Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE)

[David Hildenbrand]

> > > +
> > > +out_retry:
> > > +	put_page(page);
> > > +	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > > +		vma_end_read(vma);
> > > +	if (fault_flag_allow_retry_first(vmf->flags)) {
> > > +		err = VM_FAULT_RETRY;
> > > +	} else {
> > > +		/* Replay the fault. */
> > > +		err = 0;
> >
> > Hello!
> >
> > Unfortunately, if the page continues to be pinned, it seems like fault will continue to occur.
> > I guess it makes system stability issue. (but I'm not familiar with that, so please let me know if I'm mistaken!)
> >
> > How about migrating the page when migration problem repeats.
>
> Yes, I had the same though in the previous iteration of the series, the
> page was migrated out of the VMA if tag storage couldn't be reserved.
>
> Only short term pins are allowed on MIGRATE_CMA pages, so I expect that the
> pin will be released before the fault is replayed. Because of this, and
> because it makes the code simpler, I chose not to migrate the page if tag
> storage couldn't be reserved.

There are still some cases that are theoretically problematic: 
vmsplice() can pin pages forever and doesn't use FOLL_LONGTERM yet.

All these things also affect other users that rely on movability (e.g., 
CMA, memory hotunplug).

--
Cheers,

David / dhildenb