Documentation for userland software suspend interface
(C) 2006 Rafael J. Wysocki <firstname.lastname@example.org>
First, the warnings at the beginning of swsusp.txt still apply.
Second, you should read the FAQ in swsusp.txt _now_ if you have not
done it already.
Now, to use the userland interface for software suspend you need special
utilities that will read/write the system memory snapshot from/to the
kernel. Such utilities are available, for example, from
<http://www.sisk.pl/kernel/utilities/suspend>>. You may want to have
a look at them if you are going to develop your own suspend/resume
The interface consists of a character device providing the open(),
release(), read(), and write() operations as well as several ioctl()
commands defined in kernel/power/power.h. The major and minor
numbers of the device are, respectively, 10 and 231, and they can
be read from /sys/class/misc/snapshot/dev.
The device can be open either for reading or for writing. If open for
reading, it is considered to be in the suspend mode. Otherwise it is
assumed to be in the resume mode. The device cannot be open for reading
and writing. It is also impossible to have the device open more than once
at a time.
The ioctl() commands recognized by the device are:
SNAPSHOT_FREEZE - freeze user space processes (the current process is
not frozen); this is required for SNAPSHOT_ATOMIC_SNAPSHOT
and SNAPSHOT_ATOMIC_RESTORE to succeed
SNAPSHOT_UNFREEZE - thaw user space processes frozen by SNAPSHOT_FREEZE
SNAPSHOT_ATOMIC_SNAPSHOT - create a snapshot of the system memory; the
last argument of ioctl() should be a pointer to an int variable,
the value of which will indicate whether the call returned after
creating the snapshot (1) or after restoring the system memory state
from it (0) (after resume the system finds itself finishing the
SNAPSHOT_ATOMIC_SNAPSHOT ioctl() again); after the snapshot
has been created the read() operation can be used to transfer
it out of the kernel
SNAPSHOT_ATOMIC_RESTORE - restore the system memory state from the
uploaded snapshot image; before calling it you should transfer
the system memory snapshot back to the kernel using the write()
operation; this call will not succeed if the snapshot
image is not available to the kernel
SNAPSHOT_FREE - free memory allocated for the snapshot image
SNAPSHOT_SET_IMAGE_SIZE - set the preferred maximum size of the image
(the kernel will do its best to ensure the image size will not exceed
this number, but if it turns out to be impossible, the kernel will
create the smallest image possible)
SNAPSHOT_AVAIL_SWAP - return the amount of available swap in bytes (the last
argument should be a pointer to an unsigned int variable that will
contain the result if the call is successful).
SNAPSHOT_GET_SWAP_PAGE - allocate a swap page from the resume partition
(the last argument should be a pointer to a loff_t variable that
will contain the swap page offset if the call is successful)
SNAPSHOT_FREE_SWAP_PAGES - free all swap pages allocated with
SNAPSHOT_SET_SWAP_FILE - set the resume partition (the last ioctl() argument
should specify the device's major and minor numbers in the old
two-byte format, as returned by the stat() function in the .st_rdev
member of the stat structure); it is recommended to always use this
call, because the code to set the resume partition could be removed from
The device's read() operation can be used to transfer the snapshot image from
the kernel. It has the following limitations:
- you cannot read() more than one virtual memory page at a time
- read()s accross page boundaries are impossible (ie. if ypu read() 1/2 of
a page in the previous call, you will only be able to read()
_at_ _most_ 1/2 of the page in the next call)
The device's write() operation is used for uploading the system memory snapshot
into the kernel. It has the same limitations as the read() operation.
The release() operation frees all memory allocated for the snapshot image
and all swap pages allocated with SNAPSHOT_GET_SWAP_PAGE (if any).
Thus it is not necessary to use either SNAPSHOT_FREE or
SNAPSHOT_FREE_SWAP_PAGES before closing the device (in fact it will also
unfreeze user space processes frozen by SNAPSHOT_UNFREEZE if they are
still frozen when the device is being closed).
Currently it is assumed that the userland utilities reading/writing the
snapshot image from/to the kernel will use a swap parition, called the resume
partition, as storage space. However, this is not really required, as they
can use, for example, a special (blank) suspend partition or a file on a partition
that is unmounted before SNAPSHOT_ATOMIC_SNAPSHOT and mounted afterwards.
These utilities SHOULD NOT make any assumptions regarding the ordering of
data within the snapshot image, except for the image header that MAY be
assumed to start with an swsusp_info structure, as specified in
kernel/power/power.h. This structure MAY be used by the userland utilities
to obtain some information about the snapshot image, such as the size
of the snapshot image, including the metadata and the header itself,
contained in the .size member of swsusp_info.
The snapshot image MUST be written to the kernel unaltered (ie. all of the image
data, metadata and header MUST be written in _exactly_ the same amount, form
and order in which they have been read). Otherwise, the behavior of the
resumed system may be totally unpredictable.
While executing SNAPSHOT_ATOMIC_RESTORE the kernel checks if the
structure of the snapshot image is consistent with the information stored
in the image header. If any inconsistencies are detected,
SNAPSHOT_ATOMIC_RESTORE will not succeed. Still, this is not a fool-proof
mechanism and the userland utilities using the interface SHOULD use additional
means, such as checksums, to ensure the integrity of the snapshot image.
The suspending and resuming utilities MUST lock themselves in memory,
preferrably using mlockall(), before calling SNAPSHOT_FREEZE.
The suspending utility MUST check the value stored by SNAPSHOT_ATOMIC_SNAPSHOT
in the memory location pointed to by the last argument of ioctl() and proceed
in accordance with it:
1. If the value is 1 (ie. the system memory snapshot has just been
created and the system is ready for saving it):
(a) The suspending utility MUST NOT close the snapshot device
_unless_ the whole suspend procedure is to be cancelled, in
which case, if the snapshot image has already been saved, the
suspending utility SHOULD destroy it, preferrably by zapping
its header. If the suspend is not to be cancelled, the
system MUST be powered off or rebooted after the snapshot
image has been saved.
(b) The suspending utility SHOULD NOT attempt to perform any
file system operations (including reads) on the file systems
that were mounted before SNAPSHOT_ATOMIC_SNAPSHOT has been
called. However, it MAY mount a file system that was not
mounted at that time and perform some operations on it (eg.
use it for saving the image).
2. If the value is 0 (ie. the system state has just been restored from
the snapshot image), the suspending utility MUST close the snapshot
device. Afterwards it will be treated as a regular userland process,
so it need not exit.
The resuming utility SHOULD NOT attempt to mount any file systems that could
be mounted before suspend and SHOULD NOT attempt to perform any operations
involving such file systems.
For details, please refer to the source code.
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