mm: page_alloc: fix missed updates of lowmem_reserve in adjust_managed_page_count

mm: page_alloc: fix missed updates of lowmem_reserve in adjust_managed_page_count

In the kernel, the zone's lowmem_reserve and _watermark, and the global
variable 'totalreserve_pages' depend on the value of managed_pages, but
after running adjust_managed_page_count, these values aren't updated,
which causes some problems.

For example, in a system with six 1GB large pages, we found that the value
of protection in zoneinfo (zone->lowmem_reserve), is not right.  Its value
seems to be calculated from the initial managed_pages, but after the
managed_pages changed, was not updated.  Only after reading the file
/proc/sys/vm/lowmem_reserve_ratio, updates happen.

read file /proc/sys/vm/lowmem_reserve_ratio:

lowmem_reserve_ratio_sysctl_handler
----setup_per_zone_lowmem_reserve
--------calculate_totalreserve_pages

protection changed after reading file:

[root@test ~]# cat /proc/zoneinfo | grep protection
        protection: (0, 2719, 57360, 0)
        protection: (0, 0, 54640, 0)
        protection: (0, 0, 0, 0)
        protection: (0, 0, 0, 0)
[root@test ~]# cat /proc/sys/vm/lowmem_reserve_ratio
256     256     32      0
[root@test ~]# cat /proc/zoneinfo | grep protection
        protection: (0, 2735, 63524, 0)
        protection: (0, 0, 60788, 0)
        protection: (0, 0, 0, 0)
        protection: (0, 0, 0, 0)

lowmem_reserve increased also makes the totalreserve_pages increased,
which causes a decrease in available memory.  The one above is just a test
machine, and the increase is not significant.  On our online machine, the
reserved memory will increase by several GB due to reading this file.  It
is clearly unreasonable to cause a sharp drop in available memory just by
reading a file.

In this patch, we update reserve memory when update managed_pages, The
size of reserved memory becomes stable.  But it seems that the _watermark
should also be updated along with the managed_pages.  We have not done it
because we are unsure if it is reasonable to set the watermark through the
initial managed_pages.  If it is not reasonable, we will propose new
patch.

Link: https://lkml.kernel.org/r/20241225021034.45693-1-15645113830zzh@gmail.com
Signed-off-by: zihan zhou <15645113830zzh@gmail.com>
Signed-off-by: yaowenchao <yaowenchao@jd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>