Re: [PATCH 0/2] introduce pagetable_alloc_nolock()

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi Ryan,

> On 16/12/2025 16:52, Yeoreum Yun wrote:
> > Hi Ryan,
> >
> >> On 12/12/2025 16:18, Yeoreum Yun wrote:
> >>> Some architectures invoke pagetable_alloc() or __get_free_pages()
> >>> with preemption disabled.
> >>> For example, in arm64, linear_map_split_to_ptes() calls pagetable_alloc()
> >>> while spliting block entry to ptes and __kpti_install_ng_mappings()
> >>> calls __get_free_pages() to create kpti pagetable.
> >>>
> >>> Under PREEMPT_RT, calling pagetable_alloc() with
> >>> preemption disabled is not allowed, because it may acquire
> >>> a spin lock that becomes sleepable on RT, potentially
> >>> causing a sleep during page allocation.
> >>>
> >>> Since above two functions is called as callback of stop_machine()
> >>> where its callback is called in preemption disabled,
> >>> They could make a potential problem. (sleeping in preemption disabled).
> >>>
> >>> To address this, introduce pagetable_alloc_nolock() API.
> >>
> >> I don't really understand what the problem is that you're trying to fix. As I
> >> see it, there are 2 call sites in arm64 arch code that are calling into the page
> >> allocator from stop_machine() - one via via pagetable_alloc() and another via
> >> __get_free_pages(). But both of those calls are passing in GFP_ATOMIC. It was my
> >> understanding that the page allocator would ensure it never sleeps when
> >> GFP_ATOMIC is passed in, (even for PREEMPT_RT)?
> >
> > Although GFP_ATOMIC is specify, it only affects of "water mark" of the
> > page with __GFP_HIGH. and to get a page, it must grab the lock --
> > zone->lock or pcp_lock in the rmqueue().
> >
> > This zone->lock and pcp_lock is spin_lock and it's a sleepable in
> > PREEMPT_RT that's why the memory allocation/free using general API
> > except nolock() version couldn't be called since
> > if "contention" happens they'll sleep while waiting to get the lock.
> >
> > The reason why "nolock()" can use, it always uses "trylock" with
> > ALLOC_TRYLOCK flags. otherwise GFP_ATOMIC also can be sleepable in
> > PREEMPT_RT.
> >
> >>
> >> What is the actual symptom you are seeing?
> >
> > Since the place where called while smp_cpus_done() and there seems no
> > contention, there seems no problem. However as I mention in another
> > thread
> > (https://lore.kernel.org/all/aT%2FdrjN1BkvyAGoi@e129823.arm.com/),
> > This gives a the false impression --
> > GFP_ATOMIC are “safe to use in preemption disabled”
> > even though they are not in PREEMPT_RT case, I've changed it.
> >
> >>
> >> If the page allocator is somehow ignoring the GFP_ATOMIC request for PREEMPT_RT,
> >> then isn't that a bug in the page allocator? I'm not sure why you would change
> >> the callsites? Can't you just change the page allocator based on GFP_ATOMIC?
> >
> > It doesn't ignore the GFP_ATOMIC feature:
> >   - __GFP_HIGH: use water mark till min reserved
> >   - __GFP_KSWAPD_RECLAIM: wake up kswapd if reclaim required.
> >
> > But, it's a restriction -- "page allocation / free" API cannot be called
> > in preempt-disabled context at PREEMPT_RT.
> >
> > That's why I think it's wrong usage not a page allocator bug.
>
> I've taken a look at this and I agree with your analysis. Thanks for explaining.
>
> Looking at other stop_machine() callbacks, there are some that call printk() and
> I would assume that spinlocks could be taken there which may present the same
> kind of issue or PREEMPT_RT? (I'm guessing). I don't see any others that attempt
> to allocate memory though.

