Re: [PATCH] mm/util: fix a data race in __vm_enough_memory()

[Matthew Wilcox]

On Wed, Jan 29, 2020 at 09:51:33PM -0500, Qian Cai wrote:
> "vm_committed_as.count" could be accessed concurrently as reported by
> KCSAN,
> 
>  read to 0xffffffff923164f8 of 8 bytes by task 1268 on cpu 38:
>   __vm_enough_memory+0x43/0x280 mm/util.c:801
>   mmap_region+0x1b2/0xb90 mm/mmap.c:1726
>   do_mmap+0x45c/0x700
>   vm_mmap_pgoff+0xc0/0x130
>   vm_mmap+0x71/0x90
>   elf_map+0xa1/0x1b0
>   load_elf_binary+0x9de/0x2180
>   search_binary_handler+0xd8/0x2b0
>   __do_execve_file+0xb61/0x1080
>   __x64_sys_execve+0x5f/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
>  write to 0xffffffff923164f8 of 8 bytes by task 1265 on cpu 41:
>   percpu_counter_add_batch+0x83/0xd0 lib/percpu_counter.c:91
>   exit_mmap+0x178/0x220 include/linux/mman.h:68
>   mmput+0x10e/0x270
>   flush_old_exec+0x572/0xfe0
>   load_elf_binary+0x467/0x2180
>   search_binary_handler+0xd8/0x2b0
>   __do_execve_file+0xb61/0x1080
>   __x64_sys_execve+0x5f/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Since only the read is operating as lockless, fix it by using
> READ_ONLY() for it to avoid any possible false warning due to load

You mean READ_ONCE ...

>  {
>  	long allowed;
>  
> -	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
> +	VM_WARN_ONCE(READ_ONCE(vm_committed_as.count) <
>  			-(s64)vm_committed_as_batch * num_online_cpus(),

I'm really not a fan of exposing the internals of a percpu_counter outside
the percpu_counter.h file.  Why shouldn't this be fixed by putting the
READ_ONCE() inside percpu_counter_read()?