[PATCH] swapon.2: Document NOSWAP and additional DISCARD flags

[PATCH] swapon.2: Document NOSWAP and additional DISCARD flags

[Evan Green]

Fix the FIXME and finally document SWAP_FLAG_DISCARD_ONCE and
SWAP_FLAG_DISCARD_PAGES from Linux 3.11. Additionally, document
SWAP_FLAG_NOSWAP, a shiny new flag being introduced in Linux 5.14. See
the following link:

https://lore.kernel.org/lkml/20210721143946.v3.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid/

Signed-off-by: Evan Green <evgreen@chromium.org>
---
 man2/swapon.2 | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/man2/swapon.2 b/man2/swapon.2
index 07d5fe86c..d568da993 100644
--- a/man2/swapon.2
+++ b/man2/swapon.2
@@ -33,13 +33,10 @@
 .\" 2004-12-14 mtk, Anand Kumria: added new errors
 .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
 .\"     Update text describing limit on number of swap files.
+.\" 2021-07-22 Evan Green <evgreen@chromium.org>
+.\"     Document _DISCARD_ONCE, _DISCARD_PAGES, and _NOSWAP flags.
 .\"
-.\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
-.\"	commit dcf6b7ddd7df8965727746f89c59229b23180e5a
-.\"	Author: Rafael Aquini <aquini@redhat.com>
-.\"	Date:   Wed Jul 3 15:02:46 2013 -0700
-.\"
-.TH SWAPON 2 2021-03-22 "Linux" "Linux Programmer's Manual"
+.TH SWAPON 2 2021-07-22 "Linux" "Linux Programmer's Manual"
 .SH NAME
 swapon, swapoff \- start/stop swapping to file/device
 .SH SYNOPSIS
@@ -81,9 +78,23 @@ flag is specified in the
 argument, freed swap pages will be discarded 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.)
+but often it does not.) In conjunction with this flag, callers may add
+.B SWAP_FLAG_DISCARD_ONCE
+to perform a single discard over the entire region at swapon time, and/or
+.B SWAP_FLAG_DISCARD_PAGES
+to perform discards as swap pages are freed. If neither is specified,
+both are assumed.
 See also NOTES.
 .PP
+If the
+.B SWAP_FLAG_NOSWAP
+flag is specified in the
+.BR swapon ()
+.I swapflags
+argument, then the added swap region will not be eligible for regular
+swap activities. It can still be used for hibernate, enabling callers
+to steer the swap and hibernate regions independently.
+.PP
 These functions may be used only by a privileged process (one having the
 .B CAP_SYS_ADMIN
 capability).
@@ -201,6 +212,14 @@ which still discards the
 entire swap area when
 .BR swapon ()
 is called, even if that flag bit is not set.
+.PP
+.B SWAP_FLAG_DISCARD_ONCE
+and
+.B SWAP_FLAG_DISCARD_PAGES
+were introduced in Linux 3.11.
+.PP
+.B SWAP_FLAG_NOSWAP
+was introduced in Linux 5.14.
 .SH SEE ALSO
 .BR mkswap (8),
 .BR swapoff (8),
-- 
2.31.0

Re: [PATCH] swapon.2: Document NOSWAP and additional DISCARD flags

[Evan Green]

On Thu, Jul 22, 2021 at 2:21 PM Evan Green <evgreen@chromium.org> wrote:
>
> Fix the FIXME and finally document SWAP_FLAG_DISCARD_ONCE and
> SWAP_FLAG_DISCARD_PAGES from Linux 3.11. Additionally, document
> SWAP_FLAG_NOSWAP, a shiny new flag being introduced in Linux 5.14. See
> the following link:
>
> https://lore.kernel.org/lkml/20210721143946.v3.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid/
>
> Signed-off-by: Evan Green <evgreen@chromium.org>

Please disregard, the patch introducing the NOSWAP flag is still in flux.
-Evan

Re: [PATCH] swapon.2: Document NOSWAP and additional DISCARD flags

[Alejandro Colomar (man-pages)]

Hi Evan,

On 7/22/21 11:21 PM, Evan Green wrote:
> Fix the FIXME and finally document SWAP_FLAG_DISCARD_ONCE and
> SWAP_FLAG_DISCARD_PAGES from Linux 3.11. Additionally, document
> SWAP_FLAG_NOSWAP, a shiny new flag being introduced in Linux 5.14. See
> the following link:
> 
> https://lore.kernel.org/lkml/20210721143946.v3.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid/
> 
> Signed-off-by: Evan Green <evgreen@chromium.org>

Discarded as requested :)

However, please see a few comments below.

Thanks,

Alex

> ---
>   man2/swapon.2 | 33 ++++++++++++++++++++++++++-------
>   1 file changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/man2/swapon.2 b/man2/swapon.2
> index 07d5fe86c..d568da993 100644
> --- a/man2/swapon.2
> +++ b/man2/swapon.2
> @@ -33,13 +33,10 @@
>   .\" 2004-12-14 mtk, Anand Kumria: added new errors
>   .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
>   .\"     Update text describing limit on number of swap files.
> +.\" 2021-07-22 Evan Green <evgreen@chromium.org>
> +.\"     Document _DISCARD_ONCE, _DISCARD_PAGES, and _NOSWAP flags.
>   .\"
> -.\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
> -.\"	commit dcf6b7ddd7df8965727746f89c59229b23180e5a
> -.\"	Author: Rafael Aquini <aquini@redhat.com>
> -.\"	Date:   Wed Jul 3 15:02:46 2013 -0700
> -.\"
> -.TH SWAPON 2 2021-03-22 "Linux" "Linux Programmer's Manual"
> +.TH SWAPON 2 2021-07-22 "Linux" "Linux Programmer's Manual"

The .TH line is updated with a script, so we don't touch it manually.

