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Fri, 12 Dec 2025 23:05:39 -0800 (PST) MIME-Version: 1.0 References: <20251212161832.2067134-1-yeoreum.yun@arm.com> <20251212161832.2067134-3-yeoreum.yun@arm.com> In-Reply-To: <20251212161832.2067134-3-yeoreum.yun@arm.com> From: Brendan Jackman Date: Sat, 13 Dec 2025 16:05:20 +0900 X-Gm-Features: AQt7F2rPm7R-wCwVHPTjXe_bQj-qNecTeNkHkP5H4J7QvG1ylb2YhWsPANW--u8 Message-ID: Subject: Re: [PATCH 2/2] arm64: mmu: use pagetable_alloc_nolock() while stop_machine() 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, hannes@cmpxchg.org, ziy@nvidia.com, bigeasy@linutronix.de, clrkwllms@kernel.org, rostedt@goodmis.org, catalin.marinas@arm.com, will@kernel.org, ryan.roberts@arm.com, 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 Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: A02EC8000F X-Rspamd-Server: rspam03 X-Stat-Signature: fkx3mx34tgfcyg3mqjfursiacwxk46fq X-Rspam-User: X-HE-Tag: 1765609541-160198 X-HE-Meta: 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 Odhk1X1G VmedamvxCEm00ZIEfaE6r91F/Qv+qvpfOLqNclOWOh+yxpIBTjKv+bPFoeW/K1XsItrVflQS0kSkoApkWxDx4R4AihMe/MxHBEfUZhyEzl2nM0QBY+hnFF0C5iKwHcXuQHGCiUaZhX3Yr2RdvFJr/hCXSIMj4+Kti6NDte0IX+vMpyq5gFO3IWop62KWlk2X8Qo9yEMfHzPXtifxVPEzwWmACzcAX3wJsFKru7BqitJ39Y3nrtg7l8UiOC0g1YTtepf+G8OjjhDp4P1WB2aSdtdC3dhrhmX3HhWy9i3gM2B5XdOJ1d50B27/YK1f2/2PJY9Qx3vKrEf94sbbAqSbVEtUUIOFJR/T8TRuj4cR8fBbGJxIIniYNWE/5UQXZYQ/fPoALwckf/Nf7ya9xzYB+Vk3fuwBH7S+jN/48 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, 13 Dec 2025 at 01:18, Yeoreum Yun wrote: > > linear_map_split_to_ptes() and __kpti_install_ng_mappings() > are called as callback of stop_machine(). > That means these functions context are preemption disabled. > > Unfortunately, under PREEMPT_RT, the pagetable_alloc() or > __get_free_pages() couldn't be called in this context > since spin lock that becomes sleepable on RT, > potentially causing a sleep during page allocation. > > To address this, pagetable_alloc_nolock(). > > Signed-off-by: Yeoreum Yun > --- > arch/arm64/mm/mmu.c | 23 ++++++++++++++++++----- > 1 file changed, 18 insertions(+), 5 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 2ba01dc8ef82..0e98606d8c4c 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -475,10 +475,15 @@ static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys, > static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp, > enum pgtable_type pgtable_type) > { > - /* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */ > - struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0); > + struct ptdesc *ptdesc; > phys_addr_t pa; > > + /* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */ > + if (gfpflags_allow_spinning(gfp)) > + ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0); > + else > + ptdesc = pagetable_alloc_nolock(gfp & ~__GFP_ZERO, 0); > + > if (!ptdesc) > return INVALID_PHYS_ADDR; > > @@ -869,6 +874,7 @@ static int __init linear_map_split_to_ptes(void *__unused) > unsigned long kstart = (unsigned long)lm_alias(_stext); > unsigned long kend = (unsigned long)lm_alias(__init_begin); > int ret; > + gfp_t gfp = IS_ENABLED(CONFIG_PREEMPT_RT) ? __GFP_HIGH : GFP_ATOMIC; > > /* > * Wait for all secondary CPUs to be put into the waiting area. > @@ -881,9 +887,9 @@ static int __init linear_map_split_to_ptes(void *__unused) > * PTE. The kernel alias remains static throughout runtime so > * can continue to be safely mapped with large mappings. > */ > - ret = range_split_to_ptes(lstart, kstart, GFP_ATOMIC); > + ret = range_split_to_ptes(lstart, kstart, gfp); > if (!ret) > - ret = range_split_to_ptes(kend, lend, GFP_ATOMIC); > + ret = range_split_to_ptes(kend, lend, gfp); > if (ret) > panic("Failed to split linear map\n"); > flush_tlb_kernel_range(lstart, lend); > @@ -1207,7 +1213,14 @@ static int __init __kpti_install_ng_mappings(void *__unused) > remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings); > > if (!cpu) { > - alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order); > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + alloc = (u64) pagetable_alloc_nolock(__GFP_HIGH | __GFP_ZERO, order); > + else > + alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order); > + > + if (!alloc) > + panic("Failed to alloc kpti_ng_pgd\n"); > + I don't have the context on what this code is doing so take this with a grain of salt, but... The point of the _nolock alloc is to give the allocator an excuse to fail. Panicking on that failure doesn't seem like a great idea to me?