Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

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 = &current->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@xxxxxxxxxx/
> 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!