Re: [PATCH v3 08/16] zsmalloc: squeeze freelist into page->mapping

Sergey Senozhatsky
2016-04-17T14:58:43Z

Hello,

On (03/30/16 16:12), Minchan Kim wrote:
[..]
> +static void objidx_to_page_and_offset(struct size_class *class,
> + struct page *first_page,
> + unsigned long obj_idx,
> + struct page **obj_page,
> + unsigned long *offset_in_page)
> {
> - unsigned long obj;
> + int i;
> + unsigned long offset;
> + struct page *cursor;
> + int nr_page;
>
> - if (!page) {
> - VM_BUG_ON(obj_idx);
> - return NULL;
> - }
> + offset = obj_idx * class->size;

so we already know the `offset' before we call objidx_to_page_and_offset(),
thus we can drop `struct size_class *class' and `obj_idx', and pass
`long obj_offset' (which is `obj_idx * class->size') instead, right?

we also _may be_ can return `cursor' from the function.

static struct page *objidx_to_page_and_offset(struct page *first_page,
unsigned long obj_offset,
unsigned long *offset_in_page);

this can save ~20 instructions, which is not so terrible for a hot path
like obj_malloc(). what do you think?

well, seems that `unsigned long *offset_in_page' can be calculated
outside of this function too, it's basically

*offset_in_page = (obj_idx * class->size) & ~PAGE_MASK;

so we don't need to supply it to this function, nor modify it there.
which can save ~40 instructions on my system. does this sound silly?

-ss

> + cursor = first_page;
> + nr_page = offset >> PAGE_SHIFT;
>
> - obj = page_to_pfn(page) << OBJ_INDEX_BITS;
> - obj |= ((obj_idx) & OBJ_INDEX_MASK);
> - obj <<= OBJ_TAG_BITS;
> + *offset_in_page = offset & ~PAGE_MASK;
> +
> + for (i = 0; i < nr_page; i++)
> + cursor = get_next_page(cursor);
>
> - return (void *)obj;
> + *obj_page = cursor;
> }

-ss