Re: [PATCH] mm: remove migration for HugePage in isolate_single_pageblock()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/8/17 3:45, David Hildenbrand wrote:
> On 16.08.24 13:30, Kefeng Wang wrote:
>>
>>
>> On 2024/8/16 18:11, David Hildenbrand wrote:
>>> On 16.08.24 06:06, Kefeng Wang wrote:
>>>> The gigantic page size may larger than memory block size, so memory
>>>> offline always fails in this case after commit b2c9e2fbba32 ("mm: make
>>>> alloc_contig_range work at pageblock granularity"),
>>>>
>>>> offline_pages
>>>>     start_isolate_page_range
>>>>       start_isolate_page_range(isolate_before=true)
>>>>         isolate [isolate_start, isolate_start + pageblock_nr_pages)
>>>>       start_isolate_page_range(isolate_before=false)
>>>>         isolate [isolate_end - pageblock_nr_pages, isolate_end) 
>>>> pageblock
>>>>              __alloc_contig_migrate_range
>>>>             isolate_migratepages_range
>>>>               isolate_migratepages_block
>>>>                 isolate_or_dissolve_huge_page
>>>>                   if (hstate_is_gigantic(h))
>>>>                       return -ENOMEM;
>>>>
>>>> In fact, we don't need to migrate page in page range isolation, for
>>>> memory offline path, there is do_migrate_range() to move the pages.
>>>> For contig allocation, there is another __alloc_contig_migrate_range()
>>>> after isolation to migrate the pages. So fix issue by skipping the
>>>> __alloc_contig_migrate_range() in isolate_single_pageblock().

...

> 
> Please distill some of that in the patch description. Right now you only 
> talk about memory offlining and don't cover why alloc_contig_range() is 
> fine as well with this change.

Borrowing some word from Zi,


PageHuge(gigantic) can bigger than a pageblock, the gigantic PageHuge is 
freed as order-0. This means MIGRATE_ISOLATE pageblocks will get to the 
right free list after __alloc_contig_migrate_range(), the one after
start_isolate_page_range() for alloc_contig_range(), this is same as in
memory offline, it has own path to isolate/migrate used page and 
dissolve the free hugepages, so the migration code in 
isolate_single_pageblock() is not needed, let's cleanup it and which 
also fix the above the issue.

Please correct me or help to write better description, thanks.

> 
> Let me explore the details in the meantime ... :)
>