Re: [PATCH] sysctl: Add a group of macro functions to initcall the sysctl table of each feature

[Luis Chamberlain <mcgrof@kernel.org>]

On Wed, Dec 08, 2021 at 10:10:08AM +0800, Xiaoming Ni wrote:
> On 2021/12/8 6:39, Luis Chamberlain wrote:
> > On Tue, Dec 07, 2021 at 03:08:03PM -0600, Eric W. Biederman wrote:
> > > Luis Chamberlain <mcgrof@kernel.org> writes:
> > > 
> > > > On Mon, Dec 06, 2021 at 05:38:42PM -0800, Andrew Morton wrote:
> > > > > On Tue, 7 Dec 2021 09:13:20 +0800 Xiaoming Ni <nixiaoming@huawei.com> wrote:
> > > > > > --- a/fs/inode.c
> > > > > > +++ b/fs/inode.c
> > > > > > @@ -132,12 +132,7 @@ static struct ctl_table inodes_sysctls[] = {
> > > > > >   	{ }
> > > > > >   };
> > > > > > -static int __init init_fs_inode_sysctls(void)
> > > > > > -{
> > > > > > -	register_sysctl_init("fs", inodes_sysctls);
> > > > > > -	return 0;
> > > > > > -}
> > > > > > -early_initcall(init_fs_inode_sysctls);
> > > > > > +fs_sysctl_initcall(inodes_sysctls);
> > > > > >   #endif
> > > > > 
> > > > > Here's another, of many.
> > > > > 
> > > > > Someone made the decision to use early_initcall() here (why?) and this
> > > > > patch switches it to late_initcall()!  Worrisome.  Each such stealth
> > > > > conversion should be explained and justified, shouldn't it?
> > > > 
> > > > I made the decisions for quite a bit of the ordering and yes I agree
> > > > this need *very careful* explanation, specially if we are going to
> > > > generalize this.
> > > > 
> > > > First and foremost. git grep for sysctl_init_bases and you will see
> > > > that the bases for now are initialized on proc_sys_init() and that
> > > > gets called on proc_root_init() and that in turn on init/main.c's
> > > > start_kernel(). And so this happens *before* the init levels.
> > > > 
> > > > The proper care for what goes on top of this needs to take into
> > > > consideration the different init levels and that the if a sysctl
> > > > is using a directory *on top* of a base, then that sysctl registration
> > > > must be registered *after* that directory. The *base* directory for
> > > > "fs" is now registered through fs/sysctls.c() on init_fs_sysctls()
> > > > using register_sysctl_base(). I made these changes with these names
> > > > and requiring the DECLARE_SYSCTL_BASE() so it would be easy for us
> > > > to look at where these are declared.
> > > > 
> > > > So the next step in order to consider is *link* ordering and that
> > > > order is maintained by the Makefile. That is why I put this at the
> > > > top of the fs Makfile:
> > > > 
> > > > obj-$(CONFIG_SYSCTL)            += sysctls.o
> > > > 
> > > > So any file after this can use early_initcall(), because the base
> > > > for "fs" was declared first in link order, and it used early_initcall().
> > > > It is fine to have the other stuff that goes on top of the "fs" base
> > > > use late_initcall() but that assumes that vetting has been done so that
> > > > if a directory on "fs" was created, let's call it "foo", vetting was done
> > > > to ensure that things on top of "foo" are registered *after* the "foo"
> > > > directory.
> > > > 
> > > > We now have done the cleanup for "fs", and we can do what we see fine
> > > > for "fs", but we may run into surprises later with the other bases, so
> > > > I'd be wary of making assumptions at this point if we can use
> > > > late_initcall().
> > > > 
> > > > So, as a rule of thumb I'd like to see bases use early_initcall(). The
> > > > rest requires manual work and vetting.
> > > > 
> > > > So, how about this, we define fs_sysctl_initcall() to use also
> > > > early_initcall(), and ask susbsystems to do their vetting so that
> > > > the base also gets linked first.
> > > > 
> > > > After this, if a directory on top of a base is created we should likely create
> > > > a new init level and just bump that to use the next init level. So
> > > > something like fs_sysctl_base_initcall_subdir_1() map to core_initcall()
> > > > and so on.
> > > > 
> > > > That would allow us to easily grep for directory structures easily and
> > > > puts some implicit onus of ordering on those folks doing these conversions.
> > > > We'd document well the link order stuff for those using the base stuff
> > > > too as that is likely only where this will matter most.
> > > 
> > > I am a bit confused at this explanation of things.
> > > 
> > > Last I looked the implementation of sysctls allocated the directories
> > > independently of the sysctls entries that populated them.
> > 
> > With most sysctls being created using the same kernel/sysctl.c file and
> > structure, yes, this was true. With the changes now on linux-next things
> > change a bit. The goal is to move sysctls to be registered where they
> > are actually defined. But the directory that holds them must be
> > registered first. During the first phase of cleanups now on linux-next
> > all filesystem "fs" syscls were moved to be delcared in the kernel's
> > fs/ directory. The last part was to register the base "fs" directory.
> > For this declareres were added to simplify that and to clarify which
> > are base directories:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=ededd3fc701668743087c77ceeeb7490107cc12c
> > 
> > Then, this commit moves the "fs" base to be declared to fs/ as well:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=d0f885a73ec6e05803ce99f279232b3116061ed8
> > 
> > This used early_initcall() for the base for "fs" and that is
> > because there are no built-in sysctls for "fs" which need to
> > be exposed prior to the init levels.
> > 
> > So after this then order is important. If you are using the same
> > init level, the the next thing which will ensure order is the order
> > of things being linked, so what order they appear on the Makefile.
> > And this is why the base move for the "fs" sysctl directory is kept
> > at the top of fs/Makfile:
> > 
> > obj-$(CONFIG_SYSCTL)		+= sysctls.o
> > 
> >    Luis
> > .
> > 
> 
> Root node of the tree, using "early_initcall":
> 	Basic framework,  "fs", "kernel", "debug", "vm", "dev", "net"

register_sysctl_base() and yes these use early_initcall() as-is on
linux-next.

> Fork node. Select initcall_level based on the number of directory levels:
> 	Registration directory shared by multiple features.

Sure.

> Leaf node, use "late_initcall":
> 	File Interface

I am not sure this gives enough guidance. What is the difference between
fork node and a leaf node?

> Is this a feasible classification?

  Luis