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 85D8D2222B4 for ; Sun, 3 Aug 2025 10:27:45 +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=1754216868; cv=none; b=d/F6DV81vgoMCGBaw8Kl+FbIvmYT1ZvzKU6qu7LxTUBVZL78j+cHPZKLO4S+Oqkal1+JPtKmwHa68aH0yQ/C9iTsqTUKJCBFuBNkcQ2e13YpkjM6+bwtlSD0abUbklXjlssPoAfATvJBJ7pMjt3NhHRKfJjd0QIhEZKUmqTvex8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754216868; c=relaxed/simple; bh=sTHNqmcaHvBBrrFg/EijFSf3ApAd+tbhxiGCv0tQiAw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YAXnIUqBDfao0ghLjIk5qzkx4CZHBqSEQMx9illMlJax24vRVoivq5O39e1usZn4qc0KzN4+pGMB6OLhgA+IiYrUkQMk/RofZcCYGLGJdYBQsvRqvIW86tJxCEqY57l6aPI6VINOjwiEzE6MglizonpLkIksEdQZvownxX/tTvU= 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 6DC85150C; Sun, 3 Aug 2025 03:27:36 -0700 (PDT) Received: from [10.163.64.123] (unknown [10.163.64.123]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 25AF43F5A1; Sun, 3 Aug 2025 03:27:41 -0700 (PDT) Message-ID: <9cbc13bf-bfbf-4108-bbc0-d33d75fe7d18@arm.com> Date: Sun, 3 Aug 2025 15:57:38 +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] mm/debug_vm_pgtable: clear page table entries at destroy_args() To: Andrew Morton , "Herton R. Krzesinski" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Christophe Leroy , Gavin Shan , Gerald Schaefer References: <20250731214051.4115182-1-herton@redhat.com> <20250801135050.c9cc7226938f9f0f4fa3b83d@linux-foundation.org> Content-Language: en-US From: Anshuman Khandual In-Reply-To: <20250801135050.c9cc7226938f9f0f4fa3b83d@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 02/08/25 2:20 AM, Andrew Morton wrote: > On Thu, 31 Jul 2025 18:40:51 -0300 "Herton R. Krzesinski" wrote: > >> The mm/debug_vm_pagetable test allocates manually page table entries for the >> tests it runs, using also its manually allocated mm_struct. That in itself is >> ok, but when it exits, at destroy_args() it fails to clear those entries with >> the *_clear functions. >> >> The problem is that leaves stale entries. If another process allocates >> an mm_struct with a pgd at the same address, it may end up running into >> the stale entry. This is happening in practice on a debug kernel with >> CONFIG_DEBUG_VM_PGTABLE=y, for example this is the output with some >> extra debugging I added (it prints a warning trace if pgtables_bytes goes >> negative, in addition to the warning at check_mm() function): > > A quick shot with git-blame led me to include > > Fixes: 3c9b84f044a9e ("mm/debug_vm_pgtable: introduce struct pgtable_debug_args") > Cc: Agreed. > > And `git show 3c9b84f044a9e' tell me this email didn't have enough cc's > (added). Sure, that makes sense. > > Thanks, I'll include this in mm.git's mm-hotfixes branch and I shall > await further review activity. Right - it will be great to have this tested across other supporting platforms. > > >> [ 2.539353] debug_vm_pgtable: [get_random_vaddr ]: random_vaddr is 0x7ea247140000 >> [ 2.539366] kmem_cache info >> [ 2.539374] kmem_cachep 0x000000002ce82385 - freelist 0x0000000000000000 - offset 0x508 >> [ 2.539447] debug_vm_pgtable: [init_args ]: args->mm is 0x000000002267cc9e >> (...) >> [ 2.552800] WARNING: CPU: 5 PID: 116 at include/linux/mm.h:2841 free_pud_range+0x8bc/0x8d0 >> [ 2.552816] Modules linked in: >> [ 2.552843] CPU: 5 UID: 0 PID: 116 Comm: modprobe Not tainted 6.12.0-105.debug_vm2.el10.ppc64le+debug #1 VOLUNTARY >> [ 2.552859] Hardware name: IBM,9009-41A POWER9 (architected) 0x4e0202 0xf000005 of:IBM,FW910.00 (VL910_062) hv:phyp pSeries >> [ 2.552872] NIP: c0000000007eef3c LR: c0000000007eef30 CTR: c0000000003d8c90 >> [ 2.552885] REGS: c0000000622e73b0 TRAP: 0700 Not tainted (6.12.0-105.debug_vm2.el10.ppc64le+debug) >> [ 2.552899] MSR: 800000000282b033 CR: 24002822 XER: 0000000a >> [ 2.552954] CFAR: c0000000008f03f0 IRQMASK: 0 >> [ 2.552954] GPR00: c0000000007eef30 c0000000622e7650 c000000002b1ac00 0000000000000001 >> [ 2.552954] GPR04: 0000000000000008 0000000000000000 c0000000007eef30 ffffffffffffffff >> [ 2.552954] GPR08: 00000000ffff00f5 0000000000000001 0000000000000048 0000000000004000 >> [ 2.552954] GPR12: 00000003fa440000 c000000017ffa300 c0000000051d9f80 ffffffffffffffdb >> [ 2.552954] GPR16: 0000000000000000 0000000000000008 000000000000000a 60000000000000e0 >> [ 2.552954] GPR20: 4080000000000000 c0000000113af038 00007fffcf130000 0000700000000000 >> [ 2.552954] GPR24: c000000062a6a000 