swapon: allow a more flexible swap discard policy

Introduce the necessary changes to swapon(8) allowing a sysadmin to leverage
the new changes introduced to sys_swapon by "swap: discard while swapping
only if SWAP_FLAG_DISCARD_PAGES", therefore allowing a more flexible set of
choices when selection the discard policy for mounted swap areas.
This patch introduces the following optional arguments to the already
existent swapon(8) "--discard" option, in order to allow a discard type to
be selected at swapon time:
 * once    : only single-time area discards are issued. (swapon)
 * pages   : discard freed pages before they are reused.
If no policy is selected both discard types are enabled. (default)

[kzak@redhat.com: - support <policy> argument for short -d option too,
                  - add errx() on unsupported policy name]

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 17 insertions, 7 deletions

diff --git a/sys-utils/swapon.8 b/sys-utils/swapon.8
index 385bf5aea..836b17277 100644
--- a/sys-utils/swapon.8
+++ b/sys-utils/swapon.8
@@ -112,15 +112,25 @@ All devices marked as ``swap'' in
 are made available, except for those with the ``noauto'' option.
 Devices that are already being used as swap are silently skipped.
 .TP
-.B "\-d, \-\-discard"
-Discard freed swap pages before they are reused, if the swap
-device supports the discard or trim operation.  This may improve
-performance on some Solid State Devices, but often it does not.
+.B "\-d, \-\-discard\fR [=\fIpolicy\fR]"
+Enable swap discards, if the swap backing device supports the discard or
+trim operation. This may improve performance on some Solid State Devices,
+but often it does not. The option allows one to select between two
+available swap discard policies:
+.BI \-\-discard=once
+to perform a single-time discard operation for the whole swap area at swapon;
+or
+.BI \-\-discard=pages
+to discard freed swap pages before they are reused, while swapping.
+If no policy is selected, the default behavior is to enable both discard types.
 The
 .I /etc/fstab
-mount option
-.BI discard
-may be also used to enable discard flag.
+mount options
+.BI discard,
+.BI discard=once,
+or
+.BI discard=pages
+may be also used to enable discard flags.
 .TP
 .B "\-e, \-\-ifexists"
 Silently skip devices that do not exist.