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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Vishal Moola , Ingo Molnar , Lu Baolu , Jason Gunthorpe , Steven Rostedt , x86@kernel.org, linux-mm@kvack.org, regressions@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: don't schedule deferred kernel page table freeing while booting Message-ID: References: <20260924092307.22813-1-mikhail.v.gavrilov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924092307.22813-1-mikhail.v.gavrilov@gmail.com> X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: CCB0F140006 X-Stat-Signature: 1iu9wjoxwtxpuwrgdd3tpzgr5ckzf5jd X-Rspam-User: X-HE-Tag: 1790266766-951262 X-HE-Meta: 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 uIX7AZjP SqHvEr5aegXM1SA3TvJmCAD8Fjd69FGre/6Mut/3Cj0hoohDsfA7xiMHPBqtm0eTJQFhCZwrdjGpylTjOLK3E1G3UuvuDcCSFmTamOartKkANXmi9bFHDa7xNAAcocQWd/qpauEZMZJJhvygaEBeaie9Whwbunf3y85hEbCp5+keSDWjmzGAkktJS7/CvFy+2o3Igh/FsQ3q6Z0za+JKEcVN0hnkgMb91DC3z1H8ji0gh4KlaIRzukHD7xJ04HjClq/wfmDSxXU63zLGTVGB8C8CqrmlgFCe0VyzLBXhdf6lQsWhkm9BSQup1kH7ekRgxYWErcj4B38nkhczRywVxGjjlhJ0XOJdqNn8oSnHjoQ/RdfOYvIdNUXgU1A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 24, 2026 at 02:23:07PM +0500, Mikhail Gavrilov wrote: > Booting with a boot-time function tracer and a filter, for example > > ftrace=function ftrace_filter=pud_free_pmd_page > > panics on 7.3-rc4 as soon as the tracer starts: > > [ 23.531178] Starting tracer 'function' > [ 23.675800] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000038: 0000 [#1] SMP KASAN NOPTI > [ 23.819917] KASAN: null-ptr-deref in range [0x00000000000001c0-0x00000000000001c7] > [ 23.964025] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.3.0-rc4-fe2ec83746e5-with-fixes-v2+ #195 PREEMPT(undef) > [ 24.252248] RIP: 0010:__queue_work+0xab/0xf00 > [ 25.981629] Call Trace: > [ 26.125727] > [ 26.413912] ? pagetable_free_kernel+0x20/0x120 > [ 26.990283] queue_work_on+0x97/0xf0 > [ 27.134382] __cpa_collapse_large_pages+0x501/0x6f0 > [ 27.566662] cpa_flush+0x394/0x620 > [ 27.998953] change_page_attr_set_clr+0x321/0x4a0 > [ 29.151729] set_memory_rox+0xa2/0xf0 > [ 29.584018] create_trampoline+0x431/0x6f0 > ... > [ 44.343347] Kernel panic - not syncing: Attempted to kill the idle task! > > The boot-time tracer is started from early_trace_init(), which runs > before workqueue_init_early(). Making its trampoline read-only splits a > large page, and CPA collapses it again right away. The split table has > been a kernel page table since commit 9e4a3ec3411b > ("x86/mm/pat: Allocate split page tables as kernel page tables"), so the > collapse frees it through pagetable_free_kernel(), which queues work on > system_percpu_wq - still NULL at that point. That commit is correct in > itself; it only lets CPA reach pagetable_free_kernel() before the > workqueue that function relies on exists. > > Keep putting the table on the list, but don't schedule the work while > the system is still booting. The next kernel page table freed after > boot schedules it, and the work then frees the early table too, after > the same IOMMU flush as any other. If no kernel page table is freed > after boot, the ones freed during boot stay on the list. > > Fixes: 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel page tables") > Suggested-by: David Hildenbrand (Arm) > Cc: stable@vger.kernel.org > Signed-off-by: Mikhail Gavrilov > Link: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com > --- > v2: > - Keep the table on the list and only skip scheduling the work while > booting, instead of freeing it directly (David Hildenbrand) > - Say that 9e4a3ec3411b is correct in itself and only exposes the > problem (Lorenzo Stoakes) > v1: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com > > Tested on a Ryzen 9 7950X with a Radeon RX 7900 XTX, lockdep and KASAN > enabled, on 7.3-rc4 (fe2ec83746e5) with the same unrelated local > changes as noted for v1, booting with > > ftrace=function ftrace_filter=pud_free_pmd_page,pagetable_free_kernel,kernel_pgtable_work_func > > The boot that panicked without the fix completes. The table freed > while the tracer installs itself does not show up in the trace, since > the tracer is not live yet at that point, but the first kernel page > table freed after boot does: systemd-modules-load freeing one from > __cpa_collapse_large_pages() schedules the work, and > kernel_pgtable_work_func() runs 0.8 ms later and drains the list. From > then on every pagetable_free_kernel() in the trace (660 entries, none > lost) is followed by a work run within a few milliseconds. So on this > box the early tables wait until the first module is loaded, and no > separate drain is needed. > > mm/pgtable-generic.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index b91b1a98029c..f7f504f57914 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -444,6 +444,12 @@ void pagetable_free_kernel(struct ptdesc *pt) > list_add(&pt->pt_list, &kernel_pgtable_work.list); > spin_unlock(&kernel_pgtable_work.lock); > > - schedule_work(&kernel_pgtable_work.work); > + /* > + * The workqueue may not exist yet while the system is booting. > + * The next kernel page table freed after boot schedules the work, > + * which then frees this one as well. > + */ > + if (system_state != SYSTEM_BOOTING) > + schedule_work(&kernel_pgtable_work.work); Maybe we'll just skip collapse on boot? > } > #endif > -- > 2.55.0 > -- Sincerely yours, Mike.