linux-kernel - Re: [RFC 05/11] mm: make the try_to

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Date:   Mon, 6 Mar 2017 15:10:17 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Minchan Kim <minchan@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, kernel-team@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC 05/11] mm: make the try_to_munlock void function

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@...e.cz>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>> Signed-off-by: Minchan Kim <minchan@...nel.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.