Re: [RFC 2/2] kread: avoid duplicates

[Luis Chamberlain]

On Mon, Apr 17, 2023 at 03:08:34PM -0700, Luis Chamberlain wrote:
> On Mon, Apr 17, 2023 at 05:33:49PM +0000, Edgecombe, Rick P wrote:
> > On Sat, 2023-04-15 at 23:41 -0700, Luis Chamberlain wrote:
> > > On Sat, Apr 15, 2023 at 11:04:12PM -0700, Christoph Hellwig wrote:
> > > > On Thu, Apr 13, 2023 at 10:28:40PM -0700, Luis Chamberlain wrote:
> > > > > With this we run into 0 wasted virtual memory bytes.
> > > > 
> > > > Avoid what duplicates?
> > > 
> > > David Hildenbrand had reported that with over 400 CPUs vmap space
> > > runs out and it seems it was related to module loading. I took a
> > > look and confirmed it. Module loading ends up requiring in the
> > > worst case 3 vmalloc allocations, so typically at least twice
> > > the size of the module size and in the worst case just add
> > > the decompressed module size:
> > > 
> > > a) initial kernel_read*() call
> > > b) optional module decompression
> > > c) the actual module data copy we will keep
> > > 
> > > Duplicate module requests that come from userspace end up being
> > > thrown
> > > in the trash bin, as only one module will be allocated.  Although
> > > there
> > > are checks for a module prior to requesting a module udev still
> > > doesn't
> > > do the best of a job to avoid that and so we end up with tons of
> > > duplicate module requests. We're talking about gigabytes of vmalloc
> > > bytes just lost because of this for large systems and megabytes for
> > > average systems. So for example with just 255 CPUs we can loose about
> > > 13.58 GiB, and for 8 CPUs about 226.53 MiB.
> > > 
> > > I have patches to curtail 1/2 of that space by doing a check in
> > > kernel
> > > before we do the allocation in c) if the module is already present.
> > > For
> > > a) it is harder because userspace just passes a file descriptor. But
> > > since we can get the file path without the vmalloc this RFC suggest
> > > maybe we can add a new kernel_read*() for module loading where it
> > > makes
> > > sense to have only one read happen at a time.
> > 
> > I'm wondering how difficult it would be to just try to remove the
> > vmallocs in (a) and (b) and operate on a list of pages.
> 
> Yes I think it's worth long term to do that, if possible with seq reads.

OK here's what I suggest we do then:

I'll resubmit the first patch which allows us to prove / disprove if
module-autoloading is the culprit. With that in place folks can debug
their setup and verify how udev is to blame.

I'll drop the second kernel_read*() patch / effort and punt this as a
userspace problem as this is also not extremely pressing.

Long term should evaluate how we can avoid vmalloc for the kread and
module decompression.

If this really becomes a pressing issue we can revisit if we want an in
kernel solution, but at this point that likely would be systems with
over 400-500 CPUs with KASAN enabled. Without KASAN the issue should
eventually trigger if you're enablig modules but its hard to say at what
point you'd hit this issue.

  Luis