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Date:   Tue, 2 May 2023 13:20:40 +0200
From:   Jan Kara <jack@suse.cz>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
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Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings
Message-ID: <20230502112040.zkyogi46f3zl33he@quack3>
References: <cover.1682981880.git.lstoakes@gmail.com>
 <dee4f4ad6532b0f94d073da263526de334d5d7e0.1682981880.git.lstoakes@gmail.com>
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On Tue 02-05-23 00:11:49, Lorenzo Stoakes wrote:
> 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.
> 
> 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@redhat.com>
> Suggested-by: Kirill A . Shutemov <kirill@shutemov.name>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>  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

So I wonder is this complexity worth it over just using rcu_read_lock()
unconditionally? It is just a compilation barrier AFAIK...

Also stabilise_mapping_rcu() seems to be a bit of a misnomer since the
mapping is not stable after the function is called. Also the return value
seems a bit pointless to me since we have to check folio_mapping() for
being != NULL anyway. All in all I'd say that:

	struct address_space *mapping;

	/* Make sure mapping cannot be freed under our hands */
	rcu_read_lock();
	mapping = folio_mapping(folio);
	ret = folio_test_anon(folio) || (mapping && shmem_mapping(mapping));
	rcu_read_unlock();

looks more comprehensible...

								Honza

> +
> +/*
> + * 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.
>  		 *
> -- 
> 2.40.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR