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Message-ID: <9f749c3e-4f93-40c1-b5c6-74d2ead92d00@arm.com>
Date: Fri, 7 Nov 2025 10:16:12 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: Ritesh Harjani <ritesh.list@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Andreas Larsson <andreas@...sler.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov
<bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>,
Christophe Leroy <christophe.leroy@...roup.eu>,
Dave Hansen <dave.hansen@...ux.intel.com>,
David Hildenbrand <david@...hat.com>, "David S. Miller"
<davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Madhavan Srinivasan <maddy@...ux.ibm.com>,
Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
Peter Zijlstra <peterz@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner
<tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest
On 06/11/2025 15:33, Alexander Gordeev wrote:
>> [...]
>>>> static inline void lazy_mmu_mode_enable(void)
>>>> {
>>>> - arch_enter_lazy_mmu_mode();
>>>> + struct lazy_mmu_state *state = ¤t->lazy_mmu_state;
>>>> +
>>>> + VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
>>>> + /* enable() must not be called while paused */
>>>> + VM_WARN_ON(state->nesting_level > 0 && !state->active);
>>>> +
>>>> + if (state->nesting_level++ == 0) {
>>>> + arch_enter_lazy_mmu_mode();
>>>> + state->active = true;
>>>> + }
>>>> }
>>>>
>>>> ... I think it make more sense to enable the state after the arch_**
>>>> call right.
>>> But then in_lazy_mmu_mode() would return false if called from
>>> arch_enter_lazy_mmu_mode(). Not big problem, but still..
>> The ordering of nesting_level/active was the way you expected in v3, but
>> the conclusion of the discussion with David H [1] is that it doesn't
>> really matter so I simplified the ordering in v4 - the arch hooks
>> shouldn't call in_lazy_mmu_mode() or inspect lazy_mmu_state.
>> arch_enter()/arch_leave() shouldn't need it anyway since they're called
>> once per outer section (not in nested sections). arch_flush() could
>> potentially do something different when nested, but that seems unlikely.
>>
>> - Kevin
>>
>> [1]
>> https://lore.kernel.org/all/af4414b6-617c-4dc8-bddc-3ea00d1f6f3b@redhat.com/
> I might be misunderstand this conversation, but it looked to me as a discussion
> about lazy_mmu_state::nesting_level value, not lazy_mmu_state::active.
>
> I do use in_lazy_mmu_mode() (lazy_mmu_state::active) check from the arch-
> callbacks. Here is the example (and likely the only case so far) where it hits:
Sorry I didn't mean arch callbacks in general, I meant the ones called
from lazy_mmu_mode_*, that is arch_*_lazy_mmu_mode.
Patch 8 also makes use of in_lazy_mmu_mode() in set_pte() et al. on arm64.
- Kevin
> static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
> void *_data)
> {
> lazy_mmu_mode_pause();
> ...
> if (likely(pte_none(ptep_get(ptep)))) {
>
> /* Here set_pte() checks whether we are in lazy_mmu mode */
> set_pte_at(&init_mm, addr, ptep, pte); <--- calls set_pte()
> data->pages[index] = NULL;
> }
> ...
> lazy_mmu_mode_resume();
> ...
> }
>
> So without in_lazy_mmu_mode() check above the arch-specific set_pte()
> implementation enters a wrong branch, which ends up in:
>
> [ 394.503134] Call Trace:
> [ 394.503137] [<00007fffe01333f4>] dump_stack_lvl+0xbc/0xf0 FWIW
> [ 394.503143] [<00007fffe010298c>] vpanic+0x1cc/0x418
> [ 394.503149] [<00007fffe0102c7a>] panic+0xa2/0xa8
> [ 394.503154] [<00007fffe01e7a8a>] check_panic_on_warn+0x8a/0xb0
> [ 394.503160] [<00007fffe082d122>] end_report+0x72/0x110
> [ 394.503166] [<00007fffe082d3e6>] kasan_report+0xc6/0x100
> [ 394.503171] [<00007fffe01b9556>] ipte_batch_ptep_get+0x146/0x150
> [ 394.503176] [<00007fffe0830096>] kasan_populate_vmalloc_pte+0xe6/0x1e0
> [ 394.503183] [<00007fffe0718050>] apply_to_pte_range+0x1a0/0x570
> [ 394.503189] [<00007fffe07260fa>] __apply_to_page_range+0x3ca/0x8f0
> [ 394.503195] [<00007fffe0726648>] apply_to_page_range+0x28/0x40
> [ 394.503201] [<00007fffe082fe34>] __kasan_populate_vmalloc+0x324/0x340
> [ 394.503207] [<00007fffe076954e>] alloc_vmap_area+0x31e/0xbf0
> [ 394.503213] [<00007fffe0770106>] __get_vm_area_node+0x1a6/0x2d0
> [ 394.503218] [<00007fffe07716fa>] __vmalloc_node_range_noprof+0xba/0x260
> [ 394.503224] [<00007fffe0771970>] __vmalloc_node_noprof+0xd0/0x110
> [ 394.503229] [<00007fffe0771a22>] vmalloc_noprof+0x32/0x40
> [ 394.503234] [<00007fff604eaa42>] full_fit_alloc_test+0xb2/0x3e0 [test_vmalloc]
> [ 394.503241] [<00007fff604eb478>] test_func+0x488/0x760 [test_vmalloc]
> [ 394.503247] [<00007fffe025ad68>] kthread+0x368/0x630
> [ 394.503253] [<00007fffe01391e0>] __ret_from_fork+0xd0/0x490
> [ 394.503259] [<00007fffe24e468a>] ret_from_fork+0xa/0x30
>
> I could have cached lazy_mmu_state::active as arch-specific data
> and check it, but then what is the point to have it generalized?
>
> Thanks!
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