From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id A2F36C87FD2 for ; Fri, 1 Aug 2025 02:40:59 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CB9456B00B2; Thu, 31 Jul 2025 22:40:58 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C91366B00B9; Thu, 31 Jul 2025 22:40:58 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BA74A6B00BB; Thu, 31 Jul 2025 22:40:58 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id A83FD6B00B2 for ; Thu, 31 Jul 2025 22:40:58 -0400 (EDT) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 3D4AA160EB1 for ; Fri, 1 Aug 2025 02:40:58 +0000 (UTC) X-FDA: 83726636196.29.6863F78 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf30.hostedemail.com (Postfix) with ESMTP id 65FE580004 for ; Fri, 1 Aug 2025 02:40:56 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=none; spf=pass (imf30.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1754016056; a=rsa-sha256; cv=none; b=kND5HAvHlof3LuhzNFkALGw4fCossx6nvra5kH455JBz8ouLWLuNLi670BTfiVwlxBjZR3 6UkeQlJiz9x7PD1ToS5pnc/I7WSx/TOKQWnmEonpodUihi+5XbMhXzp1WP7IOi4bGbx9i3 UEilnHpD8v/82R64M2nDp5Aoes8GTik= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=none; spf=pass (imf30.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1754016056; h=from:from:sender: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=MmDlPPXe7TbvZXcMn5DrT5gPyqKTuiy3b6OSFccn4+M=; b=VRxDtnVOtRSHanNVkuXS/pdd7vxWvqSZeSMBQ9zfzekjN3r6oIyR1BuSg7of9GYaCSATWI iQGsCkVUaare2+MEoJG6WO1IBm4oE5yxipah8y2qBjAOh1AgKHCFEkVgG5Gj9qzaZ7bfJb 3v7NES9Bmgke11YGAOKGMyspAT8nQjU= 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 871DA16A3; Thu, 31 Jul 2025 19:40:47 -0700 (PDT) Received: from [10.164.146.15] (J09HK2D2RT.blr.arm.com [10.164.146.15]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 18C4A3F5A1; Thu, 31 Jul 2025 19:40:53 -0700 (PDT) Message-ID: Date: Fri, 1 Aug 2025 08:10:50 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/debug_vm_pgtable: clear page table entries at destroy_args() To: "Herton R. Krzesinski" , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org References: <20250731214051.4115182-1-herton@redhat.com> Content-Language: en-US From: Anshuman Khandual In-Reply-To: <20250731214051.4115182-1-herton@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 65FE580004 X-Stat-Signature: 3t5x9imjctao843shwrzbstc51sbn8rg X-Rspam-User: X-HE-Tag: 1754016056-787169 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Herton, On 01/08/25 3:10 AM, 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 Should not the allocators ensure that the allocated memory elements are all cleaned up before using them ? > 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): > > [ 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 How did you ensure that it was allocated from debug_vm_pgtable ? Trace prints during its execution and then matching up the addresses ? Just curious. > 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, So should they clear entries or doing so would add to overall latency ? > 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(). Makes sense. > > 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). Seems like a very rare case i.e both to reproduce and also to confirm if this patch here has indeed solved the problem. Just wondering - did you try to reproduce this problem on any other platform than powerpc ? > > 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. Makes sense. > > 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 A quick test on arm64 platform looked fine. It might be better to get this enabled and tested on multiple platforms via linux-next.