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

Anshuman Khandual
2017-03-03T11:45:37Z

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 ?