Re: [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite()

[Vivek Goyal]

On Wed, Jun 28, 2017 at 04:01:48PM -0600, Ross Zwisler wrote:
> To be able to use the common 4k zero page in DAX we need to have our PTE
> fault path look more like our PMD fault path where a PTE entry can be
> marked as dirty and writeable as it is first inserted, rather than waiting
> for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
> 
> Right now we can rely on having a dax_pfn_mkwrite() call because we can
> distinguish between these two cases in do_wp_page():
> 
> 	case 1: 4k zero page => writable DAX storage
> 	case 2: read-only DAX storage => writeable DAX storage
> 
> This distinction is made by via vm_normal_page().  vm_normal_page() returns
> false for the common 4k zero page, though, just as it does for DAX ptes.
> Instead of special casing the DAX + 4k zero page case, we will simplify our
> DAX PTE page fault sequence so that it matches our DAX PMD sequence, and
> get rid of dax_pfn_mkwrite() completely.
> 
> This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set insert_pfn()
> will do the work that was previously done by wp_page_reuse() as part of the
> dax_pfn_mkwrite() call path.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h |  2 ++
>  mm/memory.c        | 57 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>  2 files changed, 54 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f543a4..096052f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2293,6 +2293,8 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>  			unsigned long pfn, pgprot_t pgprot);
>  int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
>  			pfn_t pfn);
> +int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
> +			pfn_t pfn);
>  int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len);
>  
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index bb11c47..de4aa71 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1646,7 +1646,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
>  EXPORT_SYMBOL(vm_insert_page);
>  
>  static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> -			pfn_t pfn, pgprot_t prot)
> +			pfn_t pfn, pgprot_t prot, bool mkwrite)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
>  	int retval;
> @@ -1658,14 +1658,26 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	if (!pte)
>  		goto out;
>  	retval = -EBUSY;
> -	if (!pte_none(*pte))
> -		goto out_unlock;
> +	if (!pte_none(*pte)) {
> +		if (mkwrite) {
> +			entry = *pte;
> +			goto out_mkwrite;
> +		} else
> +			goto out_unlock;
> +	}
>  
>  	/* Ok, finally just insert the thing.. */
>  	if (pfn_t_devmap(pfn))
>  		entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
>  	else
>  		entry = pte_mkspecial(pfn_t_pte(pfn, prot));
> +
> +out_mkwrite:
> +	if (mkwrite) {
> +		entry = pte_mkyoung(entry);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +	}
> +
>  	set_pte_at(mm, addr, pte, entry);
>  	update_mmu_cache(vma, addr, pte); /* XXX: why not for insert_page? */
>  
> @@ -1736,7 +1748,8 @@ int vm_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>  
>  	track_pfn_insert(vma, &pgprot, __pfn_to_pfn_t(pfn, PFN_DEV));
>  
> -	ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot);
> +	ret = insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot,
> +			false);
>  
>  	return ret;
>  }
> @@ -1772,10 +1785,44 @@ int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
>  		page = pfn_to_page(pfn_t_to_pfn(pfn));
>  		return insert_page(vma, addr, page, pgprot);
>  	}
> -	return insert_pfn(vma, addr, pfn, pgprot);
> +	return insert_pfn(vma, addr, pfn, pgprot, false);
>  }
>  EXPORT_SYMBOL(vm_insert_mixed);
>  
> +int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
> +			pfn_t pfn)
> +{
> +	pgprot_t pgprot = vma->vm_page_prot;
> +
> +	BUG_ON(!(vma->vm_flags & VM_MIXEDMAP));
> +
> +	if (addr < vma->vm_start || addr >= vma->vm_end)
> +		return -EFAULT;
> +
> +	track_pfn_insert(vma, &pgprot, pfn);
> +
> +	/*
> +	 * If we don't have pte special, then we have to use the pfn_valid()
> +	 * based VM_MIXEDMAP scheme (see vm_normal_page), and thus we *must*
> +	 * refcount the page if pfn_valid is true (hence insert_page rather
> +	 * than insert_pfn).  If a zero_pfn were inserted into a VM_MIXEDMAP
> +	 * without pte special, it would there be refcounted as a normal page.
> +	 */
> +	if (!HAVE_PTE_SPECIAL && !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
> +		struct page *page;
> +
> +		/*
> +		 * At this point we are committed to insert_page()
> +		 * regardless of whether the caller specified flags that
> +		 * result in pfn_t_has_page() == false.
> +		 */
> +		page = pfn_to_page(pfn_t_to_pfn(pfn));
> +		return insert_page(vma, addr, page, pgprot);
> +	}
> +	return insert_pfn(vma, addr, pfn, pgprot, true);
> +}
> +EXPORT_SYMBOL(vm_insert_mixed_mkwrite);

Hi Ross,

vm_insert_mixed_mkwrite() is same as vm_insert_mixed() except this sets
write parameter to inser_pfn() true. Will it make sense to just add
mkwrite parameter to vm_insert_mixed() and not add a new helper function.
(like insert_pfn()).

Vivek