Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages_range

[Kemeng Shi <shikemeng@huaweicloud.com>]

on 8/19/2023 7:58 PM, Baolin Wang wrote:
> 
> 
> On 8/15/2023 6:37 PM, Kemeng Shi wrote:
>>
>>
>> on 8/15/2023 6:07 PM, Baolin Wang wrote:
>>>
>>>
>>> On 8/15/2023 5:32 PM, Kemeng Shi wrote:
>>>>
>>>>
>>>> on 8/15/2023 4:38 PM, Baolin Wang wrote:
>>>>>
>>>>>
>>>>> On 8/5/2023 7:07 PM, Kemeng Shi wrote:
>>>>>> We call isolate_freepages_block in strict mode, continuous pages in
>>>>>> pageblock will be isolated if isolate_freepages_block successed.
>>>>>> Then pfn + isolated will point to start of next pageblock to scan
>>>>>> no matter how many pageblocks are isolated in isolate_freepages_block.
>>>>>> Use pfn + isolated as start of next pageblock to scan to simplify the
>>>>>> iteration.
>>>>>
>>>>> IIUC, the isolate_freepages_block() can isolate high-order free pages, which means the pfn + isolated can be larger than the block_end_pfn. So in your patch, the 'block_start_pfn' and 'block_end_pfn' can be in different pageblocks, that will break pageblock_pfn_to_page().
>>>>>
>>>> In for update statement, we always update block_start_pfn to pfn and
>>>
>>> I mean, you changed to:
>>> 1) pfn += isolated;
>>> 2) block_start_pfn = pfn;
>>> 3) block_end_pfn = pfn + pageblock_nr_pages;
>>>
>>> But in 1) pfn + isolated can go outside of the currnet pageblock if isolating a high-order page, for example, located in the middle of the next pageblock. So that the block_start_pfn can point to the middle of the next pageblock, not the start position. Meanwhile after 3), the block_end_pfn can point another pageblock. Or I missed something else?
>>>
>> Ah, I miss to explain this in changelog.
>> In case we could we have buddy page with order higher than pageblock:
>> 1. page in buddy page is aligned with it's order
>> 2. order of page is higher than pageblock order
>> Then page is aligned with pageblock order. So pfn of page and isolated pages
>> count are both aligned pageblock order. So pfn + isolated is pageblock order
>> aligned.
> 
> That's not what I mean. pfn + isolated is not always pageblock-aligned, since the isolate_freepages_block() can isolated high-order free pages (for example: order-1, order-2 ...).
> 
> Suppose the pageblock size is 2M, when isolating a pageblock (suppose the pfn range is 0 - 511 to make the arithmetic easy) by isolate_freepages_block(), and suppose pfn 0 to pfn 510 are all order-0 page, but pfn 511 is order-1 page, so you will isolate 513 pages from this pageblock, which will make 'pfn + isolated' not pageblock aligned.
This is also no supposed to happen as low order buddy pages should never span
cross boundary of high order pages:
In buddy system, we always split order N pages into two order N - 1 pages as
following:
|        order N        |
|order N - 1|order N - 1|
So buddy pages with order N - 1 will never cross boudary of order N. Similar,
buddy pages with order N - 2 will never cross boudary of order N - 1 and so
on. Then any pages with order less than N will never cross boudary of order
N.

> 
>>>> update block_end_pfn to pfn + pageblock_nr_pages. So they should point
>>>> to the same pageblock. I guess you missed the change to update of
>>>> block_end_pfn. :)
>>>>>>
>>>>>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>>>>>> ---
>>>>>>     mm/compaction.c | 14 ++------------
>>>>>>     1 file changed, 2 insertions(+), 12 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>>>>> index 684f6e6cd8bc..8d7d38073d30 100644
>>>>>> --- a/mm/compaction.c
>>>>>> +++ b/mm/compaction.c
>>>>>> @@ -733,21 +733,11 @@ isolate_freepages_range(struct compact_control *cc,
>>>>>>         block_end_pfn = pageblock_end_pfn(pfn);
>>>>>>           for (; pfn < end_pfn; pfn += isolated,
>>>>>> -                block_start_pfn = block_end_pfn,
>>>>>> -                block_end_pfn += pageblock_nr_pages) {
>>>>>> +                block_start_pfn = pfn,
>>>>>> +                block_end_pfn = pfn + pageblock_nr_pages) {
>>>>>>             /* Protect pfn from changing by isolate_freepages_block */
>>>>>>             unsigned long isolate_start_pfn = pfn;
>>>>>>     -        /*
>>>>>> -         * pfn could pass the block_end_pfn if isolated freepage
>>>>>> -         * is more than pageblock order. In this case, we adjust
>>>>>> -         * scanning range to right one.
>>>>>> -         */
>>>>>> -        if (pfn >= block_end_pfn) {
>>>>>> -            block_start_pfn = pageblock_start_pfn(pfn);
>>>>>> -            block_end_pfn = pageblock_end_pfn(pfn);
>>>>>> -        }
>>>>>> -
>>>>>>             block_end_pfn = min(block_end_pfn, end_pfn);
>>>>>>               if (!pageblock_pfn_to_page(block_start_pfn,
>>>>>
>>>
> 
>