From: Doug Berger opendmb@gmail.com
commit e08d3fdfe2dafa0331843f70ce1ff6c1c4900bf4 upstream.
The lowmem_reserve arrays provide a means of applying pressure against allocations from lower zones that were targeted at higher zones. Its values are a function of the number of pages managed by higher zones and are assigned by a call to the setup_per_zone_lowmem_reserve() function.
The function is initially called at boot time by the function init_per_zone_wmark_min() and may be called later by accesses of the /proc/sys/vm/lowmem_reserve_ratio sysctl file.
The function init_per_zone_wmark_min() was moved up from a module_init to a core_initcall to resolve a sequencing issue with khugepaged. Unfortunately this created a sequencing issue with CMA page accounting.
The CMA pages are added to the managed page count of a zone when cma_init_reserved_areas() is called at boot also as a core_initcall. This makes it uncertain whether the CMA pages will be added to the managed page counts of their zones before or after the call to init_per_zone_wmark_min() as it becomes dependent on link order. With the current link order the pages are added to the managed count after the lowmem_reserve arrays are initialized at boot.
This means the lowmem_reserve values at boot may be lower than the values used later if /proc/sys/vm/lowmem_reserve_ratio is accessed even if the ratio values are unchanged.
In many cases the difference is not significant, but for example an ARM platform with 1GB of memory and the following memory layout
cma: Reserved 256 MiB at 0x0000000030000000 Zone ranges: DMA [mem 0x0000000000000000-0x000000002fffffff] Normal empty HighMem [mem 0x0000000030000000-0x000000003fffffff]
would result in 0 lowmem_reserve for the DMA zone. This would allow userspace to deplete the DMA zone easily.
Funnily enough
$ cat /proc/sys/vm/lowmem_reserve_ratio
would fix up the situation because as a side effect it forces setup_per_zone_lowmem_reserve.
This commit breaks the link order dependency by invoking init_per_zone_wmark_min() as a postcore_initcall so that the CMA pages have the chance to be properly accounted in their zone(s) and allowing the lowmem_reserve arrays to receive consistent values.
Fixes: bc22af74f271 ("mm: update min_free_kbytes from khugepaged after core initialization") Signed-off-by: Doug Berger opendmb@gmail.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Acked-by: Michal Hocko mhocko@suse.com Cc: Jason Baron jbaron@akamai.com Cc: David Rientjes rientjes@google.com Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/1597423766-27849-1-git-send-email-opendmb@gmail.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/page_alloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6782,7 +6782,7 @@ int __meminit init_per_zone_wmark_min(vo
return 0; } -core_initcall(init_per_zone_wmark_min) +postcore_initcall(init_per_zone_wmark_min)
/* * min_free_kbytes_sysctl_handler - just a wrapper around proc_dointvec() so