Re: [PATCH, RFC 2/2] Implement sharing/unsharing of PMDs for FS/DAX

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, May 09, 2019 at 09:05:33AM -0700, Larry Bassel wrote:
> This is based on (but somewhat different from) what hugetlbfs
> does to share/unshare page tables.
> 
> Signed-off-by: Larry Bassel <larry.bassel@oracle.com>
> ---
>  include/linux/hugetlb.h |   4 ++
>  mm/huge_memory.c        |  32 ++++++++++++++
>  mm/hugetlb.c            |  21 ++++++++--
>  mm/memory.c             | 108 +++++++++++++++++++++++++++++++++++++++++++++++-
>  4 files changed, 160 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 11943b6..9ed9542 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -142,6 +142,10 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep);
>  void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>  				unsigned long *start, unsigned long *end);
> +unsigned long page_table_shareable(struct vm_area_struct *svma,
> +				   struct vm_area_struct *vma,
> +				   unsigned long addr, pgoff_t idx);
> +bool vma_shareable(struct vm_area_struct *vma, unsigned long addr);
>  struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
>  			      int write);
>  struct page *follow_huge_pd(struct vm_area_struct *vma,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b6a34b3..e1627c3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1747,6 +1747,33 @@ static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd)
>  	mm_dec_nr_ptes(mm);
>  }
>  
> +#ifdef CONFIG_MAY_SHARE_FSDAX_PMD
> +static int unshare_huge_pmd(struct mm_struct *mm, unsigned long addr,
> +			    pmd_t *pmdp)
> +{
> +	pgd_t *pgd = pgd_offset(mm, addr);
> +	p4d_t *p4d = p4d_offset(pgd, addr);
> +	pud_t *pud = pud_offset(p4d, addr);
> +
> +	WARN_ON(page_count(virt_to_page(pmdp)) == 0);
> +	if (page_count(virt_to_page(pmdp)) == 1)
> +		return 0;
> +
> +	pud_clear(pud);

You don't have proper locking in place to do this.

> +	put_page(virt_to_page(pmdp));
> +	mm_dec_nr_pmds(mm);
> +	return 1;
> +}
> +
> +#else
> +static int unshare_huge_pmd(struct mm_struct *mm, unsigned long addr,
> +			    pmd_t *pmdp)
> +{
> +	return 0;
> +}
> +
> +#endif
> +
>  int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		 pmd_t *pmd, unsigned long addr)
>  {
> @@ -1764,6 +1791,11 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  	 * pgtable_trans_huge_withdraw after finishing pmdp related
>  	 * operations.
>  	 */
> +	if (unshare_huge_pmd(vma->vm_mm, addr, pmd)) {
> +		spin_unlock(ptl);
> +		return 1;
> +	}
> +
>  	orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
>  			tlb->fullmm);
>  	tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 641cedf..919a290 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4594,9 +4594,9 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
>  }
>  
>  #ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
> -static unsigned long page_table_shareable(struct vm_area_struct *svma,
> -				struct vm_area_struct *vma,
> -				unsigned long addr, pgoff_t idx)
> +unsigned long page_table_shareable(struct vm_area_struct *svma,
> +				   struct vm_area_struct *vma,
> +				   unsigned long addr, pgoff_t idx)
>  {
>  	unsigned long saddr = ((idx - svma->vm_pgoff) << PAGE_SHIFT) +
>  				svma->vm_start;
> @@ -4619,7 +4619,7 @@ static unsigned long page_table_shareable(struct vm_area_struct *svma,
>  	return saddr;
>  }
>  
> -static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
> +bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
>  {
>  	unsigned long base = addr & PUD_MASK;
>  	unsigned long end = base + PUD_SIZE;
> @@ -4763,6 +4763,19 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>  				unsigned long *start, unsigned long *end)
>  {
>  }
> +
> +unsigned long page_table_shareable(struct vm_area_struct *svma,
> +				   struct vm_area_struct *vma,
> +				   unsigned long addr, pgoff_t idx)
> +{
> +	return 0;
> +}
> +
> +bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
> +{
> +	return false;
> +}
> +
>  #define want_pmd_share()	(0)
>  #endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index f7d962d..4c1814c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3845,6 +3845,109 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_MAY_SHARE_FSDAX_PMD
> +static pmd_t *huge_pmd_offset(struct mm_struct *mm,
> +			      unsigned long addr, unsigned long sz)

Could you explain what this function suppose to do?

