traversing vma on nommu

[Hajime Tazaki]

Hello,

I'd like to ask your help to debug my out-of-tree kernel extension,
which is still in an RFC stage.  The RFC is about nommu extension to
UML (user momd linux).

https://lore.kernel.org/linux-um/m2y129hpx5.wl-thehajime@gmail.com/T/#t

I've been using v6.10 tag for the base branch for a while, mostly
works fine but after I rebased my branch to v6.12-rc2, I faced a crash
upon a process exit during vma iteration.

I bisected this issue and found that if I reverted the 4 commits
below, the issue is gone.

ed4dfd9aa1b1 maple_tree: make write helper functions void
c27e6183c654 maple_tree: remove unneeded mas_wr_walk() in mas_store_prealloc()
add60ea5f6d8 maple_tree: remove repeated sanity checks from write helper functions
9155e8433498 maple_tree: remove node allocations from various write helper functions

I'd like to debug what's wrong with my code but no luck so far.
I thought it is related with nommu code (mm/nommu.c) but didn't find
any useful hints for me.

It'd be very great if you have similar experience on this kind of
issue (tree iteration over vma, etc), or share some common pitfall
when using maple tree library.

below is the log of a gdb session.

```
${HOSTNAME%%.*}:$PWD# apk add se
afetch https://dl-cdn.alpinelinux.org/alpine/v3.20/main/x86_64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.20/community/x86_64/APKINDEX.tar.gz
ERROR: unable to select packages:
  se (no such package):
    required by: world[se]

Thread 1 "vmlinux" received signal SIGSEGV, Segmentation fault.
                                                               acct_collect (exitcode=exitcode at entry=256, group_dead=1) at ../kernel/acct.c:565
565      vsize += vma->vm_end - vma->vm_start;
(gdb) l
560     VMA_ITERATOR(vmi, mm, 0);
561     struct vm_area_struct *vma;
562   
563     mmap_read_lock(mm);
564     for_each_vma(vmi, vma)
565      vsize += vma->vm_end - vma->vm_start;
566     mmap_read_unlock(mm);
567    }
568   
569    spin_lock_irq(&current->sighand->siglock);
(gdb) bt
#0  acct_collect (exitcode=exitcode at entry=256, group_dead=1) at ../kernel/acct.c:565
#1  0x000000006003d5af in do_exit (code=code at entry=256) at ../kernel/exit.c:918
#2  0x000000006003deca in do_group_exit (exit_code=256) at ../kernel/exit.c:1088
#3  0x000000006003dee4 in __do_sys_exit_group (error_code=<optimized out>) at ../kernel/exit.c:1099
#4  __se_sys_exit_group (error_code=<optimized out>) at ../kernel/exit.c:1097
#5  0x000000006003840d in do_syscall_64 (regs=0x705aadd8) at ../arch/x86/um/do_syscall_64.c:83
#6  0x000000006003855e in __kernel_vsyscall () at ../arch/x86/um/entry_64.S:73
#7  0x00000087000081ed in ?? ()
#8  0x671cad19671cad19 in ?? ()
#9  0x00000000671ca871 in ?? ()
#10 0x0000000800010000 in ?? ()
#11 0x0000000400080000 in ?? ()
#12 0x000000040001f30a in ?? ()
#13 0x0000000000000000 in ?? ()
```

any suggestions are of course welcome.
thanks !

-- Hajime