On Mon, Mar 29, 2010 at 03:30:37PM -0700, Dave Hansen wrote:
> Don't forget that include/linux/memory_hotplug.h uses mapcount a bit for
> marking bootmem. So, just for clarity, we'd probably want to use -5 or
> something.
>
> /*
> * Types for free bootmem.
> * The normal smallest mapcount is -1. Here is smaller value than it.
> */
> #define SECTION_INFO (-1 - 1)
> #define MIX_SECTION_INFO (-1 - 2)
> #define NODE_INFO (-1 - 3)
So this is the memory holding the struct page and pgdat info that is
released when the memory is hot-removed? Why isn't
register_page_bootmem_info_node up to get_page_bootmem all let it go
in the __init section together with their only caller?
what is the reader of that type field? is it only put_page_bootmem?
Just for this BUG_ON?
BUG_ON(type >= -1);
and what is this about?
if (atomic_dec_return(&page->_count) == 1) {
How can this every return 0?
Yes I can use -5 no problem, that's no big deal but I don't get how
this _mapcount type info is used and why. Well the BUG_ON above is
obvious but I wonder if it's just for a BUG_ON. If it's just for a
BUG_ON can we just move the layering violation to page->lru.next and
leave mapcount -2 for PageBuddy?
> Looks like SLUB also uses _mapcount for some fun purposes:
>
> struct page {
> unsigned long flags; /* Atomic flags, some possibly
> * updated asynchronously */
> atomic_t _count; /* Usage count, see below. */
> union {
> atomic_t _mapcount; /* Count of ptes mapped in mms,
> * to show when page is mapped
> * & limit reverse map searches.
> */
> struct { /* SLUB */
> u16 inuse;
> u16 objects;
> };
> };
>
> I guess those don't *really* become a problem in practice until we get a
> really large page size that can hold >=64k objects. But, at that point,
> we're overflowing the types anyway (or really close to it).
Maybe we should add a BUG_ON in slub in case anybody runs this on
PAGE_SIZE == 2M (to avoid silent corruption).
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