On Tue, May 01, 2018 at 11:48:53AM -0500, Christopher Lameter wrote:
> On Mon, 30 Apr 2018, Matthew Wilcox wrote:
>
> > Use page->private instead, now that these two fields are in the same
> > location. Include a compile-time assert that the fields don't get out
> > of sync.
>
> Hrm. This makes the source code a bit less readable. Guess its ok.
>
> Acked-by: Christoph Lameter <cl@xxxxxxxxx>
Thanks for the ACK. I'm not thrilled with this particular patch, but
I'm not thrilled with any of the other options we've come up with either.
Option 1:
Patch as written.
Pro: Keeps struct page simple
Con: Hidden dependency on page->private and page->inuse being in the same bits
Option 2:
@@ -113,9 +113,14 @@ struct page {
struct kmem_cache *slub_cache; /* shared with slab */
/* Double-word boundary */
void *slub_freelist; /* shared with slab */
- unsigned inuse:16;
- unsigned objects:15;
- unsigned frozen:1;
+ union {
+ unsigned long counters;
+ struct {
+ unsigned inuse:16;
+ unsigned objects:15;
+ unsigned frozen:1;
+ };
+ };
};
struct { /* Tail pages of compound page */
unsigned long compound_head; /* Bit zero is set */
Pro: Expresses exactly what we do.
Con: Back to five levels of indentation in struct page
Option 3: Use -fms-extensions to create a slub_page structure.
Pro: Indentation reduced to minimum and no cross-union dependencies
Con: Nobody seemed interested in the idea
Option 4: Use explicit shifting-and-masking to combine the three counters
into one word.
Con: Lots of churn.
