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

[Minchan Kim <minchan@kernel.org>]

Hi Anshuman,

On Mon, Mar 06, 2017 at 03:10:17PM +0530, Anshuman Khandual wrote:
> On 03/06/2017 07:39 AM, Minchan Kim wrote:
> > On Fri, Mar 03, 2017 at 05:13:54PM +0530, Anshuman Khandual wrote:
> >> 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@suse.cz>
> >>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >>> Signed-off-by: Minchan Kim <minchan@kernel.org>
> >>> ---
> >>>  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 ?
> > 
> > If I read code properly,  __munlock_isolated_page calls try_to_munlock
> > always pass the TestClearPageMlocked page to try_to_munlock.
> 
> Right.
> 
> > (e.g., munlock_vma_page and __munlock_pagevec) so I thought
> > try_to_munlock should be called non-PG_mlocked page and try_to_unmap_one
> > returns PG_mlocked page once it found a VM_LOCKED VMA for a page.
> > IOW, non-PG_mlocked page is precondition for try_to_munlock.
> 
> Okay, I have missed that part. Nonetheless this is a separate issue,
> should be part of a different patch ? Not inside these cleanups.

If that precondition is not true, this patch changes the behavior
slightly.

        UNEVICTABLE_PGMUNLOCKED count mistmatch compared to old.

I wanted to catch it up. If you still think it's separate issue,
I will do. Please tell me. However, I still think it's no problem
to merge it in this clean up patch.

Thanks.

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