On 03/11/2019 05:42 PM, Mark Rutland wrote:
> Hi,
>
> On Sat, Mar 09, 2019 at 06:19:06PM -0700, Yu Zhao wrote:
>> Switch from per mm_struct to per pmd page table lock by enabling
>> ARCH_ENABLE_SPLIT_PMD_PTLOCK. This provides better granularity for
>> large system.
>>
>> I'm not sure if there is contention on mm->page_table_lock. Given
>> the option comes at no cost (apart from initializing more spin
>> locks), why not enable it now.
>>
>> We only do so when pmd is not folded, so we don't mistakenly call
>> pgtable_pmd_page_ctor() on pud or p4d in pgd_pgtable_alloc(). (We
>> check shift against PMD_SHIFT, which is same as PUD_SHIFT when pmd
>> is folded).
>
> Just to check, I take it pgtable_pmd_page_ctor() is now a NOP when the
> PMD is folded, and this last paragraph is stale?
>
>> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>> ---
>> arch/arm64/Kconfig | 3 +++
>> arch/arm64/include/asm/pgalloc.h | 12 +++++++++++-
>> arch/arm64/include/asm/tlb.h | 5 ++++-
>> 3 files changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index cfbf307d6dc4..a3b1b789f766 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -872,6 +872,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
>> config ARCH_HAS_CACHE_LINE_SIZE
>> def_bool y
>>
>> +config ARCH_ENABLE_SPLIT_PMD_PTLOCK
>> + def_bool y if PGTABLE_LEVELS > 2
>> +
>> config SECCOMP
>> bool "Enable seccomp to safely compute untrusted bytecode"
>> ---help---
>> diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
>> index 52fa47c73bf0..dabba4b2c61f 100644
>> --- a/arch/arm64/include/asm/pgalloc.h
>> +++ b/arch/arm64/include/asm/pgalloc.h
>> @@ -33,12 +33,22 @@
>>
>> static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
>> {
>> - return (pmd_t *)__get_free_page(PGALLOC_GFP);
>> + struct page *page;
>> +
>> + page = alloc_page(PGALLOC_GFP);
>> + if (!page)
>> + return NULL;
>> + if (!pgtable_pmd_page_ctor(page)) {
>> + __free_page(page);
>> + return NULL;
>> + }
>> + return page_address(page);
>> }
>>
>> static inline void pmd_free(struct mm_struct *mm, pmd_t *pmdp)
>> {
>> BUG_ON((unsigned long)pmdp & (PAGE_SIZE-1));
>> + pgtable_pmd_page_dtor(virt_to_page(pmdp));
>> free_page((unsigned long)pmdp);
>> }
>
> It looks like arm64's existing stage-2 code is inconsistent across
> alloc/free, and IIUC this change might turn that into a real problem.
> Currently we allocate all levels of stage-2 table with
> __get_free_page(), but free them with p?d_free(). We always miss the
> ctor and always use the dtor.
>
> Other than that, this patch looks fine to me, but I'd feel more
> comfortable if we could first fix the stage-2 code to free those stage-2
> tables without invoking the dtor.
Thats right. I have already highlighted this problem.
>
> Anshuman, IIRC you had a patch to fix the stage-2 code to not invoke the
> dtors. If so, could you please post that so that we could take it as a
> preparatory patch for this series?
Sure I can after fixing PTE level pte_free_kernel/__free_page which I had
missed in V2.
https://www.spinics.net/lists/arm-kernel/msg710118.html
