Re: [PATCH] mm, hotplug: protect nr_zones with pgdat_resize_lock()

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Nov 26, 2018 at 09:16:08AM +0100, Michal Hocko wrote:
>On Mon 26-11-18 10:28:40, Wei Yang wrote:
>[...]
>> But I get some difficulty to understand this TODO. You want to get rid of
>> these lock? While these locks seem necessary to protect those data of
>> pgdat/zone. Would you mind sharing more on this statement?
>
>Why do we need this lock to be irqsave? Is there any caller that uses
>the lock from the IRQ context?

Went through the code, we have totally 9 place acquire
pgdat_resize_lock:

   lib/show_mem.c:         1    show_mem()
   mm/memory_hotplug.c:    4    online/offline_pages/__remove_zone()
   mm/page_alloc.c:        2    defer_init
   mm/sparse.c:            2    not necessary

Two places I am not sure:

   * show_mem() would be called from __alloc_pages_slowpath()
   * __remove_zone() is related to acpi_scan() on x86, may related to
     other method on different arch

I am not 100% for sure, while they looks like to be called in IRQ
context.

My ugly idea is:

   * drop pgdat_resize_lock in show_mem(), we don't change the value
     here. or replace this with a read/write lock?
   * can we adjust pgdat's range in offline_pages()? This would be
     consistent since we adjust them in online_pages().


>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me