Re: [GIT PULL] Please pull hmm changes

[Jason Gunthorpe <jgg@mellanox.com>]

On Sat, Nov 30, 2019 at 10:23:31AM -0800, Linus Torvalds wrote:
> On Sat, Nov 30, 2019 at 10:03 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > I'll try to figure the code out, but my initial reaction was "yeah,
> > not in my VM".
> 
> Why is it ok to sometimes do
> 
>     WRITE_ONCE(mni->invalidate_seq, cur_seq);
> 
> (to pair with the unlocked READ_ONCE), and sometimes then do
> 
>     mni->invalidate_seq = mmn_mm->invalidate_seq;
> 
> My initial guess was that latter is only done at initialization time,
> but at least in one case it's done *after* the mni has been added to
> the mmn_mm (oh, how I despise those names - I can only repeat: WTF?).

Yes, the only occurrences are in the notifier_insert, under the
spinlock. The one case where it is out of the natural order was to
make the manipulation of seq a bit saner, but in all cases since the
spinlock is held there is no way for another thread to get the pointer
to the 'mmu_interval_notifier *' to do the unlocked read.

Regarding the ugly names.. Naming has been really hard here because
currently everything is a 'mmu notifier' and the natural abberviations
from there are crummy. Here is the basic summary:

struct mmu_notifier_mm (ie the mm->mmu_notifier_mm)
   -> mmn_mm
struct mm_struct 
   -> mm
struct mmu_notifier (ie the user subscription to the mm_struct)
   -> mn
struct mmu_interval_notifier (the other kind of user subscription)
   -> mni
struct mmu_notifier_range (ie the args to invalidate_range)
   -> range

I can send a patch to switch mmn_mm to mmu_notifier_mm, which is the
only pre-existing name for this value. But IIRC, it is a somewhat ugly
with long line wrapping. 'mni' is a pain, I have to reflect on that.
(honesly, I dislike mmu_notififer_mm quite a lot too)

I think it would be overall nicer with better names for the original
structs. Perhaps:

 mmn_* - MMU notifier prefix
 mmn_state <- struct mmu_notifier_mm
 mmn_subscription (mmn_sub) <- struct mmu_notifier
 mmn_range_subscription (mmn_range_sub) <- struct mmu_interval_notifier
 mmn_invalidate_desc <- struct mmu_notifier_range

At least this is how I describe them in my mind..  This is a lot of
churn, and spreads through many drivers. This is why I kept the names
as-is and we ended up with the also quite bad 'mmu_interval_notifier'

Maybe just switch mmu_notifier_mm for mmn_state and leave the drivers
alone?

Anyone on the CC list have advice?

Jason