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 02:46:28PM +0200, Christian Borntraeger wrote:
> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes:
> > Writing to file-backed dirty-tracked mappings via GUP is inherently broken
> > as we cannot rule out folios being cleaned and then a GUP user writing to
> > them again and possibly marking them dirty unexpectedly.
> >
> > This is especially egregious for long-term mappings (as indicated by the
> > use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as
> > we have already done in the slow path.
>
> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery?
> It would no longer work with file backed memory, correct?
>
> See
> arch/s390/kvm/pci.c
>
> kvm_s390_pci_aif_enable
> which does have
> FOLL_WRITE | FOLL_LONGTERM
> to
>

Does this memory map a dirty-tracked file? It's kind of hard to dig into where
the address originates from without going through a ton of code. In worst case
if the fast code doesn't find a whitelist it'll fall back to slow path which
explicitly checks for dirty-tracked filesystem.

We can reintroduce a flag to permit exceptions if this is really broken, are you
able to test? I don't have an s390 sat around :)

> >
> > We have access to less information in the fast path as we cannot examine
> > the VMA containing the mapping, however we can determine whether the folio
> > is anonymous and then whitelist known-good mappings - specifically hugetlb
> > and shmem mappings.
> >
> > While we obtain a stable folio for this check, the mapping might not be, as
> > a truncate could nullify it at any time. Since doing so requires mappings
> > to be zapped, we can synchronise against a TLB shootdown operation.
> >
> > For some architectures TLB shootdown is synchronised by IPI, against which
> > we are protected as the GUP-fast operation is performed with interrupts
> > disabled. However, other architectures which specify
> > CONFIG_MMU_GATHER_RCU_TABLE_FREE use an RCU lock for this operation.
> >
> > In these instances, we acquire an RCU lock while performing our checks. If
> > we cannot get a stable mapping, we fall back to the slow path, as otherwise
> > we'd have to walk the page tables again and it's simpler and more effective
> > to just fall back.
> >
> > It's important to note that there are no APIs allowing users to specify
> > FOLL_FAST_ONLY for a PUP-fast let alone with FOLL_LONGTERM, so we can
> > always rely on the fact that if we fail to pin on the fast path, the code
> > will fall back to the slow path which can perform the more thorough check.
> >
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Suggested-by: Kirill A . Shutemov <kirill@xxxxxxxxxxxxx>
> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > ---
> >   mm/gup.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 85 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 0f09dec0906c..431618048a03 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -18,6 +18,7 @@
> >   #include <linux/migrate.h>
> >   #include <linux/mm_inline.h>
> >   #include <linux/sched/mm.h>
> > +#include <linux/shmem_fs.h>
> >   #include <asm/mmu_context.h>
> >   #include <asm/tlbflush.h>
> > @@ -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);
> > +}
> > +
> > +static void unlock_rcu(void)
> > +{
> > +	rcu_read_unlock();
> > +}
> > +#else
> > +static bool stabilise_mapping_rcu(struct folio *)
> > +{
> > +	return true;
> > +}
> > +
> > +static void unlock_rcu(void)
> > +{
> > +}
> > +#endif
> > +
> > +/*
> > + * 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;
> > +
> > +	if (stabilise_mapping_rcu(folio)) {
> > +		struct address_space *mapping = folio_mapping(folio);
> > +
> > +		/*
> > +		 * 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;
> > +}
> > +
> >   /**
> >    * try_grab_folio() - Attempt to get or pin a folio.
> >    * @page:  pointer to page to be grabbed
> > @@ -123,6 +195,8 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
> >    */
> >   struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
> >   {
> > +	bool is_longterm = flags & FOLL_LONGTERM;
> > +
> >   	if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
> >   		return NULL;
> > @@ -136,8 +210,7 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
> >   		 * right zone, so fail and let the caller fall back to the slow
> >   		 * path.
> >   		 */
> > -		if (unlikely((flags & FOLL_LONGTERM) &&
> > -			     !is_longterm_pinnable_page(page)))
> > +		if (unlikely(is_longterm && !is_longterm_pinnable_page(page)))
> >   			return NULL;
> >   		/*
> > @@ -148,6 +221,16 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
> >   		if (!folio)
> >   			return NULL;
> > +		/*
> > +		 * Can this folio be safely pinned? We need to perform this
> > +		 * check after the folio is stabilised.
> > +		 */
> > +		if ((flags & FOLL_WRITE) && is_longterm &&
> > +		    !folio_longterm_write_pin_allowed(folio)) {
> > +			folio_put_refs(folio, refs);
> > +			return NULL;
> > +		}
> > +
> >   		/*
> >   		 * When pinning a large folio, use an exact count to track it.
> >   		 *