RE: [PATCH -V2] powerpc: thp: Fix crash on mremap

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> This patch fix the below crash
> 
> NIP [c00000000004cee4] .__hash_page_thp+0x2a4/0x440
> LR [c0000000000439ac] .hash_page+0x18c/0x5e0
> ...
> Call Trace:
> [c000000736103c40] [00001ffffb000000] 0x1ffffb000000(unreliable)
> [437908.479693] [c000000736103d50] [c0000000000439ac] .hash_page+0x18c/0x5e0
> [437908.479699] [c000000736103e30] [c00000000000924c] .do_hash_page+0x4c/0x58
> 
> On ppc64 we use the pgtable for storing the hpte slot information and
> store address to the pgtable at a constant offset (PTRS_PER_PMD) from
> pmd. On mremap, when we switch the pmd, we need to withdraw and deposit
> the pgtable again, so that we find the pgtable at PTRS_PER_PMD offset
> from new pmd.
> 
> We also want to move the withdraw and deposit before the set_pmd so
> that, when page fault find the pmd as trans huge we can be sure that
> pgtable can be located at the offset.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
> Changes from V1:
> * limit the withraw/deposit to only ppc64
> 
>  arch/Kconfig                           |  3 +++
>  arch/powerpc/platforms/Kconfig.cputype |  1 +
>  include/linux/huge_mm.h                |  6 ++++++
>  mm/huge_memory.c                       | 21 ++++++++++++---------
>  4 files changed, 22 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index f1cf895c040f..3759e70a649d 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -371,6 +371,9 @@ config HAVE_IRQ_TIME_ACCOUNTING
>  config HAVE_ARCH_TRANSPARENT_HUGEPAGE
>  	bool
>  
> +config ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW

I don't like name of the option, but can't find any better... :(

> +	bool
> +
>  config HAVE_ARCH_SOFT_DIRTY
>  	bool
>  
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index bca2465a9c34..5f83b4334e5f 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -71,6 +71,7 @@ config PPC_BOOK3S_64
>  	select PPC_FPU
>  	select PPC_HAVE_PMU_SUPPORT
>  	select SYS_SUPPORTS_HUGETLBFS
> +	select ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if PPC_64K_PAGES
>  
>  config PPC_BOOK3E_64
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 91672e2deec3..836242a738a5 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -230,4 +230,10 @@ static inline int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_str
>  
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> +#ifdef CONFIG_ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW
> +#define ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW 1
> +#else
> +#define ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW 0
> +#endif
> +

Just use config option directly:

	if (new_ptl != old_ptl ||
		IS_ENABLED(CONFIG_ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW))
...


Otherwise, looks good:

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

>  #endif /* _LINUX_HUGE_MM_H */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 7de1bf85f683..32006b51d102 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1505,19 +1505,22 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
>  			spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
>  		pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
>  		VM_BUG_ON(!pmd_none(*new_pmd));
> -		set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
> -		if (new_ptl != old_ptl) {
> +		/*
> +		 * Archs like ppc64 use pgtable to store per pmd
> +		 * specific information. So when we switch the pmd,
> +		 * we should also withdraw and deposit the pgtable
> +		 *
> +		 * With split pmd lock we also need to move preallocated
> +		 * PTE page table if new_pmd is on different PMD page table.
> +		 */
> +		if (new_ptl != old_ptl || ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW) {
>  			pgtable_t pgtable;
> -
> -			/*
> -			 * Move preallocated PTE page table if new_pmd is on
> -			 * different PMD page table.
> -			 */
>  			pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
>  			pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
> -
> -			spin_unlock(new_ptl);
>  		}
> +		set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
> +		if (new_ptl != old_ptl)
> +			spin_unlock(new_ptl);
>  		spin_unlock(old_ptl);
>  	}
>  out:
> -- 
> 1.8.3.2

-- 
 Kirill A. Shutemov