Re: [PATCH 2/2] module: add support to avoid duplicates early on load

Luis Chamberlain
2023-05-31T16:57:45Z

On Wed, May 31, 2023 at 09:51:41AM +0200, David Hildenbrand wrote:
> On 30.05.23 18:22, Luis Chamberlain wrote:
> > On Mon, May 29, 2023 at 09:55:15PM -0400, Linus Torvalds wrote:
> > > On Mon, May 29, 2023 at 11:18 AM Johan Hovold <johan@kernel.org> wrote:
> > > >
> > > > I took a closer look at some of the modules that failed to load and
> > > > noticed a pattern in that they have dependencies that are needed by more
> > > > than one device.
> > >
> > > Ok, this is a "maybe something like this" RFC series of two patches -
> > > one trivial one to re-organize things a bit so that we can then do the
> > > real one which uses a filter based on the inode pointer to return an
> > > "idempotent return value" for module loads that share the same inode.
> > >
> > > It's entirely untested, and since I'm on the road I'm going to not
> > > really be able to test it. It compiles for me, and the code looks
> > > fairly straightforward, but it's probably buggy.
> > >
> > > It's very loosely based on Luis' attempt, but it
> > > (a) is internal to module loading
> > > (b) uses a reliable cookie
> > > (c) doesn't leave the cookie around randomly for later
> > > (d) has seen absolutely no testing
> > >
> > > Put another way: if somebody wants to play with this, please treat it
> > > as a starting point, not the final thing. You might need to debug
> > > things, and fix silly mistakes.
> > >
> > > The idea is to just have a simple hash list of currently executing
> > > module loads, protected by a trivial spinlock. Every module loader
> > > adds itself to the right hash list, and if they were the *first* one
> > > (ie no other pending module loads for that inode), will actually do
> > > the module load.
> > >
> > > Everybody who *isn't* the first one will just wait for completion and
> > > return the same error code that the first one returned.
> >
> > That's also a hell much more snazzier MODULE_DEBUG_AUTOLOAD_DUPS if we
> > ever wanted to do something similar there if we wanted to also
> > join request_module() calls, instead of it hiding under debug.
> >
> > > This is technically bogus. The first one might fail due to arguments.
> >
> > For boot it's fine, as I can't think of boot wanting to support trying
> > to load a module with different arguments but who knows. But I can't
> > see it sensible to issue concurrent multiple requests for modules
> > with different arguments without waiting in userspace for the first
> > to fail.
> >
> > Even post-boot, doing that sounds rather insane, but it would certainly
> > be a compromise and should probably be clearly documented. I think just
> > a comment acknolwedging that corner case seems sensible.
> >
> > Because we won't be able to get the arguments until we process the
> > module, so it would be too late for this optimization on kread. So it is
> > why I had also stuck to the original feature being in kread, as then it
> > provides a uniq kread call and the caller is aware of it. But indeed I
> > had not considered the effects of arguments.
> >
> > Lucas, any thoughts from modules kmod userspace perspective into
> > supporting anyone likely issuing concurrent modules requests with
> > differing arguments?
> >
> > > So the cookie shouldn't be just the inode, it should be the inode and
> > > a hash of the arguments or something like that.
> >
> > Personally I think it's a fine optimization without the arguments.
> >
> > > But it is what it is,
> > > and apart from possible show-stopper bugs this is no worse than the
> > > failed "exclusive write deny" attempt. IOW - maybe worth trying?
> >
> > The only thing I can think of is allowing threads other than the
> > first one to complete before the one that actually loaded the
> > module. I thought about this race for module auto-loading, see
> > the comment in kmod_dup_request_announce(), so that just
> > further delays the completion to other thread with a stupid
> > queue_work(). That seems more important for module auto-loading
> > duplicates than for boot finit_module() duplicates. But not sure
> > if odering matters in the end due to a preemtible kernel and maybe
> > that concern is hysteria.
> >
> > > And if *that* didn't sell people on this patch series, I don't know
> > > what will. I should be in marketing! Two drink minimums, here I come!
> >
> > Sold:
> >
> > on 255 vcpus 0 duplicates found with this setup:
> >
> > root@kmod ~ # cat /sys/kernel/debug/modules/stats
> > Mods ever loaded 66
> > Mods failed on kread 0
> > Mods failed on decompress 0
> > Mods failed on becoming 0
> > Mods failed on load 0
> > Total module size 11268096
> > Total mod text size 4149248
> > Failed kread bytes 0
> > Failed decompress bytes 0
> > Failed becoming bytes 0
> > Failed kmod bytes 0
> > Virtual mem wasted bytes 0
> > Average mod size 170729
> > Average mod text size 62868
> >
> > So:
> >
> > Tested-by: Luis Chamberlain <mcgrof@kernel.org>
> >
> > In terms of bootup timing:
> >
> > Before:
> > Startup finished in 41.653s (kernel) + 44.305s (userspace) = 1min 25.958s
> > graphical.target reached after 44.178s in userspace.
> > After:
> > Startup finished in 23.995s (kernel) + 40.350s (userspace) = 1min 4.345s
> > graphical.target reached after 40.226s in userspace.
>
> I'll try grabbing the system where we saw the KASAN-related issues [1] and
> give it a churn with and without the two patches. Might take a bit (~1 day),
> unfortunately.
>
> [1] https://lkml.kernel.org/r/20221013180518.217405-1-david@redhat.com

Great, don't forget:

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 82b0dcc1fe77..222015093eeb 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -3066,7 +3066,7 @@ struct idempotent {

#define IDEM_HASH_BITS 8
static struct hlist_head idem_hash[1 << IDEM_HASH_BITS];
-static struct spinlock idem_lock;
+static DEFINE_SPINLOCK(idem_lock);

static bool idempotent(struct idempotent *u, const void *cookie)
{