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 85F7628506C for ; Tue, 3 Mar 2026 10:32:16 +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=1772533938; cv=none; b=Vr6h1CHU2tvQYzflljwKB7rVEyf5tg4NDf6vBoBuu6Djs0eDv0i10TcptgftP6C0485Cl10+zlFuc2wkDXelvnGzM4XMw0Y7lFO34OoD1wkw9xab/oMIHvWcB/ub++Svd5Tf9+g9IMIphE7zN0a4lYs5MLHNrKk42h/kfCAF5Gg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772533938; c=relaxed/simple; bh=ijdgVVoitEjdbH+Cq1XCJh375DyZOf3ZTLDCCXPGTvs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rMjEF70xe88f9cXnev2spyIYF77TnaLb7XSn7GgRQKhEQZbFg3Y0iVY2djjvxjDXo/tnKqhEw6/4eNTJJvEgXWYM0spGc5Z0zfu/oMid5Co2KIdC7UMqsnCsCNzt27DaBQMvT0H/ohxwstE33VQyjoLsKJdt51u1Z28+CmOLHng= 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 A34D7497; Tue, 3 Mar 2026 02:32:09 -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 BB6553F7BD; Tue, 3 Mar 2026 02:32:11 -0800 (PST) Message-ID: <56d607c0-dc72-431a-bc86-6dae100f1708@arm.com> Date: Tue, 3 Mar 2026 16:02:08 +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 v3] mm/rmap: fix incorrect pte restoration for lazyfree folios To: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com Cc: riel@surriel.com, Liam.Howlett@oracle.com, vbabka@kernel.org, harry.yoo@oracle.com, jannh@google.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable References: <20260303061528.2429162-1-dev.jain@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20260303061528.2429162-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 03/03/26 11:45 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: Andrew, could you convert the last line to the following: ..restoration path, and the kernel will crash with the trace given below. The BUG happens at: BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw); The code path is asking for anonymous page to be mapped writable into the pagetable. The BUG_ON() firing implies that such a writable page has been mapped into the pagetables of more than one process, which breaks anonymous memory/CoW semantics. > > [ 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 > --- > Patch applies on mm-unstable (9af4957ef127). > > v2->v3: > - Don't special case for anon folios > > v1->v2: > - Just respect the writable bit instead of hacking in a pte_wrprotect() in > failure path > - Also handle soft-dirty bit > > mm/rmap.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index bff8f222004e4..5a3e408e3f179 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1955,7 +1955,14 @@ 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 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); > } > > /*