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Krzesinski" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, anshuman.khandual@arm.com, Christophe Leroy , Gavin Shan , Gerald Schaefer Subject: Re: [PATCH] mm/debug_vm_pgtable: clear page table entries at destroy_args() Message-Id: <20250801135050.c9cc7226938f9f0f4fa3b83d@linux-foundation.org> In-Reply-To: <20250731214051.4115182-1-herton@redhat.com> References: <20250731214051.4115182-1-herton@redhat.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 2364514000C X-Stat-Signature: kodqfcdhbte88jzbp7y9cqrybjsniukt X-Rspam-User: X-HE-Tag: 1754081451-92608 X-HE-Meta: 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 eOnoXMW+ hi/+JonBsx5I+9tWeq1DqY4HFG0lJmwUVSpbCBQhFlPGybrd2ziogjNXAS5zuhAyrAt0OohONGHe1x1geVGqOBge0qsWcNq9yEEnfcbZPmfSehiEmwHWNEpp67Iwh9JM23NTY5di1wIedc0SERsj/fsDmSNyFsGlCsKivQMK0hapGIbQnwDIkFBMBS6NIl5ffzEMAO1kLlD3SimwXEukdYgAba1gqNRYUNMWvRUFGp67D9V/7jGm7+BA6DuNOJV2g+7iakYzDnjrLeSRBzLYxio2nNNGSArhO4/LnHV99VCPNrNpZ/Qf9M9Bnjj6b7U35W1JCl6N+bpnfUTON7/5r73+ljA== 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: 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: And `git show 3c9b84f044a9e' tell me this email didn't have enough cc's (added). Thanks, I'll include this in mm.git's mm-hotfixes branch and I shall await further review activity. > [ 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