[ofa-general] Re: [PATCH] mmu notifiers #v7

[Andrea Arcangeli]

On Thu, Feb 28, 2008 at 03:05:30PM -0800, Christoph Lameter wrote:
> Still think that the lock here is not of too much use and can be easily 
> replaced by mmap_sem.

I can use the mmap_sem.

> > +#define mmu_notifier(function, mm, args...)				\
> > +	do {								\
> > +		struct mmu_notifier *__mn;				\
> > +		struct hlist_node *__n;					\
> > +									\
> > +		if (unlikely(!hlist_empty(&(mm)->mmu_notifier.head))) { \
> > +			rcu_read_lock();				\
> > +			hlist_for_each_entry_rcu(__mn, __n,		\
> > +						 &(mm)->mmu_notifier.head, \
> > +						 hlist)			\
> > +				if (__mn->ops->function)		\
> > +					__mn->ops->function(__mn,	\
> > +							    mm,		\
> > +							    args);	\
> > +			rcu_read_unlock();				\
> > +		}							\
> > +	} while (0)
> 
> Andrew recomended local variables for parameters used multile times. This 
> means the mm parameter here.

I don't exactly see what "buggy macro" meant? I already use
parenthesis as needed to avoid the need of local variables to be
safe. Not really sure what's buggy, sorry!

> Note also Andrew's comments on the use of 0x00ff...

I thought I tried the (void) but it didn't work and your solution
worked, but perhaps I did something wrong, I'll try again with (void)
nevertheless.

> > +/*
> > + * No synchronization. This function can only be called when only a single
> > + * process remains that performs teardown.
> > + */
> > +void mmu_notifier_release(struct mm_struct *mm)
> > +{
> > +	struct mmu_notifier *mn;
> > +	struct hlist_node *n, *tmp;
> > +
> > +	if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
> > +		hlist_for_each_entry_safe(mn, n, tmp,
> > +					  &mm->mmu_notifier.head, hlist) {
> > +			hlist_del(&mn->hlist);
> > +			if (mn->ops->release)
> > +				mn->ops->release(mn, mm);
> > +		}
> > +	}
> > +}
> 
> One could avoid a hlist_for_each_entry_safe here by simply always deleting 
> the first object. 

Agreed, the current construct come from the fact we previously didn't
assume nobody could ever call mmu_notifier_unregister by the time
mm_users is 0.

> Also re the _notify variants: The binding to pte_clear_flush_young etc 
> will become a problem for notifiers that want to sleep because 
> pte_clear_flush is usually called with the pte lock held. See f.e. 
> try_to_unmap_one, page_mkclean_one etc.

Calling __free_page out of the PT lock is much bigger
change. do_wp_page will require changes anyway when the sleepable
notifiers are merged.

> It would be better if the notifier calls could be moved outside of the 
> pte lock.

The point is that it can't make a difference right now, and my
objective was to avoid unnecessary source code duplication (later it
will be necessary, right now it isn't). By the time you rework
do_wp_page, removing _notify will be a very minor detail compared to
the rest of the changes to do_wp_page IMHO. Expanding it now won't
provide a real advantage later.