Re: [PATCH v4 3/7] mm: page_isolation: check specified range for unmovable pages

[Zi Yan <ziy@nvidia.com>]

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On 24 Jan 2022, at 4:55, Oscar Salvador wrote:

> On 2022-01-19 20:06, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> Enable set_migratetype_isolate() to check specified sub-range for
>> unmovable pages during isolation. Page isolation is done
>> at max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) granularity, but not all
>> pages within that granularity are intended to be isolated. For example,
>> alloc_contig_range(), which uses page isolation, allows ranges without
>> alignment. This commit makes unmovable page check only look for
>> interesting pages, so that page isolation can succeed for any
>> non-overlapping ranges.
>
> Hi Zi Yan,
>
> I had to re-read this several times as I found this a bit misleading.
> I was mainly confused by the fact that memory_hotplug does isolation on PAGES_PER_SECTION granularity, and reading the above seems to indicate that either do it at MAX_ORDER_NR_PAGES or at pageblock_nr_pages granularity.

You are right. Sorry for the confusion. I think it should be
“Page isolation is done at least on max(MAX_ORDER_NR_PAEGS,
pageblock_nr_pages) granularity.”

memory_hotplug uses PAGES_PER_SECTION. It is greater than that.


>
> True is that start_isolate_page_range() expects the range to be pageblock aligned and works in pageblock_nr_pages chunks, but I do not think that is what you meant to say here.

Actually, start_isolate_page_range() should expect max(MAX_ORDER_NR_PAEGS,
pageblock_nr_pages) alignment instead of pageblock alignment. It seems to
be an uncovered bug in the current code, since all callers uses at least
max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) alignment.

The reason is that if start_isolate_page_range() is only pageblock aligned
and a caller wants to isolate one pageblock from a MAX_ORDER-1
(2 pageblocks on x84_64 systems) free page, this will lead to MIGRATE_ISOLATE
accounting error. To avoid it, start_isolate_page_range() needs to isolate
the max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) aligned range.


>
> Now, to the change itself, below:
>
>
>> @@ -47,8 +51,8 @@ static struct page *has_unmovable_pages(struct zone
>> *zone, struct page *page,
>>  		return page;
>>  	}
>>
>> -	for (; iter < pageblock_nr_pages - offset; iter++) {
>> -		page = pfn_to_page(pfn + iter);
>> +	for (pfn = first_pfn; pfn < last_pfn; pfn++) {
>
> You already did pfn = first_pfn before.

Got it. Will remove the redundant code.

>
>>  /**
>>   * start_isolate_page_range() - make page-allocation-type of range of pages to
>>   * be MIGRATE_ISOLATE.
>> - * @start_pfn:		The lower PFN of the range to be isolated.
>> - * @end_pfn:		The upper PFN of the range to be isolated.
>> + * @start_pfn:		The lower PFN of the range to be checked for
>> + *			possibility of isolation.
>> + * @end_pfn:		The upper PFN of the range to be checked for
>> + *			possibility of isolation.
>> + * @isolate_start:		The lower PFN of the range to be isolated.
>> + * @isolate_end:		The upper PFN of the range to be isolated.
>
> So, what does "possibility" means here. I think this need to be clarified a bit better.

start_isolate_page_range() needs to check if unmovable pages exist in the
range [start_pfn, end_pfn) but mark all pageblocks within [isolate_start,
isolate_end) MIGRATE_ISOLATE (isolate_* need to be max(MAX_ORDER_NR_PAEGS,
pageblock_nr_pages) aligned). But now I realize “possibility” here is very
confusing, since both ranges decide whether the isolation can succeed.

>
> From what you pointed out in the commit message I think what you are doing is:
>
> - alloc_contig_range() gets a range to be isolated.
> - then you pass two ranges to start_isolate_page_range()
>   (start_pfn, end_pfn]: which is the unaligned range you got in alloc_contig_range()
>   (isolate_start, isolate_end]: which got aligned to, let's say, to MAX_ORDER_NR_PAGES
>
> Now, most likely, (start_pfn, end_pfn] only covers a sub-range of (isolate_start, isolate_end], and that
> sub-range is what you really want to isolate (so (start_pfn, end_pfn])?

Correct.

I agree that isolate_start and isolate_end are pretty confusing here.
They are implementation details of start_isolate_page_range() and should
not be exposed. I will remove them from the parameter list and produce
them inside start_isolate_page_range(). They are pfn_max_align_down()
and pfn_max_align_up() of start_pfn and end_pfn, respectively.

In alloc_contig_range(), the code is still needed to save and restore
migrateypes for [isolate_start, start_pfn) and (end_pfn, isolate_end],
because [start_pfn, end_pfn) is not required to be max(MAX_ORDER_NR_PAEGS,
pageblock_nr_pages) aligned. Like I said in the patch, the code will
go away once MIGRATE_ISOLATE becomes a standalone bit without overwriting
existing migratetypes during page isolation. And then isolate_start
and isolate_end here will be completely transparent to callers of
start_isolate_page_range().

Thanks for your review and comment.

--
Best Regards,
Yan, Zi

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