Page 259 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
few_vnode vnode of the file system swap directory (/paging) under
which the swap files are created.
fsw_bsize Block size used on this file system; used to determine
how much space fsw_reserve is reserving.
fsw_head Index into swaptab [ ] of first, last entry
fsw_tail associated with this file system swap.

fsw_mntpoint File system mount point; character representation of
fsw_vnode, used for utilities (such as swapinfo (1M)
and error messages.

7.6.7.2 Swaptab and Swapmap Structures
Two structures track swap space. The swaptab[ ] array tracks a chunk of swap space.
swapmap entries hold swap information on a per-page level. swaptab defaults to track a
2MB chunk of space and swapmap tracks each page within that 2MB chunk. Each entry
in the swaptab[ ] array has a pointer (called st_swpmp) to a unique swapmap. swapmap
entries have backwards pointers to the swaptab index. There is one entry in the swapmap for
each page represented by the swaptab entry (default 2 MB, or 512 pages); that is, swapmap
conforms in size to swchunk. A linked list of free swap pages begin at the swaptab entry’s
st_free and use each free swapmap entry’s sm_next. When a page of swap is needed, the
kernel walks the structures (using the getswap( ) routine in vm_swalloc.c), which calls other
routines that actually locate the chunk, and so forth.
• Beginning with the lowest priority, we begin by examining swdev_pri[].curr, which points
to a swdevt entry.
• If sw_nfpgs is zero (no free pages), we follow the pointer sw_next to get the next swdevt
entry at this priority.
• If none of these have free pages, we move on to swfs pri[].curr, the file system swap at
this priority, checking fsw_nfpgs for free pages.
If we are still unsuccessful, we move to the next priority and try again.

• Once we find a swdevt or fswdevt with free pages, we walk that device’s swaptab list,
starting with sw head or fsw head, and using st next in each swaptab entry, until we find
a swaptab entry with non-zero st nfpgs.
• st free points to the first free swapmap entry (and thus first free page) in this swaptab
chunk.

• The swalloc( ) routine creates a disk block descriptor (dbd) using 14 bits of dbd_data
for the swaptab index and 14 bits for the swapmap index. The r_bstore in the region is
set to the disk device vnode or the file system directory vnode, and the dbd is marked
DBD_BSTORE.
When faulting in from swap, the same process is followed as for faulting in from the file system:
r_bstore and dbd_data are hashed together and checked for a soft fault, then devswap_pagein( )
is called. The devswap_pagein( ) routine uses the dbd_data as a 14-bit swaptab index and a 14-
bit swapmap index to determine the location of the page on disk. Now all information needed
to retrieve the page from swap has been stored.

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