Re: [PATCH] mm/hugetlb: fix surplus pages in dissolve_free_huge_page()

David Hildenbrand
2025-03-03T11:47:58Z

On 03.03.25 12:23, Jinjiang Tu wrote:
>
> 在 2025/3/3 19:16, Jinjiang Tu 写道:
>>
>> 在 2025/3/3 18:25, David Hildenbrand 写道:
>>> On 03.03.25 07:09, Jinjiang Tu wrote:
>>>> In dissolve_free_huge_page(), free huge pages are dissolved without
>>>> adjusting surplus count. However, free huge pages may be accounted as
>>>> surplus pages, and will lead to wrong surplus count.
>>>>
>>>> Steps to reproduce:
>>>> 1) create 64 huge pages for node1
>>>> 2) run a program to reserve (don't consume) all the huge pages
>>>> 3) echo 0 > nr_huge_pages for node1. After this step, free huge
>>>> pages in
>>>> Node1 are surplus.
>>>> 4) create 80 huge pages for node0
>>>> 5) offline memory of node1 and kill the program in step 2)
>>>
>>> Can you elaborate the "offline memory" part? How much are you
>>> offlining (the complete node vs a single memory block?)
>> I reproduce this issue on qemu. The memory onlining and offlining
>> operations are:
>>
>> 1）Node1 is memory-less at first. Hot-add memory to node1 by executing
>> the two commands in qemu monitor
>> object_add memory-backend-ram,id=mem1,size=1G
>> device_add pc-dimm,id=dimm1,memdev=mem1,node=1
>>
>> 2）online one memory block of Node1 with：
>> echo online_movable > /sys/devices/system/node/node1/memoryX/state
>>
>> 3) offline the memory block of Node1 with：
>> echo offline > /sys/devices/system/node/node1/memoryX/state
> The memory range to offline contains the free surplus huge pages created
> in step1) ~ step3)

Okay, that makes it a lot clearer. Can you make that clearer in the
patch description?

The problem appears when memory offlining stumbles over a free hugetlb
folio. We'll call
dissolve_free_hugetlb_folios()->dissolve_free_hugetlb_folio() to
dissolve it, but don't adjust accounting with surplus pages properly.

I'll note that there is another dissolve_free_hugetlb_folio() caller in
the memory-failure path that might similarly be affected.

--
Cheers,

David / dhildenb