Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock

[Daniel Jordan]

On Wed, Jul 29, 2020 at 12:21:11PM -0700, Hugh Dickins wrote:
> On Sun, 26 Jul 2020, Pengfei Li wrote:
> 
> > Since mm->locked_vm is already an atomic counter, account_locked_vm()
> > does not need to hold mmap_lock.
> 
> I am worried that this patch, already added to mmotm, along with its
> 1/2 making locked_vm an atomic64, might be rushed into v5.9 with just
> that two-line commit description, and no discussion at all.
> 
> locked_vm belongs fundamentally to mm/mlock.c, and the lock to guard
> it is mmap_lock; and mlock() has some complicated stuff to do under
> that lock while it decides how to adjust locked_vm.
>
> It is very easy to convert an unsigned long to an atomic64_t, but
> "atomic read, check limit and do stuff, atomic add" does not give
> the same guarantee as holding the right lock around it all.

Yes, this is why I withdrew my attempt to do something similar last year, I
didn't want to make the accounting racy.  Stack and heap growing and mremap
would be affected in addition to mlock.

It'd help to hear more about the motivation for this.

Daniel