Page Reclaim IV (Swap Cache)

pages in a swap cache has the following things...

page->mapping = NULL ... why is that? afaik, it should point to the swap cache addr space object
PG_swapcache flag is on
page->private = swapped-out page id

a swap cache page linked to a page slot in swap area and some processe(s).

a swapper_space address space object is used for the swap cache for the whole system. question: how is a page found inside a swap cache? ans: using the swapped out page identifier

It is possible to know how many processes were sharing a swapped out page. Until everyone gets it in, the page is temporarily stored in the swap cache.

swap area has clustering of page slots (for efficiency during r/w: swap in-out)

Page Structure Finds

PG_swapcache: the page is in the swap cache

PG_locked: undergoing IO, in swap space update, in file r/w etc.

Page Reclaim I (Basics) - Swapping Subsystem (swapfile.c, swap_state.c)

3 kinds of pages handled:
1. anon mem pages (usr stack/heap)
2. dirty pages of private mem map (does not change the files on disk)
3. IPC shared pages

note that the dirty pages are not swapped out, they are just updated in disk and removed from RAM. because this has the same effect as swapping out.

swap area can be on a physical disk partition or a separate file inside a large partition. but these data is stored only as long as the system is on and discarded otherwise. so swap area has a very little control info (unlike the filesystem info)

extents and priority -- interesting

swap_info array contains all the swap areas (disk partition,bdev or files). they are kept in swap area descriptors.

swapped out page id: page slot index[8:31], area number[1:7], 0[0]

the difference between a blank pte and swapped pte is that the id for blank pte = 0...0