The finit_module() system call can create unnecessary virtual memory
pressure for duplicate modules. This is because load_module() can in
the worse case allocate more than twice the size of a module in virtual
memory. This saves at least a full size of the module in wasted vmalloc
space memory by trying to avoid duplicates as soon as we can validate
the module name in the read module structure.

This can only be an issue if a system is getting hammered with userspace
loading modules. There are two ways to load modules typically on systems,
one is the kernel moduile auto-loading (*request_module*() calls in-kernel)
and the other is things like udev. The auto-loading is in-kernel, but that
pings back to userspace to just call modprobe. We already have a way to
restrict the amount of concurrent kernel auto-loads in a given time, however
that still allows multiple requests for the same module to go through
and force two threads in userspace racing to call modprobe for the same
exact module. Even though libkmod which both modprobe and udev does check
if a module is already loaded prior calling finit_module() races are
still possible and this is clearly evident today when you have multiple
CPUs.

To avoid memory pressure for such stupid cases put a stop gap for them.
The *earliest* we can detect duplicates from the modules side of things
is once we have blessed the module name, sadly after the first vmalloc
allocation. We can check for the module being present *before* a secondary
vmalloc() allocation.

There is a linear relationship between wasted virtual memory bytes and
the number of CPU counts. The reason is that udev ends up racing to call
tons of the same modules for each of the CPUs.

We can see the different linear relationships between wasted virtual
memory and CPU count during after boot in the following graph:

         +----------------------------------------------------------------------------+
    14GB |-+          +            +            +           +           *+          +-|
         |                                                          ****              |
         |                                                       ***                  |
         |                                                     **                     |
    12GB |-+                                                 **                     +-|
         |                                                 **                         |
         |                                               **                           |
         |                                             **                             |
         |                                           **                               |
    10GB |-+                                       **                               +-|
         |                                       **                                   |
         |                                     **                                     |
         |                                   **                                       |
     8GB |-+                               **                                       +-|
waste    |                               **                             ###           |
         |                             **                           ####              |
         |                           **                      #######                  |
     6GB |-+                     ****                    ####                       +-|
         |                      *                    ####                             |
         |                     *                 ####                                 |
         |                *****              ####                                     |
     4GB |-+            **               ####                                       +-|
         |            **             ####                                             |
         |          **           ####                                                 |
         |        **         ####                                                     |
     2GB |-+    **      #####                                                       +-|
         |     *    ####                                                              |
         |    * ####                                                   Before ******* |
         |  **##      +            +            +           +           After ####### |
         +----------------------------------------------------------------------------+
         0            50          100          150         200          250          300
                                          CPUs count

On the y-axis we can see gigabytes of wasted virtual memory during boot
due to duplicate module requests which just end up failing. Trying to
infer the slope this ends up being about ~463 MiB per CPU lost prior
to this patch. After this patch we only loose about ~230 MiB per CPU, for
a total savings of about ~233 MiB per CPU. This is all *just on bootup*!

On a 8vcpu 8 GiB RAM system using kdevops and testing against selftests
kmod.sh -t 0008 I see a saving in the *highest* side of memory
consumption of up to ~ 84 MiB with the Linux kernel selftests kmod
test 0008. With the new stress-ng module test I see a 145 MiB difference
in max memory consumption with 100 ops. The stress-ng module ops tests can be
pretty pathalogical -- it is not realistic, however it was used to
finally successfully reproduce issues which are only reported to happen on
system with over 400 CPUs [0] by just usign 100 ops on a 8vcpu 8 GiB RAM
system. Running out of virtual memory space is no surprise given the
above graph, since at least on x86_64 we're capped at 128 MiB, eventually
we'd hit a series of errors and once can use the above graph to
guestimate when. This of course will vary depending on the features
you have enabled. So for instance, enabling KASAN seems to make this
much worse.

The results with kmod and stress-ng can be observed and visualized below.
The time it takes to run the test is also not affected.

The kmod tests 0008:

The gnuplot is set to a range from 400000 KiB (390 Mib) - 580000 (566 Mib)
given the tests peak around that range.

cat kmod.plot
set term dumb
set output fileout
set yrange [400000:580000]
plot filein with linespoints title "Memory usage (KiB)"

Before:
root@kmod ~ # /data/linux-next/tools/testing/selftests/kmod/kmod.sh -t 0008
root@kmod ~ # free -k -s 1 -c 40 | grep Mem | awk '{print $3}' > log-0008-before.txt ^C
root@kmod ~ # sort -n -r log-0008-before.txt | head -1
528732

So ~516.33 MiB

After:

root@kmod ~ # /data/linux-next/tools/testing/selftests/kmod/kmod.sh -t 0008
root@kmod ~ # free -k -s 1 -c 40 | grep Mem | awk '{print $3}' > log-0008-after.txt ^C

root@kmod ~ # sort -n -r log-0008-after.txt | head -1
442516

So ~432.14 MiB

That's about 84 ~MiB in savings in the worst case. The graphs:

root@kmod ~ # gnuplot -e "filein='log-0008-before.txt'; fileout='graph-0008-before.txt'" kmod.plot
root@kmod ~ # gnuplot -e "filein='log-0008-after.txt';  fileout='graph-0008-after.txt'"  kmod.plot

