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

[Huang, Ying]

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!

Best Regards,
Huang, Ying