Re: [PATCH v3 2/2] migrate: add migrate_entry_wait_huge()

[Michal Hocko]

On Tue 28-05-13 15:52:51, Naoya Horiguchi wrote:
> When we have a page fault for the address which is backed by a hugepage
> under migration, the kernel can't wait correctly and do busy looping on
> hugepage fault until the migration finishes.
> This is because pte_offset_map_lock() can't get a correct migration entry
> or a correct page table lock for hugepage.
> This patch introduces migration_entry_wait_huge() to solve this.
> 
> Note that the caller, hugetlb_fault(), gets the pointer to the "leaf"
> entry with huge_pte_offset() inside which all the arch-dependency of
> the page table structure are. So migration_entry_wait_huge() and
> __migration_entry_wait() are free from arch-dependency.
> 
> ChangeLog v3:
>  - use huge_pte_lockptr
> 
> ChangeLog v2:
>  - remove dup in migrate_entry_wait_huge()
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 2.6.35

OK, this looks good to me and I guess you can safely replace
huge_pte_lockptr by &(mm)->page_table_lock so you can implement this
even without risky 1/2 of this series. The patch should be as simple as
possible especially when it goes to the stable.

Without 1/2 dependency
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  include/linux/swapops.h |  3 +++
>  mm/hugetlb.c            |  2 +-
>  mm/migrate.c            | 23 ++++++++++++++++++-----
>  3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git v3.10-rc3.orig/include/linux/swapops.h v3.10-rc3/include/linux/swapops.h
> index 47ead51..c5fd30d 100644
> --- v3.10-rc3.orig/include/linux/swapops.h
> +++ v3.10-rc3/include/linux/swapops.h
> @@ -137,6 +137,7 @@ static inline void make_migration_entry_read(swp_entry_t *entry)
>  
>  extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>  					unsigned long address);
> +extern void migration_entry_wait_huge(struct mm_struct *mm, pte_t *pte);
>  #else
>  
>  #define make_migration_entry(page, write) swp_entry(0, 0)
> @@ -148,6 +149,8 @@ static inline int is_migration_entry(swp_entry_t swp)
>  static inline void make_migration_entry_read(swp_entry_t *entryp) { }
>  static inline void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>  					 unsigned long address) { }
> +static inline void migration_entry_wait_huge(struct mm_struct *mm,
> +					pte_t *pte) { }
>  static inline int is_write_migration_entry(swp_entry_t entry)
>  {
>  	return 0;
> diff --git v3.10-rc3.orig/mm/hugetlb.c v3.10-rc3/mm/hugetlb.c
> index 8e1af32..d91a438 100644
> --- v3.10-rc3.orig/mm/hugetlb.c
> +++ v3.10-rc3/mm/hugetlb.c
> @@ -2877,7 +2877,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	if (ptep) {
>  		entry = huge_ptep_get(ptep);
>  		if (unlikely(is_hugetlb_entry_migration(entry))) {
> -			migration_entry_wait(mm, (pmd_t *)ptep, address);
> +			migration_entry_wait_huge(mm, ptep);
>  			return 0;
>  		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
>  			return VM_FAULT_HWPOISON_LARGE |
> diff --git v3.10-rc3.orig/mm/migrate.c v3.10-rc3/mm/migrate.c
> index 6f2df6e..64ff118 100644
> --- v3.10-rc3.orig/mm/migrate.c
> +++ v3.10-rc3/mm/migrate.c
> @@ -204,15 +204,14 @@ static void remove_migration_ptes(struct page *old, struct page *new)
>   * get to the page and wait until migration is finished.
>   * When we return from this function the fault will be retried.
>   */
> -void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
> -				unsigned long address)
> +static void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
> +				spinlock_t *ptl)
>  {
> -	pte_t *ptep, pte;
> -	spinlock_t *ptl;
> +	pte_t pte;
>  	swp_entry_t entry;
>  	struct page *page;
>  
> -	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
> +	spin_lock(ptl);
>  	pte = *ptep;
>  	if (!is_swap_pte(pte))
>  		goto out;
> @@ -240,6 +239,20 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>  	pte_unmap_unlock(ptep, ptl);
>  }
>  
> +void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
> +				unsigned long address)
> +{
> +	spinlock_t *ptl = pte_lockptr(mm, pmd);
> +	pte_t *ptep = pte_offset_map(pmd, address);
> +	__migration_entry_wait(mm, ptep, ptl);
> +}
> +
> +void migration_entry_wait_huge(struct mm_struct *mm, pte_t *pte)
> +{
> +	spinlock_t *ptl = huge_pte_lockptr(mm, pte);
> +	__migration_entry_wait(mm, pte, ptl);
> +}
> +
>  #ifdef CONFIG_BLOCK
>  /* Returns true if all buffers are successfully locked */
>  static bool buffer_migrate_lock_buffers(struct buffer_head *head,
> -- 
> 1.7.11.7
> 

-- 
Michal Hocko
SUSE Labs
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