linux-kernel - Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages

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Date:   Thu, 24 Aug 2023 10:19:10 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, david@...hat.com
Subject: Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in
 isolate_freepages_range



On 8/22/2023 9:37 AM, Kemeng Shi wrote:
> 
> 
> 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.

I realized I made a bad example, sorry for noise.

After more thinking, I agree that the 'pfn + isolated' is always 
pageblock aligned in strict mode. So feel free to add:
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

> 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@...weicloud.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,
>>>>>>
>>>>
>>
>>