Re: [RFC 05/11] mm: make the try_to_munlock void function

[Anshuman Khandual]

On 03/02/2017 12:09 PM, Minchan Kim wrote:
> try_to_munlock returns SWAP_MLOCK if the one of VMAs mapped
> the page has VM_LOCKED flag. In that time, VM set PG_mlocked to
> the page if the page is not pte-mapped THP which cannot be
> mlocked, either.

Right.

> 
> With that, __munlock_isolated_page can use PageMlocked to check
> whether try_to_munlock is successful or not without relying on
> try_to_munlock's retval. It helps to make ttu/ttuo simple with
> upcoming patches.

Right.

> 
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  include/linux/rmap.h |  2 +-
>  mm/mlock.c           |  6 ++----
>  mm/rmap.c            | 16 ++++------------
>  3 files changed, 7 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b556eef..1b0cd4c 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -235,7 +235,7 @@ int page_mkclean(struct page *);
>   * called in munlock()/munmap() path to check for other vmas holding
>   * the page mlocked.
>   */
> -int try_to_munlock(struct page *);
> +void try_to_munlock(struct page *);
>  
>  void remove_migration_ptes(struct page *old, struct page *new, bool locked);
>  
> diff --git a/mm/mlock.c b/mm/mlock.c
> index cdbed8a..d34a540 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -122,17 +122,15 @@ static bool __munlock_isolate_lru_page(struct page *page, bool getpage)
>   */
>  static void __munlock_isolated_page(struct page *page)
>  {
> -	int ret = SWAP_AGAIN;
> -
>  	/*
>  	 * Optimization: if the page was mapped just once, that's our mapping
>  	 * and we don't need to check all the other vmas.
>  	 */
>  	if (page_mapcount(page) > 1)
> -		ret = try_to_munlock(page);
> +		try_to_munlock(page);
>  
>  	/* Did try_to_unlock() succeed or punt? */
> -	if (ret != SWAP_MLOCK)
> +	if (!PageMlocked(page))

Checks if the page is still mlocked or not.

>  		count_vm_event(UNEVICTABLE_PGMUNLOCKED);
>  
>  	putback_lru_page(page);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0a48958..61ae694 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1540,18 +1540,10 @@ static int page_not_mapped(struct page *page)
>   * Called from munlock code.  Checks all of the VMAs mapping the page
>   * to make sure nobody else has this page mlocked. The page will be
>   * returned with PG_mlocked cleared if no other vmas have it mlocked.
> - *
> - * Return values are:
> - *
> - * SWAP_AGAIN	- no vma is holding page mlocked, or,
> - * SWAP_AGAIN	- page mapped in mlocked vma -- couldn't acquire mmap sem
> - * SWAP_FAIL	- page cannot be located at present
> - * SWAP_MLOCK	- page is now mlocked.
>   */
> -int try_to_munlock(struct page *page)
> -{
> -	int ret;
>  
> +void try_to_munlock(struct page *page)
> +{
>  	struct rmap_walk_control rwc = {
>  		.rmap_one = try_to_unmap_one,
>  		.arg = (void *)TTU_MUNLOCK,
> @@ -1561,9 +1553,9 @@ int try_to_munlock(struct page *page)
>  	};
>  
>  	VM_BUG_ON_PAGE(!PageLocked(page) || PageLRU(page), page);
> +	VM_BUG_ON_PAGE(PageMlocked(page), page);

We are calling on the page to see if its mlocked from any of it's
mapping VMAs. Then it is a possibility that the page is mlocked
and the above condition is true and we print VM BUG report there.
The point is if its a valid possibility why we have added the
above check ?