Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

[Huang, Ying]

David Hildenbrand <david@xxxxxxxxxx> writes:

> On 15.06.23 09:22, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>> 
>>> On 6/15/2023 11:22 AM, Huang, Ying wrote:
>>>> Hi, Mel,
>>>> Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:
>>>>
>>>>> On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote:
>>>>>> On some machines, the normal zone can have a large memory hole like
>>>>>> below memory layout, and we can see the range from 0x100000000 to
>>>>>> 0x1800000000 is a hole. So when isolating some migratable pages, the
>>>>>> scanner can meet the hole and it will take more time to skip the large
>>>>>> hole. From my measurement, I can see the isolation scanner will take
>>>>>> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].
>>>>>>
>>>>>> So adding a new helper to fast search next online memory section
>>>>>> to skip the large hole can help to find next suitable pageblock
>>>>>> efficiently. With this patch, I can see the large hole scanning only
>>>>>> takes < 1us.
>>>>>>
>>>>>> [    0.000000] Zone ranges:
>>>>>> [    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
>>>>>> [    0.000000]   DMA32    empty
>>>>>> [    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
>>>>>> [    0.000000] Movable zone start for each node
>>>>>> [    0.000000] Early memory node ranges
>>>>>> [    0.000000]   node   0: [mem 0x0000000040000000-0x0000000fffffffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
>>>>>> [    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]
>>>>>>
>>>>>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>>>>
>>>>> This may only be necessary for non-contiguous zones so a check for
>>>>> zone_contiguous could be made but I suspect the saving, if any, would be
>>>>> marginal.
>>>>>
>>>>> However, it's subtle that block_end_pfn can end up in an arbirary location
>>>>> past the end of the zone or past cc->free_pfn. As the "continue" will update
>>>>> cc->migrate_pfn, that might lead to errors in the future. It would be a
>>>>> lot safer to pass in cc->free_pfn and do two things with the value. First,
>>>>> there is no point scanning for a valid online section past cc->free_pfn so
>>>>> terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn
>>>>> does not end up with an arbitrary value which is a more defensive approach
>>>>> to any future programming errors.
>>>> I have thought about this before.  Originally, I had thought that we
>>>> were safe because cc->free_pfn should be in a online section and
>>>> block_end_pfn should reach cc->free_pfn before the end of zone.  But
>>>> after checking more code and thinking about it again, I found that the
>>>> underlying sections may go offline under us during compaction.  So that,
>>>> cc->free_pfn may be in a offline section or after the end of zone.  So,
>>>> you are right, we need to consider the range of block_end_pfn.
>>>> But, if we thought in this way (memory online/offline at any time),
>>>> it
>>>> appears that we need to check whether the underlying section was
>>>> offlined.  For example, is it safe to use "pfn_to_page()" in
>>>> "isolate_migratepages_block()"?  Is it possible for the underlying
>>>> section to be offlined under us?
>>>
>>> It is possible. There is a previous discussion[1] about the race
>>> between pfn_to_online_page() and memory offline.
>>>
>>> [1]
>>> https://lore.kernel.org/lkml/87zgc6buoq.fsf@xxxxxxxxxx/T/#m642d91bcc726437e1848b295bc57ce249c7ca399
>> Thank you very much for sharing!  That answers my questions
>> directly!
>
> I remember another discussion (but can't find it) regarding why memory
> compaction can get away without pfn_to_online_page() all over the
> place. The use is limited to __reset_isolation_pfn().

Per my understanding, isolate_migratepages() -> pageblock_pfn_to_page()
will check whether the pageblock is online.  So if the pageblock isn't
offlined afterwards, we can use pfn_to_page().

> But yes, in theory pfn_to_online_page() can race with memory offlining.

Thanks for confirmation.

Best Regards,
Huang, Ying