Re: [PATCHv5 15/28] ksm: prepare to new THP semantics

[Vlastimil Babka <vbabka@suse.cz>]

On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> We don't need special code to stabilize THP. If you've got reference to
> any subpage of THP it will not be split under you.
>
> New split_huge_page() also accepts tail pages: no need in special code
> to get reference to head page.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>   mm/ksm.c | 57 ++++++++++-----------------------------------------------
>   1 file changed, 10 insertions(+), 47 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index fe09f3ddc912..fb333d8188fc 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -441,20 +441,6 @@ static void break_cow(struct rmap_item *rmap_item)
>   	up_read(&mm->mmap_sem);
>   }
>
> -static struct page *page_trans_compound_anon(struct page *page)
> -{
> -	if (PageTransCompound(page)) {
> -		struct page *head = compound_head(page);
> -		/*
> -		 * head may actually be splitted and freed from under
> -		 * us but it's ok here.
> -		 */
> -		if (PageAnon(head))
> -			return head;
> -	}
> -	return NULL;
> -}
> -
>   static struct page *get_mergeable_page(struct rmap_item *rmap_item)
>   {
>   	struct mm_struct *mm = rmap_item->mm;
> @@ -470,7 +456,7 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
>   	page = follow_page(vma, addr, FOLL_GET);
>   	if (IS_ERR_OR_NULL(page))
>   		goto out;
> -	if (PageAnon(page) || page_trans_compound_anon(page)) {
> +	if (PageAnon(page)) {
>   		flush_anon_page(vma, page, addr);
>   		flush_dcache_page(page);
>   	} else {
> @@ -976,33 +962,6 @@ out:
>   	return err;
>   }
>
> -static int page_trans_compound_anon_split(struct page *page)
> -{
> -	int ret = 0;
> -	struct page *transhuge_head = page_trans_compound_anon(page);
> -	if (transhuge_head) {
> -		/* Get the reference on the head to split it. */
> -		if (get_page_unless_zero(transhuge_head)) {
> -			/*
> -			 * Recheck we got the reference while the head
> -			 * was still anonymous.
> -			 */
> -			if (PageAnon(transhuge_head))
> -				ret = split_huge_page(transhuge_head);
> -			else
> -				/*
> -				 * Retry later if split_huge_page run
> -				 * from under us.
> -				 */
> -				ret = 1;
> -			put_page(transhuge_head);
> -		} else
> -			/* Retry later if split_huge_page run from under us. */
> -			ret = 1;
> -	}
> -	return ret;
> -}
> -
>   /*
>    * try_to_merge_one_page - take two pages and merge them into one
>    * @vma: the vma that holds the pte pointing to page
> @@ -1023,9 +982,6 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
>
>   	if (!(vma->vm_flags & VM_MERGEABLE))
>   		goto out;
> -	if (PageTransCompound(page) && page_trans_compound_anon_split(page))
> -		goto out;
> -	BUG_ON(PageTransCompound(page));
>   	if (!PageAnon(page))
>   		goto out;
>
> @@ -1038,6 +994,13 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
>   	 */
>   	if (!trylock_page(page))
>   		goto out;
> +
> +	if (PageTransCompound(page)) {
> +		err = split_huge_page(page);
> +		if (err)
> +			goto out_unlock;
> +	}
> +
>   	/*
>   	 * If this anonymous page is mapped only here, its pte may need
>   	 * to be write-protected.  If it's mapped elsewhere, all of its
> @@ -1068,6 +1031,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
>   		}
>   	}
>
> +out_unlock:
>   	unlock_page(page);
>   out:
>   	return err;
> @@ -1620,8 +1584,7 @@ next_mm:
>   				cond_resched();
>   				continue;
>   			}
> -			if (PageAnon(*page) ||
> -			    page_trans_compound_anon(*page)) {
> +			if (PageAnon(*page)) {
>   				flush_anon_page(vma, *page, ksm_scan.address);
>   				flush_dcache_page(*page);
>   				rmap_item = get_next_rmap_item(slot,
>

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