Filippo Sironi <sir...@amazon.de> writes:
> We've seen a number of crashes with the following signature:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> ...
> Oops: 0000 [#1] SMP PTI
> ...
> RIP: 0010:__rb_erase_color+0xc2/0x260
> ...
> Call Trace:
> unlink_file_vma+0x36/0x50
> free_pgtables+0x62/0x110
> exit_mmap+0xd5/0x160
> ? put_dec+0x3a/0x90
> ? num_to_str+0xa8/0xc0
> mmput+0x11/0xb0
> do_task_stat+0x940/0xc80
> proc_single_show+0x49/0x80
> ? __check_object_size+0xcc/0x1a0
> seq_read+0xd3/0x400
> vfs_read+0x72/0xb0
> ksys_read+0x9c/0xd0
> do_syscall_64+0x69/0x400
> ? schedule+0x2a/0x90
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> ...
>
> This happens when a process goes through the tasks stats in procfs while
> another is exiting. This looks like a race where the process that's
> exiting drops the last reference on the mm (with mmput) while the other
> increases it (with mmget). By only increasing when the reference isn't
> 0 to begin with, we prevent this from happening.
For this to be a race with exit this would require racing with exit_mm
where current->mm is cleared.
Looking at exit_mm() the code does:
struct mm_struct *mm = current->mm;
mmap_read_lock(mm);
mmgrab(mm);
task_lock(current);
local_irq_disable();
current->mm = NULL;
local_irq_enable();
task_unlock(current);
mmap_read_unlock(mm);
mmput(mm);
Which seems to guarantee "mm_users > 0" if "task->mm != NULL" under
tasklist_lock.
So I suggest you instrument your failing kernels and find what is
improperly decrementing mm_users.
Eric
