Adds some explaination on how the vmemmap based struct page layout's physical mapping is allocated and tracked through linked list. It also keeps note of a possible race condition.
Signed-off-by: Anshuman Khandual <khand...@linux.vnet.ibm.com> --- Previous discussions on this http://patchwork.ozlabs.org/patch/584110/ Michael Ellerman had agreed to take the comments alone. arch/powerpc/mm/init_64.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c index 2a15986..6e5c54d 100644 --- a/arch/powerpc/mm/init_64.c +++ b/arch/powerpc/mm/init_64.c @@ -118,8 +118,28 @@ static int __meminit vmemmap_populated(unsigned long start, int page_size) return 0; } +/* + * vmemmap virtual address space management does not have a traditonal page + * table to track which virtual struct pages are backed by physical mapping. + * The virtual to physical mappings are tracked in a simple linked list + * format. 'vmemmap_list' maintains the entire vmemmap physical mapping at + * all times where as the 'next' list maintains the available + * vmemmap_backing structures which have been deleted from the + * 'vmemmap_global' list during system runtime (memory hotplug remove + * operation). The freed 'vmemmap_backing' structures are reused later when + * new requests come in without allocating fresh memory. This pointer also + * tracks the allocated 'vmemmap_backing' structures as we allocate one + * full page memory at a time when we dont have any. + */ struct vmemmap_backing *vmemmap_list; static struct vmemmap_backing *next; + +/* The same pointer 'next' tracks individual chunks inside the allocated + * full page during the boot time and again tracks the freeed nodes during + * runtime. It is racy but it does not happen as they are separated by the + * boot process. Will create problem if some how we have memory hotplug + * operation during boot !! + */ static int num_left; static int num_freed; -- 1.9.3
