Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> If we call page_mapping(page) and PageSwapCache is true, we do this:
>
> if (unlikely(PageSwapCache(page))) {
> swp_entry_t entry;
>
> entry.val = page_private(page);
> return swap_address_space(entry);
> }
>
> #define swap_address_space(entry) \
> (&swapper_spaces[swp_type(entry)][swp_offset(entry) \
> >> SWAP_ADDRESS_SPACE_SHIFT])
>
> (i think we could make that more readable by adding
> #define swp_as(entry) (swp_offset(entry) >> SWAP_ADDRESS_SPACE_SHIFT)
> but i digress)
Yes. This sounds good.
> If, instead, we call page_file_mapping(page) and PageSwapCache is true,
> we do this:
>
> return page_swap_info(page)->swap_file->f_mapping;
>
> struct swap_info_struct *page_swap_info(struct page *page)
> {
> swp_entry_t entry = { .val = page_private(page) };
> return swp_swap_info(entry);
> }
>
> struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> {
> return swap_type_to_swap_info(swp_type(entry));
> }
>
> static struct swap_info_struct *swap_type_to_swap_info(int type)
> {
> if (type >= READ_ONCE(nr_swapfiles))
> return NULL;
>
> smp_rmb(); /* Pairs with smp_wmb in alloc_swap_info. */
> return READ_ONCE(swap_info[type]);
> }
>
> So ... are these different address spaces from each other?
Yes. They are different. The address space from page_mapping(page) is
used to hold pages in swap cache, they are not associated with some
inodes in fact. While the address space from page_file_mapping(page) is
used to read/write swap pages sometimes (please check
__swap_writepage()). But page_file_mapping() isn't called there.
Best Regards,
Huang, Ying
> If not, why do we have such a complicated way of finding the address
> space for page_file_mapping()?
