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 12:25:54PM +0100, Lorenzo Stoakes wrote:
> 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.

Sorry forgot to include the [1]

[1]:https://lore.kernel.org/all/20230428234332.2vhprztuotlqir4x@xxxxxxxxxxxxxxxxx/