Alexey Gladkov [off-list ref] writes:

On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:

quoted

On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
[off-list ref] wrote:

quoted

Sorry about that. I thought that this code is not needed when switching
from int to refcount_t. I was wrong.

Well, you _may_ be right. I personally didn't check how the return
value is used.

I only reacted to "it certainly _may_ be used, and there is absolutely
no comment anywhere about why it wouldn't matter".

I have not found examples where checked the overflow after calling
refcount_inc/refcount_add.

For example in kernel/fork.c:2298 :

   current->signal->nr_threads++;                           
   atomic_inc(&current->signal->live);                      
   refcount_inc(&current->signal->sigcnt);  

$ semind search signal_struct.sigcnt
def include/linux/sched/signal.h:83  		refcount_t		sigcnt;
m-- kernel/fork.c:723 put_signal_struct 		if (refcount_dec_and_test(&sig->sigcnt))
m-- kernel/fork.c:1571 copy_signal 		refcount_set(&sig->sigcnt, 1);
m-- kernel/fork.c:2298 copy_process 				refcount_inc(&current->signal->sigcnt);

It seems to me that the only way is to use __refcount_inc and then compare
the old value with REFCOUNT_MAX

Since I have not seen examples of such checks, I thought that this is
acceptable. Sorry once again. I have not tried to hide these changes.

The current ucount code does check for overflow and fails the increment
in every case.

So arguably it will be a regression and inferior error handling behavior
if the code switches to the ``better'' refcount_t data structure.

I originally didn't use refcount_t because silently saturating and not
bothering to handle the error makes me uncomfortable.

Not having to acquire the ucounts_lock every time seems nice.  Perhaps
the path forward would be to start with stupid/correct code that always
takes the ucounts_lock for every increment of ucounts->count, that is
later replaced with something more optimal.

Not impacting performance in the non-namespace cases and having good
performance in the other cases is a fundamental requirement of merging
code like this.

Eric