On Wed, 29 Jul 2020 12:21:11 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote: Sorry for the late reply. > 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. > > (At the very least, __account_locked_vm() in 1/2 should be changed to > replace its atomic64_add by an atomic64_cmpxchg, to enforce the limit > that it just checked. But that will be no more than lipstick on a > pig, when the right lock that everyone else agrees upon is not being > held.) > Thank you for your detailed comment. You are right, I should use atomic64_cmpxchg to guarantee the limit of RLIMIT_MEMLOCK. > Now, it can be argued that our locked_vm and pinned_vm maintenance > is so random and deficient, and too difficult to keep right across > a sprawl of drivers, that we should just be grateful for those that > do volunteer to subject themselves to RLIMIT_MEMLOCK limitation, > and never mind if it's a little racy. > > And it may well be that all those who have made considerable efforts > in the past to improve the situation, have more interesting things to > devote their time to, and would prefer not to get dragged back here. > > But let's at least give this a little more visibility, and hope > to hear opinions one way or the other from those who care. Thank you. My patch should be more thoughtful. I will send an email to Stephen soon asking to remove these two patches from -mm tree. -- Pengfei
