Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

Jan Kara
2020-09-18T09:44:44Z

On Thu 17-09-20 19:09:00, Jason Gunthorpe wrote:
> On Thu, Sep 17, 2020 at 05:40:59PM -0400, Peter Xu wrote:
> > On Thu, Sep 17, 2020 at 01:35:56PM -0700, Linus Torvalds wrote:
> > > For that to happen, we'd need to have the vma flag so that we wouldn't
> > > have any worry about non-pinners, but as you suggested, I think even
> > > just a mm-wide counter - or flag - to deal with the fast-bup case is
> > > likely perfectly sufficient.
> >
> > Would mm_struct.pinned_vm suffice?
>
> I think that could be a good long term goal
>
> IIRC last time we dug into the locked_vm vs pinned_vm mess it didn't
> get fixed. There is a mix of both kinds, as you saw, and some
> resistance I don't clearly remember to changing it.
>
> My advice for this -rc fix is to go with a single bit in the mm_struct
> set on any call to pin_user_pages*
>
> Then only users using pin_user_pages and forking are the only ones who
> would ever do extra COW on fork. I think that is OK for -rc, this
> workload should be rare due to the various historical issues. Anyhow,
> a slow down regression is better than a it is broken regression.

Agreed. I really like the solution of not write-protecting pinned pages on
fork(2).

> This can be improved into a counter later. Due to the pinned_vm
> accounting all call sites should have the mm_struct at unpin, but I
> have a feeling it will take a alot of driver patches to sort it all
> out.

I somewhat fear that some of the users of pin_user_pages() don't bother
with pinned_vm accounting exactly because they don't have mm_struct on
unpin...

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR