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SubjectRe: [PATCH v2 2/2] mm,thp,compaction,cma: allow THP migration for CMA allocations
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On 2/27/20 5:21 PM, Rik van Riel wrote:
> On Thu, 2020-02-27 at 15:41 -0800, Mike Kravetz wrote:
>> On 2/27/20 1:32 PM, Rik van Riel wrote:
>>>
>>> +++ b/mm/page_alloc.c
>>> @@ -8253,14 +8253,19 @@ struct page *has_unmovable_pages(struct
>>> zone *zone, struct page *page,
>>>
>>> /*
>>> * Hugepages are not in LRU lists, but they're movable.
>>> + * THPs are on the LRU, but need to be counted as
>>> #small pages.
>>> * We need not scan over tail pages because we don't
>>> * handle each tail page individually in migration.
>>> */
>>> - if (PageHuge(page)) {
>>> + if (PageHuge(page) || PageTransCompound(page)) {
>>> struct page *head = compound_head(page);
>>> unsigned int skip_pages;
>>>
>>> - if
>>> (!hugepage_migration_supported(page_hstate(head)))
>>> + if (PageHuge(page) &&
>>> + !hugepage_migration_supported(page_hstate(h
>>> ead)))
>>> + return page;
>>> +
>>> + if (!PageLRU(head) && !__PageMovable(head))
>>
>> Pretty sure this is going to be true for hugetlb pages. So, this
>> will change
>> behavior and make all hugetlb pages look unmovable. Perhaps, only
>> check this
>> condition for THP pages?
>
> Does that need to be the following, then?
>
> if (PageTransHuge(head) && !PageHuge(page) && !PageLRU(head) &&
> !__PageMovable(head))
> return page;

Yes, that is what I would suggest. Results of a simple test on small VM.

Unmodified Kernel
-----------------
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
50
# hugeadm --hard --pool-pages-min DEFAULT:4G
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
50

V2 patches
----------
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
50
# hugeadm --hard --pool-pages-min DEFAULT:4G
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
14

V2 patches + above modification
-------------------------------
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
50
# hugeadm --hard --pool-pages-min DEFAULT:4G
# cat /sys/devices/system/memory/memory*/removable | grep 1 | wc -l
50

> That's an easy one liner I would be happy to send in
> if everybody agrees that should fix things :)

Another option might be to make hugetlb pages more like other pages and
__SetPageMovable on movable hugetlb pages. This could be used instead
of hugepage_migration_supported() calls. That is only a quick thought
and would be beyond the scope of this patch. If people think that is
worth doing, and I have not overlooked something, I could look into it
separately.
--
Mike Kravetz

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