[Buildroot] swapoff, but no swapon in initab

[Buildroot] swapoff, but no swapon in initab

[Carlos Santos]

Hi,

I'm working on a product that needs a swap partition and noticed that
both package/{busybox,sysvinit}/inittab invoke "swapoff -a" in their
shutdown sequences but do not invoke "swapon -a". Is this intentional
or the swapon call is missing (e.g. right after "mount -a")?

BTW, what about systemd?

-- 
Carlos Santos (Casantos) - DATACOM, P&D
Rua Am?rica, 1000 - Eldorado do Sul, RS, Brasil - 92990-000
casantos at datacom.com.br          +55 51 3933.3000 ext. 3627
http://www.datacom.ind.br

[Buildroot] swapoff, but no swapon in initab

[Arnout Vandecappelle]

On 06-06-18 19:14, Carlos Santos wrote:
> Hi,
> 
> I'm working on a product that needs a swap partition and noticed that
> both package/{busybox,sysvinit}/inittab invoke "swapoff -a" in their
> shutdown sequences but do not invoke "swapon -a". Is this intentional
> or the swapon call is missing (e.g. right after "mount -a")?

 It has been like that since

commit ffde94bd2ca21542ae0b4518348f8e986b9d92e0 (tag: 0_0)
Author: Eric Andersen <andersen@codepoet.org>
Date:   Sat Dec 22 01:56:11 2001

    Initial revision

so whatever reason there was, it is lost in the mists of time.

 IMO I think it's pointless (the kernel will freeze swap accesses before
shutdown) and possibly wrong (swap to file should be disabled before the
corresponding filesystem is unmounted).


 Anyway, the proper way to handle swap is by adding it to fstab, no? I just
checked and busybox seems to support it.

 Regards,
 Arnout


> BTW, what about systemd?


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7493 020B C7E3 8618 8DEC 222C 82EB F404 F9AC 0DDF

[Buildroot] swapoff, but no swapon in initab

[Carlos Santos]

> From: "Arnout Vandecappelle" <arnout@mind.be>
> To: "DATACOM" <casantos@datacom.com.br>, "buildroot" <buildroot@buildroot.org>
> Sent: Thursday, June 7, 2018 6:12:48 PM
> Subject: Re: [Buildroot] swapoff, but no swapon in initab

> On 06-06-18 19:14, Carlos Santos wrote:
>> Hi,
>> 
>> I'm working on a product that needs a swap partition and noticed that
>> both package/{busybox,sysvinit}/inittab invoke "swapoff -a" in their
>> shutdown sequences but do not invoke "swapon -a". Is this intentional
>> or the swapon call is missing (e.g. right after "mount -a")?
> 
> It has been like that since
> 
> commit ffde94bd2ca21542ae0b4518348f8e986b9d92e0 (tag: 0_0)
> Author: Eric Andersen <andersen@codepoet.org>
> Date:   Sat Dec 22 01:56:11 2001
> 
>    Initial revision
> 
> so whatever reason there was, it is lost in the mists of time.
> 
> IMO I think it's pointless (the kernel will freeze swap accesses before
> shutdown) and possibly wrong (swap to file should be disabled before the
> corresponding filesystem is unmounted).
> 
> 
> Anyway, the proper way to handle swap is by adding it to fstab, no? I just
> checked and busybox seems to support it.

The point here is that I'd like to avoid adding yet another init script
just to call swap{on,off} in its st{art,op} phase. A "swapon -a" in
inittab would inspect /etc/fstab, as stated in the man page:

  "All devices marked as ``swap'' in /etc/fstab are made available,
   except for those with the ``noauto'' option. Devices that are
   already being used as swap are silently skipped."

So even if the system already has such init script (e.g. from a
rootfs ovelay) a second call would be harmless.

-- 
Carlos Santos (Casantos) - DATACOM, P&D
?Marched towards the enemy, spear upright, armed with the certainty
that only the ignorant can have.? ? Epitaph of a volunteer

[Buildroot] swapoff, but no swapon in initab

[Arnout Vandecappelle]

