Re: [PATCH V5 1/4] mm: Refactor do_wp_page, extract the reuse case

[Michal Hocko <mhocko@suse.cz>]

On Sun 22-02-15 15:42:15, Shachar Raindel wrote:
> When do_wp_page is ending, in several cases it needs to reuse the
> existing page. This is achieved by making the page table writable,
> and possibly updating the page-cache state.
> 
> Currently, this logic was "called" by using a goto jump. This makes
> following the control flow of the function harder. It is also
> against the coding style guidelines for using goto.
> 
> As the code can easily be refactored into a specialized function,
> refactor it out and simplify the code flow in do_wp_page.
> 
> Signed-off-by: Shachar Raindel <raindel@mellanox.com>
> Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> Acked-by: Andi Kleen <ak@linux.intel.com>
> Acked-by: Haggai Eran <haggaie@mellanox.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Peter Feiner <pfeiner@google.com>
> Cc: Michel Lespinasse <walken@google.com>

Makes sense to me
Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/memory.c | 117 +++++++++++++++++++++++++++++++++++-------------------------
>  1 file changed, 68 insertions(+), 49 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 8068893..7a04414 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1983,6 +1983,65 @@ static int do_page_mkwrite(struct vm_area_struct *vma, struct page *page,
>  }
>  
>  /*
> + * Handle write page faults for pages that can be reused in the current vma
> + *
> + * This can happen either due to the mapping being with the VM_SHARED flag,
> + * or due to us being the last reference standing to the page. In either
> + * case, all we need to do here is to mark the page as writable and update
> + * any related book-keeping.
> + */
> +static int wp_page_reuse(struct mm_struct *mm, struct vm_area_struct *vma,
> +			 unsigned long address, pte_t *page_table,
> +			 spinlock_t *ptl, pte_t orig_pte,
> +			 struct page *page, int page_mkwrite,
> +			 int dirty_shared)
> +	__releases(ptl)
> +{
> +	pte_t entry;
> +	/*
> +	 * Clear the pages cpupid information as the existing
> +	 * information potentially belongs to a now completely
> +	 * unrelated process.
> +	 */
> +	if (page)
> +		page_cpupid_xchg_last(page, (1 << LAST_CPUPID_SHIFT) - 1);
> +
> +	flush_cache_page(vma, address, pte_pfn(orig_pte));
> +	entry = pte_mkyoung(orig_pte);
> +	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +	if (ptep_set_access_flags(vma, address, page_table, entry, 1))
> +		update_mmu_cache(vma, address, page_table);
> +	pte_unmap_unlock(page_table, ptl);
> +
> +	if (dirty_shared) {
> +		struct address_space *mapping;
> +		int dirtied;
> +
> +		if (!page_mkwrite)
> +			lock_page(page);
> +
> +		dirtied = set_page_dirty(page);
> +		VM_BUG_ON_PAGE(PageAnon(page), page);
> +		mapping = page->mapping;
> +		unlock_page(page);
> +		page_cache_release(page);
> +
> +		if ((dirtied || page_mkwrite) && mapping) {
> +			/*
> +			 * Some device drivers do not set page.mapping
> +			 * but still dirty their pages
> +			 */
> +			balance_dirty_pages_ratelimited(mapping);
> +		}
> +
> +		if (!page_mkwrite)
> +			file_update_time(vma->vm_file);
> +	}
> +
> +	return VM_FAULT_WRITE;
> +}
> +
> +/*
>   * This routine handles present pages, when users try to write
>   * to a shared page. It is done by copying the page to a new address
>   * and decrementing the shared-page counter for the old page.
> @@ -2008,8 +2067,6 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	struct page *old_page, *new_page = NULL;
>  	pte_t entry;
>  	int ret = 0;
> -	int page_mkwrite = 0;
> -	bool dirty_shared = false;
>  	unsigned long mmun_start = 0;	/* For mmu_notifiers */
>  	unsigned long mmun_end = 0;	/* For mmu_notifiers */
>  	struct mem_cgroup *memcg;
> @@ -2026,7 +2083,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		 */
>  		if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
>  				     (VM_WRITE|VM_SHARED))
> -			goto reuse;
> +			return wp_page_reuse(mm, vma, address, page_table, ptl,
> +					     orig_pte, old_page, 0, 0);
>  		goto gotten;
>  	}
>  
> @@ -2055,12 +2113,16 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			 */
>  			page_move_anon_rmap(old_page, vma, address);
>  			unlock_page(old_page);
> -			goto reuse;
> +			return wp_page_reuse(mm, vma, address, page_table, ptl,
> +					     orig_pte, old_page, 0, 0);
>  		}
>  		unlock_page(old_page);
>  	} else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
>  					(VM_WRITE|VM_SHARED))) {
> +		int page_mkwrite = 0;
> +
>  		page_cache_get(old_page);
> +
>  		/*
>  		 * Only catch write-faults on shared writable pages,
>  		 * read-only shared pages can get COWed by
> @@ -2091,51 +2153,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			page_mkwrite = 1;
>  		}
>  
> -		dirty_shared = true;
> -
> -reuse:
> -		/*
> -		 * Clear the pages cpupid information as the existing
> -		 * information potentially belongs to a now completely
> -		 * unrelated process.
> -		 */
> -		if (old_page)
> -			page_cpupid_xchg_last(old_page, (1 << LAST_CPUPID_SHIFT) - 1);
> -
> -		flush_cache_page(vma, address, pte_pfn(orig_pte));
> -		entry = pte_mkyoung(orig_pte);
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> -		if (ptep_set_access_flags(vma, address, page_table, entry,1))
> -			update_mmu_cache(vma, address, page_table);
> -		pte_unmap_unlock(page_table, ptl);
> -		ret |= VM_FAULT_WRITE;
> -
> -		if (dirty_shared) {
> -			struct address_space *mapping;
> -			int dirtied;
> -
> -			if (!page_mkwrite)
> -				lock_page(old_page);
> -
> -			dirtied = set_page_dirty(old_page);
> -			VM_BUG_ON_PAGE(PageAnon(old_page), old_page);
> -			mapping = old_page->mapping;
> -			unlock_page(old_page);
> -			page_cache_release(old_page);
> -
> -			if ((dirtied || page_mkwrite) && mapping) {
> -				/*
> -				 * Some device drivers do not set page.mapping
> -				 * but still dirty their pages
> -				 */
> -				balance_dirty_pages_ratelimited(mapping);
> -			}
> -
> -			if (!page_mkwrite)
> -				file_update_time(vma->vm_file);
> -		}
> -
> -		return ret;
> +		return wp_page_reuse(mm, vma, address, page_table, ptl,
> +				     orig_pte, old_page, page_mkwrite, 1);
>  	}
>  
>  	/*
> -- 
> 1.7.11.2
> 
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-- 
Michal Hocko
SUSE Labs

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