| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <85b23ed5-6219-4da6-a19a-3577ebaeac3f@arm.com>
Date: Fri, 2 Jan 2026 17:04:23 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com, will@...nel.org, akpm@...ux-foundation.org,
david@...nel.org, kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
dev.jain@....com, yang@...amperecomputing.com, chaitanyas.prakash@....com,
bigeasy@...utronix.de, clrkwllms@...nel.org, rostedt@...dmis.org,
lorenzo.stoakes@...cle.com, ardb@...nel.org, jackmanb@...gle.com,
vbabka@...e.cz, mhocko@...e.com, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v4 2/3] arm64: mmu: avoid allocating pages while splitting
the linear mapping
On 02/01/2026 16:14, Yeoreum Yun wrote:
> Hi Ryan,
>
>>> linear_map_split_to_ptes() currently allocates page tables while
>>> splitting the linear mapping into PTEs under stop_machine() using GFP_ATOMIC.
>>>
>>> This is fine for non-PREEMPT_RT configurations.
>>> However, it becomes problematic on PREEMPT_RT, because
>>> generic memory allocation/free APIs (e.g. pgtable_alloc(), __get_free_pages(), etc.)
>>> cannot be called from a non-preemptible context, except for the _nolock() variants.
>>> This is because generic memory allocation/free paths are sleepable,
>>> as they rely on spin_lock(), which becomes sleepable on PREEMPT_RT.
>>>
>>> In other words, even calling pgtable_alloc() with GFP_ATOMIC is not permitted
>>> in __linear_map_split_to_pte() when it is executed by the stopper thread,
>>> where preemption is disabled on PREEMPT_RT.
>>>
>>> To address this, the required number of page tables is first collected
>>> and preallocated, and the preallocated page tables are then used
>>> when splitting the linear mapping in __linear_map_split_to_pte().
>>>
>>> Fixes: 3df6979d222b ("arm64: mm: split linear mapping if BBML2 unsupported on secondary CPUs")
>>> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
>>
>> Looks good from my perspective.
>>
>> I have a couple more small comments below. With those addressed:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>>
>>> ---
>>> arch/arm64/mm/mmu.c | 204 +++++++++++++++++++++++++++++++++++---------
>>> 1 file changed, 166 insertions(+), 38 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index 4b4908ae189b..cc086e91a506 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -529,18 +529,14 @@ static void early_create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
>>> panic("Failed to create page tables\n");
>>> }
>>>
>>> -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);
>>> - phys_addr_t pa;
>>> -
>>> - if (!ptdesc)
>>> - return INVALID_PHYS_ADDR;
>>> -
>>> - pa = page_to_phys(ptdesc_page(ptdesc));
>>> +static struct ptdesc **split_pgtables;
>>> +static unsigned long split_pgtables_count;
>>> +static unsigned long split_pgtables_idx;
>>
>> I think these could all be __initdata, if you make
>> pgd_pgtable_get_preallocated() __init (see below) ?
>
> I don't think so since range_split_to_ptes() couldn't be __init.
> That's why there is warning while compiling below:
>
> WARNING: modpost: vmlinux: section mismatch in reference: range_split_to_ptes+0x3c (section: .text) -> pgd_pgtable_get_preallocated (section: .init.text)
Ahh ok, the compiler beats me :)
>>
>>>
>>> +static __always_inline void __pgd_pgtable_init(struct mm_struct *mm,
>>
>> Is there a reason for __always_inline? If not, I think it's preferable to just
>> leave it static and let the compiler decide.
>
> Okay. I'll remove __always_inline. Thanks.
