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SubjectRe: [PATCH] mm: migrate: handle freed page at the first place
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On 11/11/19 3:18 PM, Andrew Morton wrote:
> On Tue, 12 Nov 2019 06:09:25 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> When doing migration if the freed page is met, we just return without
>> migrating it since it is pointless to migrate a freed page. But, the
>> current code did two things before handling freed page:
>>
>> 1. Return -ENOMEM if the page is THP and THP migration is not supported.
>> 2. Allocate target page unconditionally.
>>
>> Both makes not too much sense. If we handle freed page at the first place
>> we don't have to worry about allocating/freeing target page and split
>> THP at all.
>>
>> For example (worst case) if we are trying to migrate a freed THP without
>> THP migration supported, the migrate_pages() would just split the THP then
>> retry to migrate base pages one by one by pointless allocating and freeing
>> pages, this is just waste of time.
>>
>> I didn't run into any actual problem with the current code (or I may
>> just not notice it yet), it was found by visual inspection.
>>
>>
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>> int rc = MIGRATEPAGE_SUCCESS;
>> struct page *newpage;
>>
>> - if (!thp_migration_supported() && PageTransHuge(page))
>> - return -ENOMEM;
>> -
>> - newpage = get_new_page(page, private);
>> - if (!newpage)
>> - return -ENOMEM;
>> -
>> if (page_count(page) == 1) {
> Is it possible to have (!thp_migration_supported() &&
> PageTransHuge(page) && page_count(page) == 1)? If so, isn't this new
> behviour?

IMHO it should be possible on some architectures, i.e. aarch64, with
anonymous THP. I just saw PowerPC and x86_64 have
CONFIG_ARCH_ENABLE_THP_MIGRATION selected. I'm not quite sure if I miss
something.

It should be not new behavior since migrate_pages() should just split
the THP then retry with base pages one by one. Even though it returns
-EBUSY due to THP split failure in the current code, the behavior sounds
problematic. We should not return errno for a freed page, right?

>
>> /* page was freed from under us. So we are done. */
>> ClearPageActive(page);
>> @@ -1187,13 +1180,16 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>> __ClearPageIsolated(page);
>> unlock_page(page);
>> }
>> - if (put_new_page)
>> - put_new_page(newpage, private);
>> - else
>> - put_page(newpage);
>> goto out;
>> }
>>
>> + if (!thp_migration_supported() && PageTransHuge(page))
>> + return -ENOMEM;
>> +
>> + newpage = get_new_page(page, private);
>> + if (!newpage)
>> + return -ENOMEM;
>> +
>> rc = __unmap_and_move(page, newpage, force, mode);
>> if (rc == MIGRATEPAGE_SUCCESS)
>> set_page_owner_migrate_reason(newpage, reason);

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