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

Brendan Jackman
2025-12-17T13:35:40Z

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@google.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.

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