Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes]

On Tue, May 02, 2023 at 01:13:34PM +0200, Peter Zijlstra wrote:
> On Tue, May 02, 2023 at 12:11:49AM +0100, Lorenzo Stoakes wrote:
> > @@ -95,6 +96,77 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
> >  	return folio;
> >  }
> >
> > +#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
> > +static bool stabilise_mapping_rcu(struct folio *folio)
> > +{
> > +	struct address_space *mapping = READ_ONCE(folio->mapping);
> > +
> > +	rcu_read_lock();
> > +
> > +	return mapping == READ_ONCE(folio->mapping);
>
> This doesn't make sense; why bother reading the same thing twice?

The intent is to see whether the folio->mapping has been truncated from
underneath us, as per the futex code that Kirill referred to which does
something similar [1].

>
> Who cares if the thing changes from before; what you care about is that
> the value you see has stable storage, this doesn't help with that.
>
> > +}
> > +
> > +static void unlock_rcu(void)
> > +{
> > +	rcu_read_unlock();
> > +}
> > +#else
> > +static bool stabilise_mapping_rcu(struct folio *)
> > +{
> > +	return true;
> > +}
> > +
> > +static void unlock_rcu(void)
> > +{
> > +}
> > +#endif
>
> Anyway, this all can go away. RCU can't progress while you have
> interrupts disabled anyway.

There seems to be other code in the kernel that assumes that this is not
the case, i.e. the futex code, though not sure if that's being run with
IRQs disabled... if not and it's absolutely certain that we need no special
handling for the RCU case, then happy days and more than glad to remove
this bit.

I'm far from an expert on RCU (I need to gain a better understanding of it)
so I'm deferring how best to proceed on _this part_ to the community.

>
> > +/*
> > + * Used in the GUP-fast path to determine whether a FOLL_PIN | FOLL_LONGTERM |
> > + * FOLL_WRITE pin is permitted for a specific folio.
> > + *
> > + * This assumes the folio is stable and pinned.
> > + *
> > + * Writing to pinned file-backed dirty tracked folios is inherently problematic
> > + * (see comment describing the writeable_file_mapping_allowed() function). We
> > + * therefore try to avoid the most egregious case of a long-term mapping doing
> > + * so.
> > + *
> > + * This function cannot be as thorough as that one as the VMA is not available
> > + * in the fast path, so instead we whitelist known good cases.
> > + *
> > + * The folio is stable, but the mapping might not be. When truncating for
> > + * instance, a zap is performed which triggers TLB shootdown. IRQs are disabled
> > + * so we are safe from an IPI, but some architectures use an RCU lock for this
> > + * operation, so we acquire an RCU lock to ensure the mapping is stable.
> > + */
> > +static bool folio_longterm_write_pin_allowed(struct folio *folio)
> > +{
> > +	bool ret;
> > +
> > +	/* hugetlb mappings do not require dirty tracking. */
> > +	if (folio_test_hugetlb(folio))
> > +		return true;
> > +
>
> This:
>
> > +	if (stabilise_mapping_rcu(folio)) {
> > +		struct address_space *mapping = folio_mapping(folio);
>
> And this is 3rd read of folio->mapping, just for giggles?

I like to giggle :)

Actually this is to handle the various cases in which the mapping might not
be what we want (i.e. have PAGE_MAPPING_FLAGS set) which doesn't appear to
have a helper exposed for a check. Given previous review about duplication
I felt best to reuse this even though it does access again... yes I felt
weird about doing that.

>
> > +
> > +		/*
> > +		 * Neither anonymous nor shmem-backed folios require
> > +		 * dirty tracking.
> > +		 */
> > +		ret = folio_test_anon(folio) ||
> > +			(mapping && shmem_mapping(mapping));
> > +	} else {
> > +		/* If the mapping is unstable, fallback to the slow path. */
> > +		ret = false;
> > +	}
> > +
> > +	unlock_rcu();
> > +
> > +	return ret;
>
> then becomes:
>
>
> 	if (folio_test_anon(folio))
> 		return true;

This relies on the mapping so belongs below the lockdep assert imo.

>
> 	/*
> 	 * Having IRQs disabled (as per GUP-fast) also inhibits RCU
> 	 * grace periods from making progress, IOW. they imply
> 	 * rcu_read_lock().
> 	 */
> 	lockdep_assert_irqs_disabled();
>
> 	/*
> 	 * Inodes and thus address_space are RCU freed and thus safe to
> 	 * access at this point.
> 	 */
> 	mapping = folio_mapping(folio);
> 	if (mapping && shmem_mapping(mapping))
> 		return true;
>
> 	return false;
>
> > +}

I'm more than happy to do this (I'd rather drop the RCU bits if possible)
but need to be sure it's safe.