On Tue, Apr 10, 2018 at 05:03:17PM -0500, Christopher Lameter wrote:
> On Tue, 10 Apr 2018, Matthew Wilcox wrote:
>
> > > Is this aligned on a doubleword boundary? Maybe move the refcount below
> > > the flags field?
> >
> > You need freelist and _mapcount to be in the same dword. There's no
> > space to put them both in dword 0, so that's used for flags and mapping
> > / s_mem. Then freelist, mapcount and refcount are in dword 1 (on 64-bit),
> > or freelist & mapcount are in dword 1 on 32-bit. After that, 32 and 64-bit
> > no longer line up on the same dword boundaries.
>
> Well its no longer clear from the definitions that this must be the case.
> Clarify that in the next version?
I had a Thought. And it seems to work:
struct page {
unsigned long flags;
union { /* Five words */
struct { /* Page cache & anonymous pages */
struct list_head lru;
unsigned long private;
struct address_space *mapping;
pgoff_t index;
};
struct { /* Slab / Slob / Slub */
struct page *next;
void *freelist;
union {
unsigned int active; /* slab */
struct { /* slub */
unsigned inuse:16;
unsigned objects:15;
unsigned frozen:1;
};
int units; /* slob */
};
#ifdef CONFIG_64BIT
int pages;
#endif
void *s_mem;
struct kmem_cache *slab_cache;
};
struct rcu_head rcu_head;
... tail pages, page tables, etc, etc ...
};
union {
atomic_t _mapcount;
unsigned int page_type;
#ifdef CONFIG_64BIT
unsigned int pobjects; /* slab */
#else
struct {
short int pages;
short int pobjects;
};
#endif
};
atomic_t _refcount;
struct mem_cgroup *mem_cgroup;
};
Now everybody gets 5 contiguous words to use as they want with the only
caveat that they can't use bit 0 of the first word (PageTail). It looks
a little messy to split up pages & pobjects like that -- as far as I
can see there's no reason we couldn't make them unsigned short on 64BIT?
pages is always <= pobjects, and pobjects is limited to 2^15.
