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

[Pengfei Li]

On Thu, 30 Jul 2020 16:57:05 -0400
Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> wrote:

> 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.
> 

Thanks for your comments.

My motivation is to allow mm related counters to be safely read and
written without holding mmap_lock. But sorry i didn't do well.

-- 
Pengfei