D 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 */ =20 pte =3D pte_offset_map_lock(walk->mm, pmd, addr, &ptl); - if (!pte) { - walk->action =3D ACTION_AGAIN; + if (!pte) return 0; - } ptent =3D ptep_get(pte); if (!pte_present(ptent)) goto out; base-commit: 3169a901e935bc1f2d2eec0171abcf524b7747e4 --=20 2.39.5