nixos/vm-tests: Remove msize mount option

This seems to be the root cause of the random page allocation failures
and @wizeman did a very good job on not only finding the root problem
but also giving a detailed explanation of it in #10828.

Here is an excerpt:

  The problem here is that the kernel is trying to allocate a contiguous
  section of 2^7=128 pages, which is 512 KB. This is way too much:
  kernel pages tend to get fragmented over time and kernel developers
  often go to great lengths to try allocating at most only 1 contiguous
  page at a time whenever they can.

  From the error message, it looks like the culprit is unionfs, but this
  is misleading: unionfs is the name of the userspace process that was
  running when the system ran out of memory, but it wasn't unionfs who
  was allocating the memory: it was the kernel; specifically it was the
  v9fs_dir_readdir_dotl() function, which is the code for handling the
  readdir() function in the 9p filesystem (the filesystem that is used
  to share a directory structure between a qemu host and its VM).

  If you look at the code, here's what it's doing at the moment it tries
  to allocate memory:

    buflen = fid->clnt->msize - P9_IOHDRSZ;

    rdir = v9fs_alloc_rdir_buf(file, buflen);

  If you look into v9fs_alloc_rdir_buf(), you will see that it will try
  to allocate a contiguous buffer of memory (using kzalloc(), which is a
  wrapper around kmalloc()) of size buflen + 8 bytes or so.

  So in reality, this code actually allocates a buffer of size
  proportional to fid->clnt->msize. What is this msize? If you follow
  the definition of the structures, you will see that it's the
  negotiated buffer transfer size between 9p client and 9p server. On
  the client side, it can be controlled with the msize mount option.

  What this all means is that, the reason for running out of memory is
  that the code (which we can't easily change) tries to allocate a
  contiguous buffer of size more or less equal to "negotiated 9p
  protocol buffer size", which seems to be way too big (in our NixOS
  tests, at least).

After that initial finding, @lethalman tested the gnome3 gdm test
without setting the msize parameter at all and it seems to have resolved
the problem.

The reason why I'm committing this without testing against all of the
NixOS VM test is basically that I think we can only go better but not
worse than the current state.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/pkgs/build-support/vm/default.nix b/pkgs/build-support/vm/default.nix
index 250f2b0fbce7..ebcbf8c87b5e 100644
--- a/pkgs/build-support/vm/default.nix
+++ b/pkgs/build-support/vm/default.nix
@@ -118,7 +118,7 @@ rec {
 
     echo "mounting Nix store..."
     mkdir -p /fs/nix/store
-    mount -t 9p store /fs/nix/store -o trans=virtio,version=9p2000.L,msize=262144,cache=loose
+    mount -t 9p store /fs/nix/store -o trans=virtio,version=9p2000.L,cache=loose
 
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
@@ -127,7 +127,7 @@ rec {
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,msize=262144,cache=loose
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,cache=loose
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc
diff --git a/pkgs/build-support/vm/windows/controller/default.nix b/pkgs/build-support/vm/windows/controller/default.nix
index 0beaf401758a..1c8e6af83b86 100644
--- a/pkgs/build-support/vm/windows/controller/default.nix
+++ b/pkgs/build-support/vm/windows/controller/default.nix
@@ -48,11 +48,11 @@ let
     mount -t proc none /fs/proc
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      -o trans=virtio,version=9p2000.L,cache=loose \
       store /fs/nix/store
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      -o trans=virtio,version=9p2000.L,cache=loose \
       xchg /fs/xchg
 
     echo root:x:0:0::/root:/bin/false > /fs/etc/passwd