Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()

[Matthew Wilcox]

On Sat, May 09, 2020 at 11:25:16AM +0200, Peter Zijlstra wrote:
> On Fri, May 08, 2020 at 11:34:07PM +0200, Joerg Roedel wrote:
> > On Fri, May 08, 2020 at 09:20:00PM +0200, Peter Zijlstra wrote:
> > > The only concern I have is the pgd_lock lock hold times.
> > > 
> > > By not doing on-demand faults anymore, and consistently calling
> > > sync_global_*(), we iterate that pgd_list thing much more often than
> > > before if I'm not mistaken.
> > 
> > Should not be a problem, from what I have seen this function is not
> > called often on x86-64.  The vmalloc area needs to be synchronized at
> > the top-level there, which is by now P4D (with 4-level paging). And the
> > vmalloc area takes 64 entries, when all of them are populated the
> > function will not be called again.
> 
> Right; it's just that the moment you do trigger it, it'll iterate that
> pgd_list and that is potentially huge. Then again, that's not a new
> problem.
> 
> I suppose we can deal with it if/when it becomes an actual problem.
> 
> I had a quick look and I think it might be possible to make it an RCU
> managed list. We'd have to remove the pgd_list entry in
> put_task_struct_rcu_user(). Then we can play games in sync_global_*()
> and use RCU iteration. New tasks (which can be missed in the RCU
> iteration) will already have a full clone of the PGD anyway.

One of the things on my long-term todo list is to replace mm_struct.mmlist
with an XArray containing all mm_structs.  Then we can use one mark
to indicate maybe-swapped and another mark to indicate ... whatever it
is pgd_list indicates.  Iterating an XArray (whether the entire thing
or with marks) is RCU-safe and faster than iterating a linked list,
so this should solve the problem?