Re: [PATCH V8 38/44] memremap_pages: Define pgmap_mk_{readwrite|noaccess}() calls

[Dan Williams]

On Fri, Feb 4, 2022 at 4:25 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Fri, Feb 4, 2022 at 4:19 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > On Fri, Feb 4, 2022 at 4:10 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
> > >
> > > On Fri, Feb 04, 2022 at 10:35:59AM -0800, Dan Williams wrote:
> > > > On Thu, Jan 27, 2022 at 9:55 AM <ira.weiny@xxxxxxxxx> wrote:
> > > > >
> > >
> > > [snip]
> > >
> > > I'll address the other comments later but wanted to address the idea below.
> > >
> > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > > index f5b2be39a78c..5020ed7e67b7 100644
> > > > > --- a/include/linux/sched.h
> > > > > +++ b/include/linux/sched.h
> > > > > @@ -1492,6 +1492,13 @@ struct task_struct {
> > > > >         struct callback_head            l1d_flush_kill;
> > > > >  #endif
> > > > >
> > > > > +#ifdef CONFIG_DEVMAP_ACCESS_PROTECTION
> > > > > +       /*
> > > > > +        * NOTE: pgmap_prot_count is modified within a single thread of
> > > > > +        * execution.  So it does not need to be atomic_t.
> > > > > +        */
> > > > > +       u32                             pgmap_prot_count;
> > > > > +#endif
> > > >
> > > > It's not at all clear why the task struct needs to be burdened with
> > > > this accounting. Given that a devmap instance is needed to manage page
> > > > protections, why not move the nested protection tracking to a percpu
> > > > variable relative to an @pgmap arg? Something like:
> > > >
> > > > void __pgmap_mk_readwrite(struct dev_pagemap *pgmap)
> > > > {
> > > >        migrate_disable();
> > > >        preempt_disable();
> > >
> > > Why burden threads like this?  kmap_local_page() is perfectly able to migrate
> > > or be preempted?
> > >
> > > I think this is way to restrictive.
> >
> > kmap_local_page() holds migrate_disable() over the entire mapping, so
> > we're only talking about preempt_disable(). I tend to think that
> > bloating task_struct for something that is rarely used "kmap on dax
> > pmem pages" is not the right tradeoff.
>
> Now, I can see an argument that promoting kmap_local_page() to
> preempt_disable() could cause problems, but I'd like help confirming
> that before committing to extending task_struct.

...as I say that it occurs to me that the whole point of
kmap_local_page() is to be better than kmap_atomic() and this undoes
that. I'd at least like that documented as the reason that task_struct
needs to carry a new field.