0000000000000001 8000000062a68000 0000000000000001 >> [ 2.552954] GPR28: 000000000000000a c000000062ebc600 0000000000002000 c000000062ebc760 >> [ 2.553170] NIP [c0000000007eef3c] free_pud_range+0x8bc/0x8d0 >> [ 2.553185] LR [c0000000007eef30] free_pud_range+0x8b0/0x8d0 >> [ 2.553199] Call Trace: >> [ 2.553207] [c0000000622e7650] [c0000000007eef30] free_pud_range+0x8b0/0x8d0 (unreliable) >> [ 2.553229] [c0000000622e7750] [c0000000007f40b4] free_pgd_range+0x284/0x3b0 >> [ 2.553248] [c0000000622e7800] [c0000000007f4630] free_pgtables+0x450/0x570 >> [ 2.553274] [c0000000622e78e0] [c0000000008161c0] exit_mmap+0x250/0x650 >> [ 2.553292] [c0000000622e7a30] [c0000000001b95b8] __mmput+0x98/0x290 >> [ 2.558344] [c0000000622e7a80] [c0000000001d1018] exit_mm+0x118/0x1b0 >> [ 2.558361] [c0000000622e7ac0] [c0000000001d141c] do_exit+0x2ec/0x870 >> [ 2.558376] [c0000000622e7b60] [c0000000001d1ca8] do_group_exit+0x88/0x150 >> [ 2.558391] [c0000000622e7bb0] [c0000000001d1db8] sys_exit_group+0x48/0x50 >> [ 2.558407] [c0000000622e7be0] [c00000000003d810] system_call_exception+0x1e0/0x4c0 >> [ 2.558423] [c0000000622e7e50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec >> (...) >> [ 2.558892] ---[ end trace 0000000000000000 ]--- >> [ 2.559022] BUG: Bad rss-counter state mm:000000002267cc9e type:MM_ANONPAGES val:1 >> [ 2.559037] BUG: non-zero pgtables_bytes on freeing mm: -6144 >> >> Here the modprobe process ended up with an allocated mm_struct from the >> mm_struct slab that was used before by the debug_vm_pgtable test. That is not a >> problem, since the mm_struct is initialized again etc., however, if it ends up >> using the same pgd table, it bumps into the old stale entry when clearing/freeing >> the page table entries, so it tries to free an entry already gone (that one >> which was allocated by the debug_vm_pgtable test), which also explains the >> negative pgtables_bytes since it's accounting for not allocated entries in the >> current process. As far as I looked pgd_{alloc,free} etc. does not clear entries, >> and clearing of the entries is explicitly done in the free_pgtables-> >> free_pgd_range->free_p4d_range->free_pud_range->free_pmd_range-> >> free_pte_range path. However, the debug_vm_pgtable test does not call >> free_pgtables, since it allocates mm_struct and entries manually for its test >> and eg. not goes through page faults. So it also should clear manually the >> entries before exit at destroy_args(). >> >> This problem was noticed on a reboot X number of times test being done >> on a powerpc host, with a debug kernel with CONFIG_DEBUG_VM_PGTABLE >> enabled. Depends on the system, but on a 100 times reboot loop the >> problem could manifest once or twice, if a process ends up getting the >> right mm->pgd entry with the stale entries used by mm/debug_vm_pagetable. >> After using this patch, I couldn't reproduce/experience the problems >> anymore. I was able to reproduce the problem as well on latest upstream >> kernel (6.16). >> >> I also modified destroy_args() to use mmput() instead of mmdrop(), there >> is no reason to hold mm_users reference and not release the mm_struct >> entirely, and in the output above with my debugging prints I already >> had patched it to use mmput, it did not fix the problem, but helped >> in the debugging as well. >> >> Signed-off-by: Herton R. Krzesinski >> --- >> mm/debug_vm_pgtable.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c >> index 7731b238b534..0f5ddefd128a 100644 >> --- a/mm/debug_vm_pgtable.c >> +++ b/mm/debug_vm_pgtable.c >> @@ -1041,29 +1041,34 @@ static void __init destroy_args(struct pgtable_debug_args *args) >> >> /* Free page table entries */ >> if (args->start_ptep) { >> + pmd_clear(args->pmdp); >> pte_free(args->mm, args->start_ptep); >> mm_dec_nr_ptes(args->mm); >> } >> >> if (args->start_pmdp) { >> + pud_clear(args->pudp); >> pmd_free(args->mm, args->start_pmdp); >> mm_dec_nr_pmds(args->mm); >> } >> >> if (args->start_pudp) { >> + p4d_clear(args->p4dp); >> pud_free(args->mm, args->start_pudp); >> mm_dec_nr_puds(args->mm); >> } >> >> - if (args->start_p4dp) >> + if (args->start_p4dp) { >> + pgd_clear(args->pgdp); >> p4d_free(args->mm, args->start_p4dp); >> + } >> >> /* Free vma and mm struct */ >> if (args->vma) >> vm_area_free(args->vma); >> >> if (args->mm) >> - mmdrop(args->mm); >> + mmput(args->mm); >> } >> >> static struct page * __init >> -- >> 2.47.1