IIRC, there was a problem related for printk while try to grab
pl011_console related lock (spin_lock) while holding
console_lock(raw_spin_lock) in v6.10.0-rc7 at rpi5:

    [  230.381263] CPU: 2 PID: 5574 Comm: syz.4.1695 Not tainted 6.10.0-rc7-01903-g52828ea60dfd #3
    [  230.381479] Hardware name: linux,dummy-virt (DT)
    [  230.381565] Call trace:
    [  230.381607]  dump_backtrace+0x318/0x348
    [  230.381727]  show_stack+0x4c/0x80
    [  230.381875]  dump_stack_lvl+0x214/0x328
    [  230.382159]  dump_stack+0x3c/0x58
    [  230.382456]  __lock_acquire+0x4398/0x4720
    [  230.382683]  lock_acquire+0x648/0xb70
    [  230.382928]  _raw_spin_lock_irqsave+0x138/0x240
    [  230.383121]  pl011_console_write+0x240/0x8a0
    [  230.383356]  console_flush_all+0x708/0x1368
    [  230.383571]  console_unlock+0x180/0x440
    [  230.383742]  vprintk_emit+0x1f8/0x9d0
    [  230.383832]  vprintk_default+0x64/0x90
    [  230.383914]  vprintk+0x2d0/0x400
    [  230.383971]  _printk+0xdc/0x128
    [  230.384229]  hrtimer_interrupt+0x8f0/0x920
    [  230.384414]  arch_timer_handler_virt+0xc0/0x100
    [  230.384812]  handle_percpu_devid_irq+0x20c/0x4e0
    [  230.385053]  generic_handle_domain_irq+0xc0/0x120
    [  230.385367]  gic_handle_irq+0x88/0x360
    [  230.385559]  call_on_irq_stack+0x24/0x70
    [  230.385801]  do_interrupt_handler+0xf8/0x200
    [  230.386092]  el1_interrupt+0x68/0xc0
    [  230.386434]  el1h_64_irq_handler+0x18/0x28
    [  230.386716]  el1h_64_irq+0x64/0x68
    [  230.386853]  __sanitizer_cov_trace_const_cmp2+0x30/0x68
    [  230.387026]  alloc_pages_mpol_noprof+0x170/0x698
    [  230.387309]  vma_alloc_folio_noprof+0x128/0x2a8
    [  230.387610]  vma_alloc_zeroed_movable_folio+0xa0/0xe0
    [  230.387822]  folio_prealloc+0x5c/0x280
    [  230.388008]  do_wp_page+0xc30/0x3bc0
    [  230.388206]  __handle_mm_fault+0xdb8/0x2ba0
    [  230.388448]  handle_mm_fault+0x194/0x8a8
    [  230.388676]  do_page_fault+0x6bc/0x1030
    [  230.388924]  do_mem_abort+0x8c/0x240
    [  230.389056]  el0_da+0xf0/0x3f8
    [  230.389178]  el0t_64_sync_handler+0xb4/0x130
    [  230.389452]  el0t_64_sync+0x190/0x198

But this problem is gone when I try with some of patches in rt-tree
related for printk which are merged in current tree
(https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/log/?h=linux-6.10.y-rt-rebase).

So I think printk() wouldn't be a problem.

>
> Anyway, to fix the 2 arm64 callsites, I see 2 possible approaches:
>
> - Call the nolock variant (as you are doing). But that would just convert a
> deadlock to a panic; if the lock is held when stop_machine() runs, without your
> change, we now have a deadlock due to waiting on the lock inside stop_machine().
> With your change, we notice the lock is already taken and panic. I guess it is
> marginally better, but not by much. Certainly I would just _always_ call the
> nolock variant regardless of PREEMPT_RT if we take this route; For !PREEMPT_RT,
> the lock is guarranteed to be free so nolock will always succeed.
>
> - Preallocate the memory before entering stop_machine(). I think this would be
> much more robust. For kpti_install_ng_mappings() I think you could hoist the
> allocation/free out of stop_machine() and pass the pointer in pretty easily. For
> linear_map_split_to_ptes() its a bit more complex; Perhaps, we need to walk the
> pgtable to figure out how much to preallocate, allocate it, then set it up as a
> special allocator, wrapped by an allocation function and modify the callchain to
> take a callback function instead of gfp flags.
>
> What do you think?

Definitely, second suggestoin is much better.
My question is whether *memory contention* really happen in the point
both functions are called.

Above two functions are called as last step of "smp_init()" -- smp_cpus_done().
If we can be sure, I think we don't need to go to complex way and
I believe the reason why we couldn't find out this problem,
even using GFP_ATOMIC in PREEMPT_RT since there was *no contection*
in this time of both functions are called.

That's why I first try with the "simple way".

What do you think?

--
Sincerely,
Yeoreum Yun