From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta1.migadu.com (out-170.mta1.migadu.com [95.215.58.170]) (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 9D846134CF for ; Mon, 20 Jul 2026 02:46:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784515580; cv=none; b=YLarrpdgpuEjoTjDuYz1sKRYrRwnbkefs0wG8BeEnPj03WRP9K26bcZkXu0Q9PXPkdlVKQiefzBDQvCSSnLSA7cLetEZcnqCnR1T+XsjwUvtA2u5b7UzdW9FyEBNbnhp3yE8Al44xc90QZPkjJcRxkOgpB+DAXdRzIahMkdH0Ec= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784515580; c=relaxed/simple; bh=y8Rn6cR7ilu+lBQrnhmaK0qpy8px3TUB3t8SJRjNb8s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ChY9inE1NEeL8NHLN8th2Oc7xMYxsqpaQH66TBuYr0eWxCrKnt8eGF+yPBOyAgnU4hn8TFiC3GGS0TqvXiYoRdIJ7Hk1ug83OB04HPUi2b40RUG1BOLShHfwoPjDnFUqEcM6Uhd3Zqf3+nTp6GNrWWCATv7IpSoL0gNN89T050M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=VthVF208; arc=none smtp.client-ip=95.215.58.170 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="VthVF208" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784515575; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QXQ5xmSSPrBQjMJEqhXYcrMLUG5j6Qkd/yAMgAIwkNw=; b=VthVF208Rz9Zsj4OxKww1Mh1aAqYZh8ODArvhqsNu/s9Ske8sgST5cTVQhcrBJSwI3uBXA 8JB3rppGUV3NYJ1/DPE/YZSx27jeYEnZxvFC9vJU/oQaZecnU0TYfcouoK5EtVUvnMZeDp 20xvsq1APwou9w69K59p8VOMpwoy8r4= Date: Mon, 20 Jul 2026 10:45:56 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v6 9/9] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg() To: "Vlastimil Babka (SUSE)" , Andrew Morton Cc: Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260714015117.78351-1-ye.liu@linux.dev> <20260714015117.78351-10-ye.liu@linux.dev> <21007313-40dc-46fb-bbf2-453c8eedf3e8@kernel.org> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Ye Liu In-Reply-To: <21007313-40dc-46fb-bbf2-453c8eedf3e8@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2026/7/14 16:24, Vlastimil Babka (SUSE) 写道: > On 7/14/26 03:51, Ye Liu wrote: >> print_page_owner_memcg() takes a snapshot of page->memcg_data via >> READ_ONCE at the top of the function and guards against tail pages and >> NULL memcg_data. However, it later calls two functions that re-read >> page->memcg_data locklessly: >> >> 1) page_memcg_check(page) — re-reads page->memcg_data; >> 2) PageMemcgKmem(page) — calls folio_memcg_kmem(), which re-reads >> folio->memcg_data and folio->page->compound_head, wrapping both >> in VM_BUG_ON assertions: >> >> VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page); >> VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio); >> >> If the page is concurrently freed and reallocated as a THP tail page >> or a slab page between the initial guards and these later calls, the >> VM_BUG_ON assertions can fire on debug builds (CONFIG_DEBUG_VM=y), >> causing a kernel panic. >> >> Fix both TOCTOU issues by using the memcg_data snapshot throughout: >> - Extract objcg from the snapshot via objcg = (void *)(memcg_data & >> ~OBJEXTS_FLAGS_MASK) instead of calling page_memcg_check(page); >> - Test (memcg_data & MEMCG_DATA_KMEM) instead of calling >> PageMemcgKmem(page), which is semantically equivalent: >> PageMemcgKmem()->folio_memcg_kmem()->folio->memcg_data & >> MEMCG_DATA_KMEM. >> - When memcg_data has MEMCG_DATA_OBJEXTS set, early-return after >> printing "Slab cache page\n" since objcg != memcg for slab pages >> and there is no meaningful cgroup to look up. > > These points have too much detail that's already in the code. Would just > mention that we opencode applicable parts of page_memcg_check() and > PageMemcgKmem() using the snapshot? Yes,It's a bit wordy. > >> This avoids both TOCTOU windows and the assertions entirely. >> >> Signed-off-by: Ye Liu > > Reviewed-by: Vlastimil Babka (SUSE) > > Was all of this reported by sashiko? At least the new-in-v6 was? > Then: > > Reported-by: Sashiko > > But it's no longer a cleanup but a fix, so probably this? > > Fixes: fcf8935832b8 ("mm/page_owner: print memcg information") > Cc: stable@vger.kernel.org > > It's not fixing a new regression so I think it's fine to keep it part of > this series for next release and not need to split out for mm-hotfixes. > Hi Andrew, Could you please help me revise the above? Should I send you V7? >> --- >> Changes in v6: >> - Rename patch to cover both TOCTOU fixes rather than only >> PageMemcgKmem(). >> - Also replace page_memcg_check(page) with extracting objcg from the >> memcg_data snapshot to fix a second TOCTOU issue. >> - Add early return for the MEMCG_DATA_OBJEXTS (slab) case since >> objcg != memcg for slab pages and there is no cgroup to look up. >> - Update commit message to cover all changes. >> - Link: https://lore.kernel.org/all/20260701061101.344679-10-ye.liu@linux.dev/ >> mm/page_owner.c | 10 +++++++--- >> 1 file changed, 7 insertions(+), 3 deletions(-) >> >> diff --git a/mm/page_owner.c b/mm/page_owner.c >> index 2e3880053a34..e18512a49e38 100644 >> --- a/mm/page_owner.c >> +++ b/mm/page_owner.c >> @@ -540,6 +540,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> struct page *page) >> { >> unsigned long memcg_data; >> + struct obj_cgroup *objcg; >> struct mem_cgroup *memcg; >> bool online; >> char name[80]; >> @@ -549,11 +550,14 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> if (!memcg_data || PageTail(page)) >> goto out_unlock; >> >> - if (memcg_data & MEMCG_DATA_OBJEXTS) >> + if (memcg_data & MEMCG_DATA_OBJEXTS) { >> ret += scnprintf(kbuf + ret, count - ret, >> "Slab cache page\n"); >> + goto out_unlock; >> + } >> >> - memcg = page_memcg_check(page); >> + objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK); >> + memcg = objcg ? obj_cgroup_memcg(objcg) : NULL; >> if (!memcg) >> goto out_unlock; >> >> @@ -561,7 +565,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> cgroup_name(memcg->css.cgroup, name, sizeof(name)); >> ret += scnprintf(kbuf + ret, count - ret, >> "Charged %sto %smemcg %s\n", >> - PageMemcgKmem(page) ? "(via objcg) " : "", >> + (memcg_data & MEMCG_DATA_KMEM) ? "(via objcg) " : "", >> online ? "" : "offline ", >> name); >> out_unlock: > -- Thanks, Ye Liu