From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id F05801E1DE5 for ; Mon, 2 Mar 2026 09:06:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772442393; cv=none; b=ordf07Zjc+n2NtEovp/fQ2Yw5MKeVI46D0SSdEKFQBqiZp1YmwjZGVwfrgDM0vUi7oC90JgeM7mMjDr5i4TAlLYBTyYkQuibnUtmVMkYQ7OKbaoUTChP73G+/U5ZGQ652tezcbVVhg3QcQdxfcwjxkAc4W1GQQVkbBM8uG7e3qw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772442393; c=relaxed/simple; bh=qC0RkfzOFY3WZ6KgC1qwmL+kKJT2sC92giCkTDCW2l8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ryzghLWUmPj0eyKi5lWM9yg57YL9vY4jQWzXPjzF2atiqHU/yjgbRp2DzEuckkFOxsW+j1WRDg++VsLviqtYICNHMaLQuQtP7BCcA1sGuOudyjzaZLZq/yarzi4aKbN9f766LOPrjgQOZ0wDNv4sjv8KqQcIeocRC230qZA0/lg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9832B14BF; Mon, 2 Mar 2026 01:06:24 -0800 (PST) Received: from [10.164.19.52] (unknown [10.164.19.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 881973F73B; Mon, 2 Mar 2026 01:06:26 -0800 (PST) Message-ID: <99e4c72f-dc2f-4b26-9b32-0dd716789060@arm.com> Date: Mon, 2 Mar 2026 14:36:23 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/rmap: fix incorrect pte restoration for lazyfree folios To: "David Hildenbrand (Arm)" , Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@kernel.org, harry.yoo@oracle.com, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable References: <20260228140540.1774748-1-dev.jain@arm.com> <2703daae-3854-492a-a3d8-8bedd6a58aaf@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: <2703daae-3854-492a-a3d8-8bedd6a58aaf@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 02/03/26 2:14 pm, David Hildenbrand (Arm) wrote: > On 2/28/26 19:34, Barry Song wrote: >> On Sun, Mar 1, 2026 at 3:06 AM Dev Jain wrote: >>> >>> We batch unmap anonymous lazyfree folios by folio_unmap_pte_batch. >>> If the batch has a mix of writable and non-writable bits, we may end up >>> setting the entire batch writable. Fix this by respecting writable bit >>> during batching. >>> Although on a successful unmap of a lazyfree folio, the soft-dirty bit is >>> lost, preserve it on pte restoration by respecting the bit during batching, >>> to make the fix consistent w.r.t both writable bit and soft-dirty bit. >>> >>> I was able to write the below reproducer and crash the kernel. >>> Explanation of reproducer (set 64K mTHP to always): >>> >>> Fault in a 64K large folio. Split the VMA at mid-point with MADV_DONTFORK. >>> fork() - parent points to the folio with 8 writable ptes and 8 non-writable >>> ptes. Merge the VMAs with MADV_DOFORK so that folio_unmap_pte_batch() can >>> determine all the 16 ptes as a batch. Do MADV_FREE on the range to mark >>> the folio as lazyfree. Write to the memory to dirty the pte, eventually >>> rmap will dirty the folio. Then trigger reclaim, we will hit the pte >>> restoration path, and the kernel will crash with the following trace: >>> >>> [ 21.134473] kernel BUG at mm/page_table_check.c:118! >>> [ 21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP >>> [ 21.135917] Modules linked in: >>> [ 21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT >>> [ 21.136858] Hardware name: linux,dummy-virt (DT) >>> [ 21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) >>> [ 21.137308] pc : page_table_check_set+0x28c/0x2a8 >>> [ 21.137607] lr : page_table_check_set+0x134/0x2a8 >>> [ 21.137885] sp : ffff80008a3b3340 >>> [ 21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000 >>> [ 21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001 >>> [ 21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30 >>> [ 21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000 >>> [ 21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff >>> [ 21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020 >>> [ 21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0 >>> [ 21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff >>> [ 21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002 >>> [ 21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0 >>> [ 21.141991] Call trace: >>> [ 21.142093] page_table_check_set+0x28c/0x2a8 (P) >>> [ 21.142265] __page_table_check_ptes_set+0x144/0x1e8 >>> [ 21.142441] __set_ptes_anysz.constprop.0+0x160/0x1a8 >>> [ 21.142766] contpte_set_ptes+0xe8/0x140 >>> [ 21.142907] try_to_unmap_one+0x10c4/0x10d0 >>> [ 21.143177] rmap_walk_anon+0x100/0x250 >>> [ 21.143315] try_to_unmap+0xa0/0xc8 >>> [ 21.143441] shrink_folio_list+0x59c/0x18a8 >>> [ 21.143759] shrink_lruvec+0x664/0xbf0 >>> [ 21.144043] shrink_node+0x218/0x878 >>> [ 21.144285] __node_reclaim.constprop.0+0x98/0x338 >>> [ 21.144763] user_proactive_reclaim+0x2a4/0x340 >>> [ 21.145056] reclaim_store+0x3c/0x60 >>> [ 21.145216] dev_attr_store+0x20/0x40 >>> [ 21.145585] sysfs_kf_write+0x84/0xa8 >>> [ 21.145835] kernfs_fop_write_iter+0x130/0x1c8 >>> [ 21.145994] vfs_write+0x2b8/0x368 >>> [ 21.146119] ksys_write+0x70/0x110 >>> [ 21.146240] __arm64_sys_write+0x24/0x38 >>> [ 21.146380] invoke_syscall+0x50/0x120 >>> [ 21.146513] el0_svc_common.constprop.0+0x48/0xf8 >>> [ 21.146679] do_el0_svc+0x28/0x40 >>> [ 21.146798] el0_svc+0x34/0x110 >>> [ 21.146926] el0t_64_sync_handler+0xa0/0xe8 >>> [ 21.147074] el0t_64_sync+0x198/0x1a0 >>> [ 21.147225] Code: f9400441 b4fff241 17ffff94 d4210000 (d4210000) >>> [ 21.147440] ---[ end trace 0000000000000000 ]--- >>> >>> >>> #define _GNU_SOURCE >>> #include >>> #include >>> #include >>> #include >>> #include >>> #include >>> #include >>> #include >>> >>> void write_to_reclaim() { >>> const char *path = "/sys/devices/system/node/node0/reclaim"; >>> const char *value = "409600000000"; >>> int fd = open(path, O_WRONLY); >>> if (fd == -1) { >>> perror("open"); >>> exit(EXIT_FAILURE); >>> } >>> >>> if (write(fd, value, sizeof("409600000000") - 1) == -1) { >>> perror("write"); >>> close(fd); >>> exit(EXIT_FAILURE); >>> } >>> >>> printf("Successfully wrote %s to %s\n", value, path); >>> close(fd); >>> } >>> >>> int main() >>> { >>> char *ptr = mmap((void *)(1UL << 30), 1UL << 16, PROT_READ | PROT_WRITE, >>> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); >>> if ((unsigned long)ptr != (1UL << 30)) { >>> perror("mmap"); >>> return 1; >>> } >>> >>> /* a 64K folio gets faulted in */ >>> memset(ptr, 0, 1UL << 16); >>> >>> /* 32K half will not be shared into child */ >>> if (madvise(ptr, 1UL << 15, MADV_DONTFORK)) { >>> perror("madvise madv dontfork"); >>> return 1; >>> } >>> >>> pid_t pid = fork(); >>> >>> if (pid < 0) { >>> perror("fork"); >>> return 1; >>> } else if (pid == 0) { >>> sleep(15); >>> } else { >>> /* merge VMAs. now first half of the 16 ptes are writable, the other half not. */ >>> if (madvise(ptr, 1UL << 15, MADV_DOFORK)) { >>> perror("madvise madv fork"); >>> return 1; >>> } >>> if (madvise(ptr, (1UL << 16), MADV_FREE)) { >>> perror("madvise madv free"); >>> return 1; >>> } >>> >>> /* dirty the large folio */ >>> (*ptr) += 10; >>> >>> write_to_reclaim(); >>> // sleep(10); >>> waitpid(pid, NULL, 0); >>> >>> } >>> } >>> >>> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") >>> Cc: stable >>> Signed-off-by: Dev Jain >>> --- >>> v1->v2: >>> - Just respect the writable bit instead of hacking in a pte_wrprotect() in >>> failure path >>> - Also handle soft-dirty bit >>> >>> Based on mm-unstable (df9c51269a5e). >>> >>> mm/rmap.c | 12 +++++++++++- >>> 1 file changed, 11 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index bff8f222004e4..fb64829913052 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1955,7 +1955,17 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >>> if (userfaultfd_wp(vma)) >>> return 1; >>> >>> - return folio_pte_batch(folio, pvmw->pte, pte, max_nr); >>> + if (!folio_test_anon(folio)) >>> + return folio_pte_batch(folio, pvmw->pte, pte, max_nr); >>> + >>> + /* >>> + * For anon folios, if unmap fails, we need to restore the ptes. >>> + * To avoid accidentally upgrading write permissions for ptes that >>> + * were not originally writable, and to avoid losing the soft-dirty >>> + * bit, use the appropriate FPB flags. >>> + */ >>> + return folio_pte_batch_flags(folio, vma, pvmw->pte, &pte, max_nr, >>> + FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY); >> >> Do we really need to differentiate between file and anon? >> I’d rather just return unconditionally by removing the >> if (!folio_test_anon(folio)) check above. >> >> If we do want to keep two branches, why not use a flag variant instead? > > I suspect Dev's code might generate better code, as the compiler might > not want to provide two variants of folio_pte_batch_flags() where it > propagates all constants; and even if it does, we'd end up with two > essentially identical functions in the kernel binary. Interesting. > > So if we want to special-case anon folios, I think we should use Dev's > variant. > > But I also wonder whether we just want to keep it simple for now and > just unconditionally check FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY. > > I'd vote for simplicity at this point. Fair enough. I'll do that. >