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

[David Hildenbrand <david@redhat.com>]

On 15.06.23 09:22, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> On 6/15/2023 11:22 AM, Huang, Ying wrote:
>>> Hi, Mel,
>>> Mel Gorman <mgorman@techsingularity.net> 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@linux.alibaba.com>
>>>>
>>>> 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@nvidia.com/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().

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

-- 
Cheers,

David / dhildenb