From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 50AA833B6E0; Wed, 17 Dec 2025 12:04:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765973101; cv=none; b=QGhhYhOatCsHnOpf02FVajyCnFnNbRfka/xQBEPrufctvJVWTIro2Ft3wzWC8TlzWKrIez2Af5tn17m0Asky7oqx2zBDpMp8ypGWmf6kfPWpaMbLpKDHJz9ohjfP2d3IaUK46NRtNRRgbkLCeS3vO1S0D3g9Fc/xBUDpqtmoSPI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765973101; c=relaxed/simple; bh=W8Mf+S5hozPR7/qnVI2Jb/1medprDBxv/TPTmrWgO8g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VxpcGRAeZAQmCIS5sXxoZGe2S7zpGkoPPryQlMBegNdlpfF2owvftKltzS2Rx1zPg4gOeFk1URdXWyGi43TDSDMCzeaPZCiNF8BEESsezIHjV+O2gpzh3nPLCUqmIPQjqL987BZZJN8iSoG0Y0Gm0kggJU/fxHkLRTO7/rSlR0o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9267414BF; Wed, 17 Dec 2025 04:04:51 -0800 (PST) Received: from [10.57.91.77] (unknown [10.57.91.77]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7DC733F73F; Wed, 17 Dec 2025 04:04:50 -0800 (PST) Message-ID: Date: Wed, 17 Dec 2025 12:04:48 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] introduce pagetable_alloc_nolock() Content-Language: en-GB To: Yeoreum Yun Cc: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, bigeasy@linutronix.de, clrkwllms@kernel.org, rostedt@goodmis.org, catalin.marinas@arm.com, will@kernel.org, kevin.brodsky@arm.com, dev.jain@arm.com, yang@os.amperecomputing.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-rt-devel@lists.linux.dev, linux-arm-kernel@lists.infradead.org References: <20251212161832.2067134-1-yeoreum.yun@arm.com> <916c17ba-22b1-456e-a184-cb3f60249af7@arm.com> <100cc8da-b826-4fc2-a624-746bf6fb049d@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. Thanks, Ryan > > -- > Sincerely, > Yeoreum Yun