>
>>
>>> + struct ptdesc *ptdesc,
>>> + enum pgtable_type pgtable_type)
>>> +{
>>> switch (pgtable_type) {
>>> case TABLE_PTE:
>>> BUG_ON(!pagetable_pte_ctor(mm, ptdesc));
>>> @@ -555,26 +551,49 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
>>> pagetable_p4d_ctor(ptdesc);
>>> break;
>>> }
>>> -
>>> - return pa;
>>> }
>>>
>>> -static phys_addr_t
>>> -pgd_pgtable_alloc_init_mm_gfp(enum pgtable_type pgtable_type, gfp_t gfp)
>>> +static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm,
>>> + enum pgtable_type pgtable_type)
>>> {
>>> - return __pgd_pgtable_alloc(&init_mm, gfp, pgtable_type);
>>> + /* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
>>> + struct ptdesc *ptdesc = pagetable_alloc(GFP_PGTABLE_KERNEL & ~__GFP_ZERO, 0);
>>> +
>>> + if (!ptdesc)
>>> + return INVALID_PHYS_ADDR;
>>> +
>>> + __pgd_pgtable_init(mm, ptdesc, pgtable_type);
>>> +
>>> + return page_to_phys(ptdesc_page(ptdesc));
>>> }
>>>
>>> -static phys_addr_t __maybe_unused
>>> +static phys_addr_t
>>> pgd_pgtable_alloc_init_mm(enum pgtable_type pgtable_type)
>>> {
>>> - return pgd_pgtable_alloc_init_mm_gfp(pgtable_type, GFP_PGTABLE_KERNEL);
>>> + return __pgd_pgtable_alloc(&init_mm, pgtable_type);
>>> }
>>>
>>> static phys_addr_t
>>> pgd_pgtable_alloc_special_mm(enum pgtable_type pgtable_type)
>>> {
>>> - return __pgd_pgtable_alloc(NULL, GFP_PGTABLE_KERNEL, pgtable_type);
>>> + return __pgd_pgtable_alloc(NULL, pgtable_type);
>>> +}
>>> +
>>> +static phys_addr_t
>>> +pgd_pgtable_get_preallocated(enum pgtable_type pgtable_type)
>>
>> I think this could probably be __init?
>
> See above.
>
>>
>>> +{
>>> + struct ptdesc *ptdesc;
>>> +
>>> + if (WARN_ON(split_pgtables_idx >= split_pgtables_count))
>>> + return INVALID_PHYS_ADDR;
>>> +
>>> + ptdesc = split_pgtables[split_pgtables_idx++];
>>> + if (!ptdesc)
>>> + return INVALID_PHYS_ADDR;
>>> +
>>> + __pgd_pgtable_init(&init_mm, ptdesc, pgtable_type);
>>> +
>>> + return page_to_phys(ptdesc_page(ptdesc));
>>> }
>>>
>>> static void split_contpte(pte_t *ptep)
>>> @@ -586,7 +605,9 @@ static void split_contpte(pte_t *ptep)
>>> __set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
>>> }
>>>
>>> -static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>>> +static int split_pmd(pmd_t *pmdp, pmd_t pmd,
>>> + pgtable_alloc_t pgtable_alloc,
>>> + bool to_cont)
>>
>> nit: this will easily fit in 2 lines and still be within 80 chars:
>
> Okay. I'll change it.
>
>>
>> static int split_pmd(pmd_t *pmdp, pmd_t pmd, pgtable_alloc_t pgtable_alloc,
>> bool to_cont)
>>
>>> {
>>> pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
>>> unsigned long pfn = pmd_pfn(pmd);
>>> @@ -595,7 +616,7 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>>> pte_t *ptep;
>>> int i;
>>>
>>> - pte_phys = pgd_pgtable_alloc_init_mm_gfp(TABLE_PTE, gfp);
>>> + pte_phys = pgtable_alloc(TABLE_PTE);
>>> if (pte_phys == INVALID_PHYS_ADDR)
>>> return -ENOMEM;
>>> ptep = (pte_t *)phys_to_virt(pte_phys);
>>> @@ -630,7 +651,9 @@ static void split_contpmd(pmd_t *pmdp)
>>> set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
>>> }
>>>
>>> -static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>>> +static int split_pud(pud_t *pudp, pud_t pud,
>>> + pgtable_alloc_t pgtable_alloc,
>>> + bool to_cont)
>>
>> nit: same comment.
>
> Thanks for your review :D
> BTW, except the __init related comments, Could I add R-b tag
> after fixing others?
Yes, please add my R-b.
>
> --
> Sincerely,
> Yeoreum Yun
Powered by blists - more mailing lists