linux-kernel - Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

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Message-ID: <20170719022800.vpi2lcpqylkcz4j6@treble>
Date:   Tue, 18 Jul 2017 21:28:00 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jessica Yu <jeyu@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

On Tue, Jul 18, 2017 at 04:21:07PM -0400, Joe Lawrence wrote:
> On Mon, Jul 17, 2017 at 05:29:41PM +0200, Miroslav Benes wrote:
> >
> > On Wed, 28 Jun 2017, Joe Lawrence wrote:
> > 
> > > +Brief API summary
> > > +-----------------
> > > + [ ... snip ...]
> > > +* klp_shadow_detach() - detach and free all <*, num> shadow variables
> > > +  - find and remove any <*, num> references from hashtable
> > > +    - if found, release shadow variable
> > 
> > I think that the second one should be klp_shadow_detach_all(), shouldn't 
> > it?
> 
> Good catch, I'll fixup in v3.
> 
> > > +static DEFINE_HASHTABLE(klp_shadow_hash, 12);
> > 
> > Is there a reason, why you pick 12? I'm just curious.
>
> The hashtable bit-size was inherited from the kpatch implementation.
> Perhaps Josh knows why this value was picked?

My thinking was that it gives you about 4096 unique hash table entries
for 32k of RAM.  It was a rough guess.  It's hard to really predict what
size you need.

> Aside: we could have per-livepatch hashtables if that was desired, this
> value could be then adjusted accordingly.  We haven't needed them for
> kpatch, so I didn't see good reason to complicate things.

I think a global hash table is much better because it allows you to deal
more gracefully with patch upgrades.

> > > + *
> > > + * Note: allocates @new_size space for shadow variable data and copies
> > > + * @new_size bytes from @new_data into the shadow varaible's own @new_data
> > > + * space.  If @new_data is NULL, @new_size is still allocated, but no
> > > + * copy is performed.
> > 
> > I must say I'm not entirely happy with this. I don't know if this is what 
> > Petr had in mind (I'm sure he'll get to the patch set soon). Calling 
> > memcpy instead of a simple assignment in v1 seems worse. 
> 
> This change was a bit of a experiment on my part in reaction to
> adding klp_shadow_get_or_attach().
> 
> I like the simplicity of v1's pointer assignment -- in fact, moving all
> allocation responsiblity (klp_shadow meta-data and data[] area) out to
> the caller is doable, though implementing klp_shadow_get_or_attach() and
> and klp_shadow_detach_all() complicates matters, for example, adding an
> alloc/release callback.  I originally attempted this for v2, but turned
> back when the API and implementation grew complicated.  If the memcpy
> and gfp_flag restrictions are too ugly, I can try revisting that
> approach.  Ideas welcome :)

Personally I'm not a fan of the callbacks, I like the v2 API.

-- 
Josh