Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

[Ahmed S. Darwish]

On Fri, Oct 30, 2020 at 08:51:21PM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 30, 2020 at 06:52:50PM -0400, Peter Xu wrote:

...

>
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index c48f8df6e50268..294c2c3c4fe00d 100644
> > > +++ b/mm/memory.c
> > > @@ -1171,6 +1171,12 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
> > >  		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
> > >  					0, src_vma, src_mm, addr, end);
> > >  		mmu_notifier_invalidate_range_start(&range);
> > > +		/*
> > > +		 * The read side doesn't spin, it goes to the mmap_lock, so the
> > > +		 * raw version is used to avoid disabling preemption here
> > > +		 */
> > > +		mmap_assert_write_locked(src_mm);
> > > +		raw_write_seqcount_t_begin(&src_mm->write_protect_seq);
> >
> > Would raw_write_seqcount_begin() be better here?
>
> Hum..
>
> I felt no because it had the preempt stuff added into it, however it
> would work - __seqcount_lock_preemptible() == false for the seqcount_t
> case (see below)
>
> Looking more closely, maybe the right API to pick is
> write_seqcount_t_begin() and write_seqcount_t_end() ??
>

No, that's not the right API: it is also internal to seqlock.h.

Please stick with the official exported API: raw_write_seqcount_begin().

It should satisfy your needs, and the raw_*() variant is created exactly
for contexts wishing to avoid the lockdep checks (e.g. NMI handlers
cannot invoke lockdep, etc.)

> However, no idea what the intention of the '*_seqcount_t_*' family is
> - it only seems to be used to implement the seqlock..
>

Exactly. '*_seqcount_t_*' is a seqlock.h implementation detail, and it
has _zero_ relevance to what is discussed in this thread actually.

...

> Ahmed explained in commit 8117ab508f the reason the seqcount_t write
> side has preemption disabled is because it can livelock RT kernels if
> the read side is spinning after preempting the write side. eg look at
> how __read_seqcount_begin() is implemented:
>
> 	while ((seq = __seqcount_sequence(s)) & 1)			\
> 		cpu_relax();						\
>
> However, in this patch, we don't spin on the read side.
>
> If the read side collides with a writer it immediately goes to the
> mmap_lock, which is sleeping, and so it will sort itself out properly,
> even if it was preempted.
>

Correct.

Thanks,

--
Ahmed Darwish
Linutronix GmbH