linux-kernel - Re: [patch 3/3] mm: memory: merge shared-writable dirtying branches in do_wp

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Date:	Tue, 9 Dec 2014 19:22:19 +0100
From:	Jan Kara <jack@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>, Jan Kara <jack@...e.cz>,
	"Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 3/3] mm: memory: merge shared-writable dirtying branches
 in do_wp_page()

On Fri 05-12-14 09:52:46, Johannes Weiner wrote:
> Whether there is a vm_ops->page_mkwrite or not, the page dirtying is
> pretty much the same.  Make sure the page references are the same in
> both cases, then merge the two branches.
> 
> It's tempting to go even further and page-lock the !page_mkwrite case,
> to get it in line with everybody else setting the page table and thus
> further simplify the model.  But that's not quite compelling enough to
> justify dropping the pte lock, then relocking and verifying the entry
> for filesystems without ->page_mkwrite, which notably includes tmpfs.
> Leave it for now and lock the page late in the !page_mkwrite case.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
  The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  mm/memory.c | 48 +++++++++++++++++-------------------------------
>  1 file changed, 17 insertions(+), 31 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 5640a718ac58..df47fd0a4b7f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2046,7 +2046,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	pte_t entry;
>  	int ret = 0;
>  	int page_mkwrite = 0;
> -	struct page *dirty_page = NULL;
> +	bool dirty_shared = false;
>  	unsigned long mmun_start = 0;	/* For mmu_notifiers */
>  	unsigned long mmun_end = 0;	/* For mmu_notifiers */
>  	struct mem_cgroup *memcg;
> @@ -2097,6 +2097,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		unlock_page(old_page);
>  	} else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
>  					(VM_WRITE|VM_SHARED))) {
> +		page_cache_get(old_page);
>  		/*
>  		 * Only catch write-faults on shared writable pages,
>  		 * read-only shared pages can get COWed by
> @@ -2104,7 +2105,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		 */
>  		if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
>  			int tmp;
> -			page_cache_get(old_page);
> +
>  			pte_unmap_unlock(page_table, ptl);
>  			tmp = do_page_mkwrite(vma, old_page, address);
>  			if (unlikely(!tmp || (tmp &
> @@ -2124,11 +2125,10 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  				unlock_page(old_page);
>  				goto unlock;
>  			}
> -
>  			page_mkwrite = 1;
>  		}
> -		dirty_page = old_page;
> -		get_page(dirty_page);
> +
> +		dirty_shared = true;
>  
>  reuse:
>  		/*
> @@ -2147,43 +2147,29 @@ reuse:
>  		pte_unmap_unlock(page_table, ptl);
>  		ret |= VM_FAULT_WRITE;
>  
> -		if (!dirty_page)
> -			return ret;
> -
> -		if (!page_mkwrite) {
> +		if (dirty_shared) {
>  			struct address_space *mapping;
>  			int dirtied;
>  
> -			lock_page(dirty_page);
> -			dirtied = set_page_dirty(dirty_page);
> -			VM_BUG_ON_PAGE(PageAnon(dirty_page), dirty_page);
> -			mapping = dirty_page->mapping;
> -			unlock_page(dirty_page);
> +			if (!page_mkwrite)
> +				lock_page(old_page);
>  
> -			if (dirtied && mapping) {
> -				/*
> -				 * Some device drivers do not set page.mapping
> -				 * but still dirty their pages
> -				 */
> -				balance_dirty_pages_ratelimited(mapping);
> -			}
> +			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);
>  
> -			file_update_time(vma->vm_file);
> -		}
> -		put_page(dirty_page);
> -		if (page_mkwrite) {
> -			struct address_space *mapping = dirty_page->mapping;
> -
> -			set_page_dirty(dirty_page);
> -			unlock_page(dirty_page);
> -			page_cache_release(dirty_page);
> -			if (mapping)	{
> +			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;
> -- 
> 2.1.3
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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