On 08-06-18 13:38, Carlos Santos wrote:
>> From: "Arnout Vandecappelle" <arnout@mind.be>
>> To: "DATACOM" <casantos@datacom.com.br>, "buildroot" <buildroot@buildroot.org>
>> Sent: Thursday, June 7, 2018 6:12:48 PM
>> Subject: Re: [Buildroot] swapoff, but no swapon in initab
> 
>> On 06-06-18 19:14, Carlos Santos wrote:
>>> Hi,
>>>
>>> I'm working on a product that needs a swap partition and noticed that
>>> both package/{busybox,sysvinit}/inittab invoke "swapoff -a" in their
>>> shutdown sequences but do not invoke "swapon -a". Is this intentional
>>> or the swapon call is missing (e.g. right after "mount -a")?
>>
>> It has been like that since
>>
>> commit ffde94bd2ca21542ae0b4518348f8e986b9d92e0 (tag: 0_0)
>> Author: Eric Andersen <andersen@codepoet.org>
>> Date:   Sat Dec 22 01:56:11 2001
>>
>>    Initial revision
>>
>> so whatever reason there was, it is lost in the mists of time.
>>
>> IMO I think it's pointless (the kernel will freeze swap accesses before
>> shutdown) and possibly wrong (swap to file should be disabled before the
>> corresponding filesystem is unmounted).
>>
>>
>> Anyway, the proper way to handle swap is by adding it to fstab, no? I just
>> checked and busybox seems to support it.
> 
> The point here is that I'd like to avoid adding yet another init script
> just to call swap{on,off} in its st{art,op} phase. A "swapon -a" in
> inittab would inspect /etc/fstab, as stated in the man page:

 Ah, yes, I thought mount -a would do that.

> 
>   "All devices marked as ``swap'' in /etc/fstab are made available,
>    except for those with the ``noauto'' option. Devices that are
>    already being used as swap are silently skipped."

 So bottom line: it would make sense to add a swapon -a to the default inittabs
(both busybox and sysv).

 For system, BTW, I would expect it would already do the right thing based on
fstab, no?

 Regards,
 Arnout

> 
> So even if the system already has such init script (e.g. from a
> rootfs ovelay) a second call would be harmless.
> 

-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7493 020B C7E3 8618 8DEC 222C 82EB F404 F9AC 0DDF

[Buildroot] swapoff, but no swapon in initab

[Peter Korsgaard]

>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:

Hi,

 >> "All devices marked as ``swap'' in /etc/fstab are made available,
 >> except for those with the ``noauto'' option. Devices that are
 >> already being used as swap are silently skipped."

 >  So bottom line: it would make sense to add a swapon -a to the default inittabs
 > (both busybox and sysv).

Don't we then end up with an ugly error message / somewhat needlessly bloat
everyones busybox config for the (very common) situation where swap is
not used and disabled in the kernel?

-- 
Bye, Peter Korsgaard

[Buildroot] swapoff, but no swapon in initab

[Carlos Santos]

> From: "Peter Korsgaard" <peter@korsgaard.com>
> To: "Arnout Vandecappelle" <arnout@mind.be>
> Cc: "DATACOM" <casantos@datacom.com.br>, "buildroot" <buildroot@buildroot.org>
> Sent: Tuesday, June 12, 2018 6:06:13 AM
> Subject: Re: swapoff, but no swapon in initab

>>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:
> 
> Hi,
> 
> >> "All devices marked as ``swap'' in /etc/fstab are made available,
> >> except for those with the ``noauto'' option. Devices that are
> >> already being used as swap are silently skipped."
> 
> >  So bottom line: it would make sense to add a swapon -a to the default inittabs
> > (both busybox and sysv).
> 
> Don't we then end up with an ugly error message / somewhat needlessly bloat
> everyones busybox config for the (very common) situation where swap is
> not used and disabled in the kernel?

A "swapon -a" does nothing if a swap entry is not found in /etc/fstab,
so there is no error message. Conversely, a "swapoff -a" does nothing
if a mounted swap is not found but I did not test if it issues some
error message due to a failing system call when swap is disabled in
the kernel. I will run some tests and submit an additional patch, if
necessary.

-- 
Carlos Santos (Casantos) - DATACOM, P&D
?Marched towards the enemy, spear upright, armed with the certainty
that only the ignorant can have.? ? Epitaph of a volunteer

[Buildroot] swapoff, but no swapon in initab

[Peter Korsgaard]

>>>>> "Carlos" == Carlos Santos <casantos@datacom.com.br> writes:

Hi,

 >> >  So bottom line: it would make sense to add a swapon -a to the default inittabs
 >> > (both busybox and sysv).
 >> 
 >> Don't we then end up with an ugly error message / somewhat needlessly bloat
 >> everyones busybox config for the (very common) situation where swap is
 >> not used and disabled in the kernel?

 > A "swapon -a" does nothing if a swap entry is not found in /etc/fstab,
 > so there is no error message. Conversely, a "swapoff -a" does nothing
 > if a mounted swap is not found but I did not test if it issues some
 > error message due to a failing system call when swap is disabled in
 > the kernel. I will run some tests and submit an additional patch, if
 > necessary.

