Chapter 18: Page Reclaim and Swapping
A dedicated partition and two files are used to accommodate the swap areas in this example. The swap
partition has the highest priority and is therefore used preferentially by the kernel. Both files have
priority 0 and are used on the basis of a round robin process when no more space is available on the
partition with priority 0. (How it can nevertheless occur that there are data in the swap files although the
swap partition is not completely full, as indicated by theprocoutput, is explained below.)
What is the meaning of the various elements in theswap_info_structstructure? The first entries are
used to hold the classical management data required for swap areas:
❑ The state of the swap area can be described with various flags stored in theflagselement.
SWP_USEDspecifies that the entry in the swap array is used. Since the array is otherwise filled
with zeros, a distinction can easily be made between used and unused elements.SWP_WRITEOK
specifies that the swap area may be written to. Both flags are set after a swap area has been
inserted into the kernel; the abbreviation for this state isSWP_ACTIVE.
❑ swap_filepoints to thefilestructure associated with the swap area (the layout and contents of
the structure are discussed in Chapter 8). With swap partitions, there is a pointer to the device
file of the partition on the block device (in our example,/dev/hda5). With swap files, this pointer
is to thefileinstance of the relevant file, that is,/mnt/swap1or/tmp/swap2in our example.
❑ bdevpoints to theblock_devicestructure of the underlying block device.
Even if all swap areas in our example are located on the same block device (/dev/hda), all
three entries point to different instances of the data structure. This is because the two files are
on different partitions of the hard disk and the swap partition is a separate partition anyway.
Since, in structural terms, the kernel manages partitions essentially as if they were autonomous
block devices, this results in three different pointers to the three swap areas, although all are
located on the same disk.
❑ The relative priority of a swap area is held in theprioelement. Since this is a signed data type,
both positive and negative priorities are possible. As already noted, the higher a swap partition’s
priority is, the more important the swap partition is.
❑ The total number of usable page slots, each of which can store a complete memory page, is held
inpages. For example, the swap partition in our sample mapping has space for 34,128 pages,
which, given a page size of 4 KiB in the IA-32 system used for the mapping, corresponds to a
memory volume of≈128 MiB.
❑ maxyields the total number of page slots that the swap area contains. In contrast topages,not
just usable pages butallpages are counted here — including those that (owing to block device
faults, e.g.) are defective or are used for management purposes. Because defective blocks are
extremely rare on state-of-the-art hard disks (and swap partitions need not necessarily be created
in such an area),maxis typically only 1 greater thanpages, as is the case with all three swap areas
in the example above. There are two reasons for this one-page difference. First, the very first
page of a swap area is used by the kernel for identification purposes (after all, totally random
parts of the disk should not be overwritten with swap data). Second, the kernel also uses the first
slot to store state information, such as the size of the area and a list of defective section, and this
information must be permanently retained.
❑ swap_mapis a pointer to an array of short integers(which is unsurprisingly referred to asswap
mapin the following) that contains as many elements as there are page slots in the swap area.
It is used as an access counter for the individual slots to indicate the number of processes that
share the swapped-out pages.
❑ The kernel uses a somewhat unusual method to link the various elements in the swap list
according to priority. Since the data of the various areas are arranged in the elements of a linear
