Message-ID: <d5354d96-6f28-41d7-9f66-64c254d9477b@suse.cz>
Date: Wed, 17 Dec 2025 16:10:54 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Brendan Jackman <jackmanb@...gle.com>, Yeoreum Yun <yeoreum.yun@....com>,
 Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, david@...nel.org, lorenzo.stoakes@...cle.com,
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Subject: Re: [PATCH 0/2] introduce pagetable_alloc_nolock()

On 12/17/25 14:35, Brendan Jackman wrote:
> On Wed Dec 17, 2025 at 1:15 PM UTC, Vlastimil Babka wrote:
>> On 12/17/25 13:52, Yeoreum Yun wrote:
>>>> On 17/12/2025 10:48, Yeoreum Yun wrote:
>>>> > 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.
>>>>
>>>> My guess would be that it's unlikely, but not impossible. The secondary CPUs are
>>>> up, and presumably running their idle thread. I think various power management
>>>> things can be plugged into the idle thread; if so, then I guess it's possible
>>>> that the CPU could be running some hook as part of a power state transition, and
>>>> that could be dynamically allocating memory? That's all just a guess though; I
>>>> don't know the details of that part of the system.
>>>>
>>>> >
>>>> > 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?
>>>>
>>>> As far as linear_map_split_to_ptes() is concerned, it was implemented under the
>>>> impression that doing allocation with GFP_ATOMIC was safe, even in
>>>> stop_machine(). Given that's an incorrect assumption, I think we should fix it
>>>> to pre-allocate outside of stop_machine() regardless of the likelihood of
>>>> actually hitting the race.
>>>>
>>> 
>>> Yeap. It’s better to be certain than uncertain. Thanks for checking.
>>> I'll repsin with the preallocate way.
>>
>> Note this is explained in Documentation/core-api/real-time/differences.rst:
>>
>> Memory allocation
>> -----------------
>>
>> The memory allocation APIs, such as kmalloc() and alloc_pages(), require a
>> gfp_t flag to indicate the allocation context. On non-PREEMPT_RT kernels, it is
>> necessary to use GFP_ATOMIC when allocating memory from interrupt context or
>> from sections where preemption is disabled. This is because the allocator must
>> not sleep in these contexts waiting for memory to become available.
>>
>> However, this approach does not work on PREEMPT_RT kernels. The memory
>> allocator in PREEMPT_RT uses sleeping locks internally, which cannot be
>> acquired when preemption is disabled. Fortunately, this is generally not a
>> problem, because PREEMPT_RT moves most contexts that would traditionally run
>> with preemption or interrupts disabled into threaded context, where sleeping is
>> allowed.
>>
>> What remains problematic is code that explicitly disables preemption or
>> interrupts. In such cases, memory allocation must be performed outside the
>> critical section.
>>
>> This restriction also applies to memory deallocation routines such as kfree()
>> and free_pages(), which may also involve internal locking and must not be
>> called from non-preemptible contexts.
> 
> Oh, thanks for pointing to that, I had never read that before (oops).
> 
> Shall we point to this from the doc-comment? Something like the below.
> 
> BTW, Yeorum, assuming you care about PREEMPT_RT, maybe you can get
> Sparse to find some other bugs of this nature? Or if not, plain old
> Coccinelle would probably find a few.
> 
> ---
> 
> From 4c6b4d4cb08aee9559d02a348b9ecf799142c96f Mon Sep 17 00:00:00 2001
> From: Brendan Jackman <jackmanb@...gle.com>
> Date: Wed, 17 Dec 2025 13:26:28 +0000
> Subject: [PATCH] mm: clarify GFP_ATOMIC/GFP_NOWAIT doc-comment
> 
> The current description of contexts where it's invalid to make
> GFP_ATOMIC and GFP_NOWAIT calls is rather vague.
> 
> Replace this with a direct description of the actual contexts of concern
> and refer to the RT docs where this is explained more discursively.
> 
> While rejigging this prose, also move the documentation of GFP_NOWAIT to
> the GFP_NOWAIT section.

There doesn't seem to be any move?

> 
> Link: https://lore.kernel.org/all/d912480a-5229-4efe-9336-b31acded30f5@suse.cz/
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>
Nit below:

> ---
>  include/linux/gfp_types.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> index 3de43b12209ee..07a378542caf2 100644
> --- a/include/linux/gfp_types.h
> +++ b/include/linux/gfp_types.h
> @@ -309,8 +309,10 @@ enum {
>   *
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
>   * watermark is applied to allow access to "atomic reserves".
> - * The current implementation doesn't support NMI and few other strict
> - * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.
> + * The current implementation doesn't support NMI, nor contexts that disable
> + * preemption under PREEMPT_RT. This includes raw_spin_lock() and plain
> + * preempt_disable() - see Documentation/core-api/real-time/differences.rst for
> + * more info.

Can we reference the "Memory allocation" section directly?

>   *
>   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> @@ -321,6 +323,7 @@ enum {
>   * %GFP_NOWAIT is for kernel allocations that should not stall for direct
>   * reclaim, start physical IO or use any filesystem callback.  It is very
>   * likely to fail to allocate memory, even for very small allocations.
> + * The same restrictions on calling contexts apply as for %GFP_ATOMIC.
>   *
>   * %GFP_NOIO will use direct reclaim to discard clean pages or slab pages
>   * that do not require the starting of any physical IO.
> --
> 2.50.1

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