It does not:

grep CONFIG_SWAP .config
# CONFIG_SWAP is not set

# swapon -a ; echo $?
0

But it does require people to add swapon/swapoff to their busybox config
even if they aren't using swap, otherwise they would get error messages
at bootup/shutdown (I know they already get it at shutdown, but an
orderly shutdown is a lot more rare than bootup).

But ok, the swapon/swapoff applet is very small, so I guess that is OK.

size util-linux/swaponoff.o
   text    data     bss     dec     hex filename
    597       0       0     597     255 util-linux/swaponoff.o

-- 
Bye, Peter Korsgaard

[Buildroot] swapoff, but no swapon in initab

[Arnout Vandecappelle]

On 12-06-18 14:55, Peter Korsgaard wrote:
>>>>>> "Carlos" == Carlos Santos <casantos@datacom.com.br> writes:
> 
> Hi,
> 
>  >> >  So bottom line: it would make sense to add a swapon -a to the default inittabs
>  >> > (both busybox and sysv).
>  >> 
>  >> Don't we then end up with an ugly error message / somewhat needlessly bloat
>  >> everyones busybox config for the (very common) situation where swap is
>  >> not used and disabled in the kernel?
> 
>  > A "swapon -a" does nothing if a swap entry is not found in /etc/fstab,
>  > so there is no error message. Conversely, a "swapoff -a" does nothing
>  > if a mounted swap is not found but I did not test if it issues some
>  > error message due to a failing system call when swap is disabled in
>  > the kernel. I will run some tests and submit an additional patch, if
>  > necessary.
> 
> It does not:
> 
> grep CONFIG_SWAP .config
> # CONFIG_SWAP is not set
> 
> # swapon -a ; echo $?
> 0
> 
> But it does require people to add swapon/swapoff to their busybox config
> even if they aren't using swap, otherwise they would get error messages

 I think if people start customizing the busybox config and remove stuff from
it, then it's only reasonable that they also modify the default inittab. Swapon
and swapoff are both enabled in our busybox.config and busybox-minimal.config.

 Regards,
 Arnout

> at bootup/shutdown (I know they already get it at shutdown, but an
> orderly shutdown is a lot more rare than bootup).
> 
> But ok, the swapon/swapoff applet is very small, so I guess that is OK.
> 
> size util-linux/swaponoff.o
>    text    data     bss     dec     hex filename
>     597       0       0     597     255 util-linux/swaponoff.o
> 

-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7493 020B C7E3 8618 8DEC 222C 82EB F404 F9AC 0DDF

[Buildroot] swapoff, but no swapon in initab

[Peter Korsgaard]

>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:

 >> But it does require people to add swapon/swapoff to their busybox config
 >> even if they aren't using swap, otherwise they would get error messages

 >  I think if people start customizing the busybox config and remove stuff from
 > it, then it's only reasonable that they also modify the default
 > inittab.

Correct.

 > Swaponand swapoff are both enabled in our busybox.config and busybox-minimal.config.

Hmm, does swap work on nommu? I would guess not. Looking at the kernel
option:

git grep -A3 'config SWAP' init/Kconfig
init/Kconfig:config SWAP
init/Kconfig-   bool "Support for paging of anonymous memory (swap)"
init/Kconfig-   depends on MMU && BLOCK
init/Kconfig-   default y

-- 
Bye, Peter Korsgaard

[Buildroot] swapoff, but no swapon in initab

[Arnout Vandecappelle]

On 12-06-18 16:50, Peter Korsgaard wrote:
>>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:
> 
>  >> But it does require people to add swapon/swapoff to their busybox config
>  >> even if they aren't using swap, otherwise they would get error messages
> 
>  >  I think if people start customizing the busybox config and remove stuff from
>  > it, then it's only reasonable that they also modify the default
>  > inittab.
> 
> Correct.
> 
>  > Swaponand swapoff are both enabled in our busybox.config and busybox-minimal.config.
> 
> Hmm, does swap work on nommu? I would guess not. Looking at the kernel
> option:

 Doesn't matter if it works or not; if the swapon executable exists, then the
swapon in inittab will pass without error (if you don't have swap configured in
fstab). Clearly, the swapon will fail if you indeed have swap enabled in fstab
on a NOMMU system, but I don't think we have to protect against that situation,
right? It will anyway also fail if you don't have swap enabled in the kernel.
Just like mounting the ext2 rootfs will fail if you don't have ext2/3/4 enabled
in your kernel...

