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

[Zi Yan <ziy@nvidia.com>]

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On 2 Feb 2022, at 7:25, David Hildenbrand wrote:

> On 02.02.22 13:18, Oscar Salvador wrote:
>> On Wed, Jan 19, 2022 at 02:06:19PM -0500, 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.
>>
>> Another thing that came to my mind.
>> Prior to this patch, has_unmovable_pages() was checking on pageblock
>> granularity, starting at pfn#0 of the pageblock.
>> With this patch, you no longer check on pageblock granularity, which
>> means you might isolate a pageblock, but some pages that sneaked in
>> might actually be unmovable.
>>
>> E.g:
>>
>> Let's say you have a pageblock that spans (pfn#512,pfn#1024),
>> and you pass alloc_contig_range() (pfn#514,pfn#1024).
>> has_unmovable_pages() will start checking the pageblock at pfn#514,
>> and so it will mis pfn#512 and pfn#513. Isn't that a problem, if those
>> pfn turn out to be actually unmovable?
>
> That's the whole idea for being able to allocate parts of an unmovable
> pageblock that are movable.
>
> If the first part is unmovable but the second part is movable, nothing
> should stop us from trying to allocate the second part.
>
> Of course, we want to remember the original migratetype of the
> pageblock, to restore that after isolation -- otherwise we'll end up
> converting all such pageblocks to MIGRATE_MOVABLE. The next patch does
> that IIRC.

Yes. A desirable optimization is to make MIGRATE_ISOLATE a standalone bit,
so isolating a pageblock will not remove its original migratetype. It is
on my todo list.

>
> However, devil is in the detail, and I still have to review those parts
> of this series.

Thanks. You can wait for my next version. I am planning to make
start_isolate_page_range() accept any address range and move migratetype
save and restore into it, so that the caller do not need to worry about
alignment.

Basically, start_isolate_page_range() will need to migrate compound pages
at the beginning and/or the end of the given range [start_pfn, end_pfn) if
start_pfn and/or end_pfn-1 is in the middle of a compound page.
If start_pfn and/or end_pfn-1 is in the middle of a free page, the free
page will need to be split and put into separate migratetype lists.

>
>
> Note that there are no current users of alloc_contig_range() that
> allocate < MAX_ORDER - 1 -- except CMA, but for CMA we immediately exit
> has_unmovable_pages() either way.
>
> -- 
> Thanks,
>
> David / dhildenb


--
Best Regards,
Yan, Zi

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