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 3734ACA0EDC for ; Thu, 14 Aug 2025 10:46:12 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C235A90013E; Thu, 14 Aug 2025 06:46:11 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id BD4FD900121; Thu, 14 Aug 2025 06:46:11 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A75A090013E; Thu, 14 Aug 2025 06:46:11 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 8BB87900121 for ; Thu, 14 Aug 2025 06:46:11 -0400 (EDT) Received: from smtpin15.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 430DF117B75 for ; Thu, 14 Aug 2025 10:46:11 +0000 (UTC) X-FDA: 83775033342.15.F6D8163 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf10.hostedemail.com (Postfix) with ESMTP id 3F5C2C000A for ; Thu, 14 Aug 2025 10:46:09 +0000 (UTC) Authentication-Results: imf10.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf10.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1755168369; 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=IIVK3+/oQmeFPwI4Ut9SelhyWIwxiHv6uaV97+15kV4=; b=0hqfuNaKCaRgzYBmtfFn72UXfPrMB4gXnjmxx8SDA2WpRdmFp9twlPEwN+GQk+k4kTAZjy UaAfh5MW5XEP1F0tdkUheBZnRPI22JvgsZjNJtZHUeD+Tk433eKCQ0hvDFEkevFieW4xLy OuHw7DwTXEAbZLAtkmNrh1Qln1cghwM= ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf10.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1755168369; a=rsa-sha256; cv=none; b=d/a80o2N4M4rAe0jOruv+gXqoh4niDDGkehv8IsxJM2EhIA0EZW7SOouSxktWD9HNXg0Gh jj5MTVgK/dKlM3wGA+Y+CjwTuXUb5RQsBZhq4wULQLOrHuJdVDH54M00Yv7m4FR2PvgfdF XyelhI1CBIZ1N7fR2mXRTmCO8NQuAkY= 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 E56F2113E; Thu, 14 Aug 2025 03:45:59 -0700 (PDT) Received: from [10.163.65.236] (unknown [10.163.65.236]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C23BE3F738; Thu, 14 Aug 2025 03:46:06 -0700 (PDT) Message-ID: <5e5f45c1-e813-4900-8fad-2ed0dc067468@arm.com> Date: Thu, 14 Aug 2025 16:16:03 +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: 3F5C2C000A X-Stat-Signature: o5gph6whurk4jahqdsakdirtdewcene4 X-Rspam-User: X-HE-Tag: 1755168369-586572 X-HE-Meta: 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 7etEBYU9 S5Gs0FXelRZGQEoBuE1jJ3m8EDR28pW04pGVvxWCIvdHTn/8rwZghbp7HMvAD7SubXdeY/l09tgQJ2Yh6msJ7cxJa/Sex0vl8kD0qQTk2H2SMlIVJ3DTGtC50d3hqJLAR5otmUve6Rhv7nKuHsfLYuOqZMFkTfJoIZIHv+IhMUTg1niDroCYEa0znJi/ctbiuGVExbM1ci8Dh5G4Fuc9XuM3Bww== 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: This has been on linux-next for almost last two weeks now and no problem has been reported. So I guess it's all good. 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 > 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 > 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