linux-kernel - Re: [v2 PATCH 2/2] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind

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Message-ID: <a5c48e13-68a3-ae0f-6554-d06fc79b2fe4@linux.alibaba.com>
Date:   Wed, 17 Jul 2019 12:25:07 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>, mhocko@...nel.org,
        mgorman@...hsingularity.net, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [v2 PATCH 2/2] mm: mempolicy: handle vma with unmovable pages
 mapped correctly in mbind



On 7/17/19 11:50 AM, Vlastimil Babka wrote:
> On 7/17/19 8:23 PM, Yang Shi wrote:
>>
>> On 7/16/19 10:28 AM, Yang Shi wrote:
>>>
>>> On 7/16/19 5:07 AM, Vlastimil Babka wrote:
>>>> On 6/22/19 2:20 AM, Yang Shi wrote:
>>>>> @@ -969,10 +975,21 @@ static long do_get_mempolicy(int *policy,
>>>>> nodemask_t *nmask,
>>>>>    /*
>>>>>     * page migration, thp tail pages can be passed.
>>>>>     */
>>>>> -static void migrate_page_add(struct page *page, struct list_head
>>>>> *pagelist,
>>>>> +static int migrate_page_add(struct page *page, struct list_head
>>>>> *pagelist,
>>>>>                    unsigned long flags)
>>>>>    {
>>>>>        struct page *head = compound_head(page);
>>>>> +
>>>>> +    /*
>>>>> +     * Non-movable page may reach here.  And, there may be
>>>>> +     * temporaty off LRU pages or non-LRU movable pages.
>>>>> +     * Treat them as unmovable pages since they can't be
>>>>> +     * isolated, so they can't be moved at the moment.  It
>>>>> +     * should return -EIO for this case too.
>>>>> +     */
>>>>> +    if (!PageLRU(head) && (flags & MPOL_MF_STRICT))
>>>>> +        return -EIO;
>>>>> +
>>>> Hm but !PageLRU() is not the only way why queueing for migration can
>>>> fail, as can be seen from the rest of the function. Shouldn't all cases
>>>> be reported?
>>> Do you mean the shared pages and isolation failed pages? I'm not sure
>>> whether we should consider these cases break the semantics or not, so
>>> I leave them as they are. But, strictly speaking they should be
>>> reported too, at least for the isolation failed page.
> CC'd linux-api, should be done on v3 posting also.
>
>> By reading mbind man page, it says:
>>
>> If MPOL_MF_MOVE is specified in flags, then the kernel will attempt to
>> move all the existing pages in the memory range so that they follow the
>> policy.  Pages that are shared with other processes will not be moved.
>> If MPOL_MF_STRICT is also specified, then the call fails with the error
>> EIO if some pages could not be moved.
> I don't think this means that for shared pages, -EIO should not be
> reported. I can imagine both interpretations of the paragraph. I guess
> we can be conservative and keep not reporting them, if that was always
> the case - but then perhaps clarify the man page?

Yes, I agree the man page does looks ambiguous.  Anyway, I think we 
could add a patch later to kernel or manpage for either interpretations 
once it gets clarified.

>
>> It looks the code already handles shared page correctly, we just need
>> return -EIO for isolation failed page if MPOL_MF_STRICT is specified.
>>
>>> Thanks,
>>> Yang
>>>
>>>>>        /*
>>>>>         * Avoid migrating a page that is shared with others.
>>>>>         */
>>>>> @@ -984,6 +1001,8 @@ static void migrate_page_add(struct page *page,
>>>>> struct list_head *pagelist,
>>>>>                    hpage_nr_pages(head));
>>>>>            }
>>>>>        }
>>>>> +
>>>>> +    return 0;
>>>>>    }
>>>>>      /* page allocation callback for NUMA node migration */
>>>>> @@ -1186,9 +1205,10 @@ static struct page *new_page(struct page
>>>>> *page, unsigned long start)
>>>>>    }
>>>>>    #else
>>>>>    -static void migrate_page_add(struct page *page, struct list_head
>>>>> *pagelist,
>>>>> +static int migrate_page_add(struct page *page, struct list_head
>>>>> *pagelist,
>>>>>                    unsigned long flags)
>>>>>    {
>>>>> +    return -EIO;
>>>>>    }
>>>>>      int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
>>>>>