 Regards,
 Arnout

> 
> git grep -A3 'config SWAP' init/Kconfig
> init/Kconfig:config SWAP
> init/Kconfig-   bool "Support for paging of anonymous memory (swap)"
> init/Kconfig-   depends on MMU && BLOCK
> init/Kconfig-   default y
> 

-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7493 020B C7E3 8618 8DEC 222C 82EB F404 F9AC 0DDF

[Buildroot] swapoff, but no swapon in initab

[Peter Korsgaard]

>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:

 > On 12-06-18 16:50, Peter Korsgaard wrote:
 >>>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:
 >> 
 >> >> But it does require people to add swapon/swapoff to their busybox config
 >> >> even if they aren't using swap, otherwise they would get error messages
 >> 
 >> >  I think if people start customizing the busybox config and remove stuff from
 >> > it, then it's only reasonable that they also modify the default
 >> > inittab.
 >> 
 >> Correct.
 >> 
 >> > Swaponand swapoff are both enabled in our busybox.config and busybox-minimal.config.
 >> 
 >> Hmm, does swap work on nommu? I would guess not. Looking at the kernel
 >> option:

 >  Doesn't matter if it works or not; if the swapon executable exists, then the
 > swapon in inittab will pass without error (if you don't have swap configured in
 > fstab). Clearly, the swapon will fail if you indeed have swap enabled in fstab
 > on a NOMMU system, but I don't think we have to protect against that situation,
 > right? It will anyway also fail if you don't have swap enabled in the kernel.
 > Just like mounting the ext2 rootfs will fail if you don't have ext2/3/4 enabled
 > in your kernel...

Yes, so the only minor issue is that we need to have swapon/swapoff
enabled in busybox-minimal.config even though it isn't useful just to
not get an error message during bootup/shutdown.

-- 
Bye, Peter Korsgaard

[Buildroot] swapoff, but no swapon in initab

[Arnout Vandecappelle]

On 20-06-18 10:42, Peter Korsgaard wrote:
>>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:
> 
>  > On 12-06-18 16:50, Peter Korsgaard wrote:
>  >>>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:
>  >> 
>  >> >> But it does require people to add swapon/swapoff to their busybox config
>  >> >> even if they aren't using swap, otherwise they would get error messages
>  >> 
>  >> >  I think if people start customizing the busybox config and remove stuff from
>  >> > it, then it's only reasonable that they also modify the default
>  >> > inittab.
>  >> 
>  >> Correct.
>  >> 
>  >> > Swaponand swapoff are both enabled in our busybox.config and busybox-minimal.config.
>  >> 
>  >> Hmm, does swap work on nommu? I would guess not. Looking at the kernel
>  >> option:
> 
>  >  Doesn't matter if it works or not; if the swapon executable exists, then the
>  > swapon in inittab will pass without error (if you don't have swap configured in
>  > fstab). Clearly, the swapon will fail if you indeed have swap enabled in fstab
>  > on a NOMMU system, but I don't think we have to protect against that situation,
>  > right? It will anyway also fail if you don't have swap enabled in the kernel.
>  > Just like mounting the ext2 rootfs will fail if you don't have ext2/3/4 enabled
>  > in your kernel...
> 
> Yes, so the only minor issue is that we need to have swapon/swapoff
> enabled in busybox-minimal.config even though it isn't useful just to
> not get an error message during bootup/shutdown.

 Well, it currently _is_ enabled in busybox-minimal.config... Admittedly, this
adds almost 1KB to the busybox size so perhaps it should be removed from
-minimal. But the we probably should also remove swapoff from inittab. Hell,
perhaps we should do that anyway, since doing swapoff for reboot is utterly
useless IMO.

 Regards,
 Arnout


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7493 020B C7E3 8618 8DEC 222C 82EB F404 F9AC 0DDF

[Buildroot] swapoff, but no swapon in initab

[Peter Korsgaard]

>>>>> "Arnout" == Arnout Vandecappelle <arnout@mind.be> writes:

Hi,

 >> Yes, so the only minor issue is that we need to have swapon/swapoff
 >> enabled in busybox-minimal.config even though it isn't useful just to
 >> not get an error message during bootup/shutdown.

 >  Well, it currently _is_ enabled in busybox-minimal.config... Admittedly, this
 > adds almost 1KB to the busybox size so perhaps it should be removed from
 > -minimal. But the we probably should also remove swapoff from inittab. Hell,
 > perhaps we should do that anyway, since doing swapoff for reboot is utterly
 > useless IMO.

Yes, I'm not sure what good swapoff would do just before rebooting,
except if you are using a swapfile and keeping it mounted would cause
umounting that filesystem to fail - But I've never used swap in a
Buildroot based project.

Notice that we will still get the error message from the swapon call
during startup if we drop the swapon applet.

Maybe we should just leave it as is.

-- 
Bye, Peter Korsgaard