linux-kernel - Re: [RFC PATCH 2/5] lib/memregion: Uplevel the pmem "region" ida to a global allocator

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Date:   Thu, 4 Apr 2019 14:02:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Keith Busch <keith.busch@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        X86 ML <x86@...nel.org>, Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [RFC PATCH 2/5] lib/memregion: Uplevel the pmem "region" ida to a
 global allocator

On Thu, Apr 4, 2019 at 12:32 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Apr 04, 2019 at 12:08:38PM -0700, Dan Williams wrote:
> > +++ b/lib/Kconfig
> > @@ -318,6 +318,12 @@ config DECOMPRESS_LZ4
> >  config GENERIC_ALLOCATOR
> >       bool
> >
> > +#
> > +# Generic IDA for memory regions
> > +#
>
> Leaky abstraction -- nobody needs know that it's implemented as an IDA.
> Suggest:
>
> # Memory region ID allocation
>

Looks good to me.

> ...
>
> > +++ b/lib/memregion.c
> > @@ -0,0 +1,22 @@
> > +#include <linux/idr.h>
> > +#include <linux/module.h>
> > +
> > +static DEFINE_IDA(region_ida);
> > +
> > +int memregion_alloc(void)
> > +{
> > +     return ida_simple_get(&region_ida, 0, 0, GFP_KERNEL);
> > +}
> > +EXPORT_SYMBOL(memregion_alloc);
> > +
> > +void memregion_free(int id)
> > +{
> > +     ida_simple_remove(&region_ida, id);
> > +}
> > +EXPORT_SYMBOL(memregion_free);
> > +
> > +static void __exit memregion_exit(void)
> > +{
> > +     ida_destroy(&region_ida);
> > +}
> > +module_exit(memregion_exit);
>
>  - Should these be EXPORT_SYMBOL_GPL?

I don't see the need. These are simple wrappers around existing
EXPORT_SYMBOL() exports, and there's little concern that these
interfaces might disappear in the future causing us pain with out of
tree modules as these don't touch anything in the core.

>  - Can we use the new interface, ida_alloc() and ida_free()?

Sure.

>  - Do we really want memregion_exit() to happen while there are still IDs
>    allocated in the IDA?  I think this might well be better as:
>
>         BUG_ON(!ida_empty(&region_ida));

True, or just delete the module_exit because this functionality can't
be built as a module, so the exit path is already dead code.

> Also, do we really want to call the structure the region_ida?  Why not
> region_ids?

Sure, sounds good.