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

[Greg KH]

On Sat, Apr 15, 2023 at 11:41:28PM -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.

How does the memory get "lost"?  Shouldn't it be properly freed when the
duplicate module load fails?

thanks,

greg k-h