As far as I can see vma_mmu_pagesize() is always PAGE_SIZE of DAX
filesystem. So we have 'sz' == PAGE_SIZE here.

So this function can pointer to PMD of PUD page table entry casted to
pmd_t*.

Why?

> +{
> +	pgd_t *pgd;
> +	p4d_t *p4d;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +
> +	pgd = pgd_offset(mm, addr);
> +	if (!pgd_present(*pgd))
> +		return NULL;
> +	p4d = p4d_offset(pgd, addr);
> +	if (!p4d_present(*p4d))
> +		return NULL;
> +
> +	pud = pud_offset(p4d, addr);
> +	if (sz != PUD_SIZE && pud_none(*pud))
> +		return NULL;
> +	/* hugepage or swap? */
> +	if (pud_huge(*pud) || !pud_present(*pud))
> +		return (pmd_t *)pud;
> +
> +	pmd = pmd_offset(pud, addr);
> +	if (sz != PMD_SIZE && pmd_none(*pmd))
> +		return NULL;
> +	/* hugepage or swap? */
> +	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> +		return pmd;
> +
> +	return NULL;
> +}
> +
> +static pmd_t *pmd_share(struct mm_struct *mm, pud_t *pud, unsigned long addr)
> +{
> +	struct vm_area_struct *vma = find_vma(mm, addr);

Why? Caller has vma on hands.

> +	struct address_space *mapping = vma->vm_file->f_mapping;
> +	pgoff_t idx = ((addr - vma->vm_start) >> PAGE_SHIFT) +
> +			vma->vm_pgoff;

linear_page_index()?

> +	struct vm_area_struct *svma;
> +	unsigned long saddr;
> +	pmd_t *spmd = NULL;
> +	pmd_t *pmd;
> +	spinlock_t *ptl;
> +
> +	if (!vma_shareable(vma, addr))
> +		return pmd_alloc(mm, pud, addr);
> +
> +	i_mmap_lock_write(mapping);
> +
> +	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
> +		if (svma == vma)
> +			continue;
> +
> +		saddr = page_table_shareable(svma, vma, addr, idx);
> +		if (saddr) {
> +			spmd = huge_pmd_offset(svma->vm_mm, saddr,
> +					       vma_mmu_pagesize(svma));
> +			if (spmd) {
> +				get_page(virt_to_page(spmd));

So, here we get a pin on a page table page. And we don't know if it's PMD
or PUD page table.

And we only checked one entry in the page table.

What if the page table mixes huge-PMD/PUD entries with pointers to page
table.

> +				break;
> +			}
> +		}
> +	}
> +
> +	if (!spmd)
> +		goto out;
> +
> +	ptl = pmd_lockptr(mm, spmd);
> +	spin_lock(ptl);

You take lock on PMD page table...

> +
> +	if (pud_none(*pud)) {
> +		pud_populate(mm, pud,
> +			    (pmd_t *)((unsigned long)spmd & PAGE_MASK));

... and modify PUD page table.

> +		mm_inc_nr_pmds(mm);
> +	} else {
> +		put_page(virt_to_page(spmd));
> +	}
> +	spin_unlock(ptl);
> +out:
> +	pmd = pmd_alloc(mm, pud, addr);
> +	i_mmap_unlock_write(mapping);
> +	return pmd;
> +}
> +
> +static bool may_share_pmd(struct vm_area_struct *vma)
> +{
> +	if (vma_is_fsdax(vma))
> +		return true;
> +	return false;
> +}
> +#else
> +static pmd_t *pmd_share(struct mm_struct *mm, pud_t *pud, unsigned long addr)
> +{
> +	return pmd_alloc(mm, pud, addr);
> +}
> +
> +static bool may_share_pmd(struct vm_area_struct *vma)
> +{
> +	return false;
> +}
> +#endif
> +
>  /*
>   * By the time we get here, we already hold the mm semaphore
>   *
> @@ -3898,7 +4001,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  		}
>  	}
>  
> -	vmf.pmd = pmd_alloc(mm, vmf.pud, address);
> +	if (unlikely(may_share_pmd(vma)))
> +		vmf.pmd = pmd_share(mm, vmf.pud, address);
> +	else
> +		vmf.pmd = pmd_alloc(mm, vmf.pud, address);
>  	if (!vmf.pmd)
>  		return VM_FAULT_OOM;
>  	if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
> -- 
> 1.8.3.1
>