From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Joe Lawrence <joe.lawrence@redhat.com>
Cc: Miroslav Benes <mbenes@suse.cz>,
live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
Jessica Yu <jeyu@redhat.com>, Jiri Kosina <jikos@kernel.org>,
Petr Mladek <pmladek@suse.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API
Date: Tue, 18 Jul 2017 21:28:00 -0500 [thread overview]
Message-ID: <20170719022800.vpi2lcpqylkcz4j6@treble> (raw)
In-Reply-To: <20170718202107.3hsptpdspr26snxc@redhat.com>
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
next prev parent reply other threads:[~2017-07-19 2:28 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-28 15:37 [PATCH v2 0/2] livepatch: add shadow variable API Joe Lawrence
2017-06-28 15:37 ` [PATCH v2 1/2] livepatch: introduce " Joe Lawrence
2017-06-30 13:49 ` kbuild test robot
2017-07-07 18:05 ` Joe Lawrence
2017-07-14 0:41 ` Josh Poimboeuf
2017-07-17 15:35 ` Miroslav Benes
2017-07-18 13:00 ` Petr Mladek
2017-07-18 19:36 ` Joe Lawrence
2017-07-19 15:19 ` Petr Mladek
2017-07-19 18:50 ` Miroslav Benes
2017-07-17 15:29 ` Miroslav Benes
2017-07-18 20:21 ` Joe Lawrence
2017-07-19 2:28 ` Josh Poimboeuf [this message]
2017-07-19 19:01 ` Miroslav Benes
2017-07-20 14:45 ` Miroslav Benes
2017-07-20 15:48 ` Joe Lawrence
2017-07-20 20:23 ` Josh Poimboeuf
2017-07-21 8:42 ` Petr Mladek
2017-07-21 8:59 ` Miroslav Benes
2017-07-18 12:45 ` Petr Mladek
2017-07-20 20:30 ` Joe Lawrence
2017-07-21 9:12 ` Miroslav Benes
2017-07-21 9:27 ` Petr Mladek
2017-07-21 9:13 ` Petr Mladek
2017-07-21 13:55 ` Joe Lawrence
2017-07-24 15:04 ` Josh Poimboeuf
2017-06-28 15:37 ` [PATCH v2 2/2] livepatch: add shadow variable sample programs Joe Lawrence
2017-07-18 14:47 ` Petr Mladek
2017-07-18 19:15 ` Joe Lawrence
2017-07-19 14:44 ` Petr Mladek
2017-07-19 15:06 ` Petr Mladek
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