Re: [PATCH v2 1/4] mm/hugetlb: fix the inconsistency of /proc/sys/vm/nr_huge_pages

[Xueshi Hu <xueshi.hu@smartx.com>]

On 8/7/23 23:15, David Hildenbrand wrote:
> On 06.08.23 09:48, Xueshi Hu wrote:
>> There are currently three 'nr_hugepages' used to export the number of 
>> huge
>> pages:
>> 1. /proc/sys/vm/nr_hugepages
>> 2. /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
>> 3. /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>>
>> For consistency, all three 'nr_hugepages' should return the total number
>> of huge pages. When written, the number of persistent huge pages will be
>> adjusted to the specified value.
>>
>> But, /proc/sys/vm/nr_hugepages returns the number of persistent huge
>> pages.
>>
> 
> But that's documented behavior, no?
> 
> Documentation/admin-guide/mm/hugetlbpage.rst
> 
> ``/proc/sys/vm/nr_hugepages`` indicates the current number of 
> "persistent" huge
> pages in the kernel's huge page pool.  "Persistent" huge pages will be
> returned to the huge page pool when freed by a task.  A user with root
> privileges can dynamically allocate more or free some persistent huge pages
> by increasing or decreasing the value of ``nr_hugepages``.
> 

Actually, Documentation/admin-guide/mm/hugetlbpage.rst is contradictory
about the definition of /proc/sys/vm/nr_hugepages.

The documentation says:

- ``/proc/sys/vm/nr_hugepages`` indicates the current number of
"persistent" huge.

But, the documentation also says:

- The ``/proc`` interfaces discussed above have been retained for
backwards compatibility.

- The ``nr_hugepages`` attribute returns the total number of huge pages on
the specified node.  When this attribute is written, the number of
persistent huge pages on the parent node will be adjusted to the specified
value, if sufficient resources exist, regardless of the task's mempolicy
or cpuset constraints.

So, I create the patch 4 to make the documentation more clear.

If such subtle inconsistencies result in unexpected behavior, it can be
challenging for a system administrator to detect.

Thanks,
Hu