root@kmod ~ # cat graph-0008-before.txt

  580000 +-----------------------------------------------------------------+
         |       +        +       +       +       +        +       +       |
  560000 |-+                                    Memory usage (KiB) ***A***-|
         |                                                                 |
  540000 |-+                                                             +-|
         |                                                                 |
         |        *A     *AA*AA*A*AA          *A*AA    A*A*A *AA*A*AA*A  A |
  520000 |-+A*A*AA  *AA*A           *A*AA*A*AA     *A*A     A          *A+-|
         |*A                                                               |
  500000 |-+                                                             +-|
         |                                                                 |
  480000 |-+                                                             +-|
         |                                                                 |
  460000 |-+                                                             +-|
         |                                                                 |
         |                                                                 |
  440000 |-+                                                             +-|
         |                                                                 |
  420000 |-+                                                             +-|
         |       +        +       +       +       +        +       +       |
  400000 +-----------------------------------------------------------------+
         0       5        10      15      20      25       30      35      40

root@kmod ~ # cat graph-0008-after.txt

  580000 +-----------------------------------------------------------------+
         |       +        +       +       +       +        +       +       |
  560000 |-+                                    Memory usage (KiB) ***A***-|
         |                                                                 |
  540000 |-+                                                             +-|
         |                                                                 |
         |                                                                 |
  520000 |-+                                                             +-|
         |                                                                 |
  500000 |-+                                                             +-|
         |                                                                 |
  480000 |-+                                                             +-|
         |                                                                 |
  460000 |-+                                                             +-|
         |                                                                 |
         |          *A              *A*A                                   |
  440000 |-+A*A*AA*A  A       A*A*AA    A*A*AA*A*AA*A*AA*A*AA*AA*A*AA*A*AA-|
         |*A           *A*AA*A                                             |
  420000 |-+                                                             +-|
         |       +        +       +       +       +        +       +       |
  400000 +-----------------------------------------------------------------+
         0       5        10      15      20      25       30      35      40

The stress-ng module tests:

This is used to run the test to try to reproduce the vmap issues
reported by David:

  echo 0 > /proc/sys/vm/oom_dump_tasks
  ./stress-ng --module 100 --module-name xfs

Prior to this commit:
root@kmod ~ # free -k -s 1 -c 40 | grep Mem | awk '{print $3}' > baseline-stress-ng.txt
root@kmod ~ # sort -n -r baseline-stress-ng.txt | head -1
5046456

After this commit:
root@kmod ~ # free -k -s 1 -c 40 | grep Mem | awk '{print $3}' > after-stress-ng.txt
root@kmod ~ # sort -n -r after-stress-ng.txt | head -1
4896972

5046456 - 4896972
149484
149484/1024
145.98046875000000000000

So this commit using stress-ng reveals saving about 145 MiB in memory
using 100 ops from stress-ng which reproduced the vmap issue reported.

cat kmod.plot
set term dumb
set output fileout
set yrange [4700000:5070000]
plot filein with linespoints title "Memory usage (KiB)"

root@kmod ~ # gnuplot -e "filein='baseline-stress-ng.txt'; fileout='graph-stress-ng-before.txt'"  kmod-simple-stress-ng.plot
root@kmod ~ # gnuplot -e "filein='after-stress-ng.txt'; fileout='graph-stress-ng-after.txt'"  kmod-simple-stress-ng.plot

root@kmod ~ # cat graph-stress-ng-before.txt

           +---------------------------------------------------------------+
  5.05e+06 |-+     + A     +       +       +       +       +       +     +-|
           |         *                          Memory usage (KiB) ***A*** |
           |         *                             A                       |
     5e+06 |-+      **                            **                     +-|
           |        **                            * *    A                 |
  4.95e+06 |-+      * *                          A  *   A*               +-|
           |        * *      A       A           *  *  *  *             A  |
           |       *  *     * *     * *        *A   *  *  *      A      *  |
   4.9e+06 |-+     *  *     * A*A   * A*AA*A  A      *A    **A   **A*A  *+-|
           |       A  A*A  A    *  A       *  *      A     A *  A    * **  |
           |      *      **      **         * *              *  *    * * * |
  4.85e+06 |-+   A       A       A          **               *  *     ** *-|
           |     *                           *               * *      ** * |
           |     *                           A               * *      *  * |
   4.8e+06 |-+   *                                           * *      A  A-|
           |     *                                           * *           |
  4.75e+06 |-+  *                                            * *         +-|
           |    *                                            **            |
           |    *  +       +       +       +       +       + **    +       |
   4.7e+06 +---------------------------------------------------------------+
           0       5       10      15      20      25      30      35      40

root@kmod ~ # cat graph-stress-ng-after.txt

           +---------------------------------------------------------------+
  5.05e+06 |-+     +       +       +       +       +       +       +     +-|
           |                                    Memory usage (KiB) ***A*** |
           |                                                               |
     5e+06 |-+                                                           +-|
           |                                                               |
  4.95e+06 |-+                                                           +-|
           |                                                               |
           |                                                               |
   4.9e+06 |-+                                      *AA                  +-|
           |  A*AA*A*A  A  A*AA*AA*A*AA*A  A  A  A*A   *AA*A*A  A  A*AA*AA |
           |  *      * **  *            *  *  ** *            ***  *       |
  4.85e+06 |-+*       ***  *            * * * ***             A *  *     +-|
           |  *       A *  *             ** * * A               *  *       |
           |  *         *  *             *  **                  *  *       |
   4.8e+06 |-+*         *  *             A   *                  *  *     +-|
           | *          * *                  A                  * *        |
  4.75e+06 |-*          * *                                     * *      +-|
           | *          * *                                     * *        |
           | *     +    * *+       +       +       +       +    * *+       |
   4.7e+06 +---------------------------------------------------------------+
           0       5       10      15      20      25      30      35      40

[0] https://lkml.kernel.org/r/20221013180518.217405-1-david@redhat.comReported-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>