>   .SH NAME
>   swapon, swapoff \- start/stop swapping to file/device
>   .SH SYNOPSIS
> @@ -81,9 +78,23 @@ flag is specified in the
>   argument, freed swap pages will be discarded 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.)
> +but often it does not.) In conjunction with this flag, callers may add


See the following extract from man-pages(7):

$ man 7 man-pages | sed -n '/Use semantic newlines/,/^$/p';
    Use semantic newlines
        In the source of a manual page,  new  sentences  should  be
        started  on new lines, and long sentences should split into
        lines at clause breaks (commas, semicolons, colons, and  so
        on).   This  convention,  sometimes known as "semantic new‐
        lines", makes it easier to see the effect of patches, which
        often  operate at the level of individual sentences or sen‐
        tence clauses.


> +.B SWAP_FLAG_DISCARD_ONCE
> +to perform a single discard over the entire region at swapon time, and/or
> +.B SWAP_FLAG_DISCARD_PAGES
> +to perform discards as swap pages are freed. If neither is specified,
> +both are assumed.
>   See also NOTES.
>   .PP
> +If the
> +.B SWAP_FLAG_NOSWAP
> +flag is specified in the
> +.BR swapon ()
> +.I swapflags
> +argument, then the added swap region will not be eligible for regular
> +swap activities. It can still be used for hibernate, enabling callers
> +to steer the swap and hibernate regions independently.
> +.PP
>   These functions may be used only by a privileged process (one having the
>   .B CAP_SYS_ADMIN
>   capability).
> @@ -201,6 +212,14 @@ which still discards the
>   entire swap area when
>   .BR swapon ()
>   is called, even if that flag bit is not set.
> +.PP
> +.B SWAP_FLAG_DISCARD_ONCE
> +and
> +.B SWAP_FLAG_DISCARD_PAGES
> +were introduced in Linux 3.11.
> +.PP
> +.B SWAP_FLAG_NOSWAP
> +was introduced in Linux 5.14.
>   .SH SEE ALSO
>   .BR mkswap (8),
>   .BR swapoff (8),
> 


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/

Re: [PATCH] swapon.2: Document NOSWAP and additional DISCARD flags

[Evan Green]

On Sun, Jul 25, 2021 at 1:50 PM Alejandro Colomar (man-pages)
<alx.manpages@gmail.com> wrote:
>
> Hi Evan,
>
> On 7/22/21 11:21 PM, Evan Green wrote:
> > Fix the FIXME and finally document SWAP_FLAG_DISCARD_ONCE and
> > SWAP_FLAG_DISCARD_PAGES from Linux 3.11. Additionally, document
> > SWAP_FLAG_NOSWAP, a shiny new flag being introduced in Linux 5.14. See
> > the following link:
> >
> > https://lore.kernel.org/lkml/20210721143946.v3.1.I09866d90c6de14f21223a03e9e6a31f8a02ecbaf@changeid/
> >
> > Signed-off-by: Evan Green <evgreen@chromium.org>
>
> Discarded as requested :)
>
> However, please see a few comments below.
>
> Thanks,
>
> Alex
>
> > ---
> >   man2/swapon.2 | 33 ++++++++++++++++++++++++++-------
> >   1 file changed, 26 insertions(+), 7 deletions(-)
> >
> > diff --git a/man2/swapon.2 b/man2/swapon.2
> > index 07d5fe86c..d568da993 100644
> > --- a/man2/swapon.2
> > +++ b/man2/swapon.2
> > @@ -33,13 +33,10 @@
> >   .\" 2004-12-14 mtk, Anand Kumria: added new errors
> >   .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
> >   .\"     Update text describing limit on number of swap files.
> > +.\" 2021-07-22 Evan Green <evgreen@chromium.org>
> > +.\"     Document _DISCARD_ONCE, _DISCARD_PAGES, and _NOSWAP flags.
> >   .\"
> > -.\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
> > -.\"  commit dcf6b7ddd7df8965727746f89c59229b23180e5a
> > -.\"  Author: Rafael Aquini <aquini@redhat.com>
> > -.\"  Date:   Wed Jul 3 15:02:46 2013 -0700
> > -.\"
> > -.TH SWAPON 2 2021-03-22 "Linux" "Linux Programmer's Manual"
> > +.TH SWAPON 2 2021-07-22 "Linux" "Linux Programmer's Manual"
>
> The .TH line is updated with a script, so we don't touch it manually.

Ack.

>
> >   .SH NAME
> >   swapon, swapoff \- start/stop swapping to file/device
> >   .SH SYNOPSIS
> > @@ -81,9 +78,23 @@ flag is specified in the
> >   argument, freed swap pages will be discarded 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.)
> > +but often it does not.) In conjunction with this flag, callers may add
>
>
> See the following extract from man-pages(7):
>
> $ man 7 man-pages | sed -n '/Use semantic newlines/,/^$/p';
>     Use semantic newlines
>         In the source of a manual page,  new  sentences  should  be
>         started  on new lines, and long sentences should split into
>         lines at clause breaks (commas, semicolons, colons, and  so
>         on).   This  convention,  sometimes known as "semantic new‐
>         lines", makes it easier to see the effect of patches, which
>         often  operate at the level of individual sentences or sen‐
>         tence clauses.
>

Gotcha, thanks for the gentle education. I'll hopefully have a revised
version of this patch out at some point once the main change is
stabilized. I'll be sure to fix these issues, if you notice anything
else in this patch that would need to be corrected I can include that
in the next spin too.
-Evan