Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[David Hildenbrand <david@redhat.com>]

[...]

> +{
> +	struct address_space *mapping;
> +
> +	/*
> +	 * GUP-fast disables IRQs - this prevents IPIs from causing page tables
> +	 * to disappear from under us, as well as preventing RCU grace periods
> +	 * from making progress (i.e. implying rcu_read_lock()).
> +	 *
> +	 * This means we can rely on the folio remaining stable for all
> +	 * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +	 * and those that do not.
> +	 *
> +	 * We get the added benefit that given inodes, and thus address_space,
> +	 * objects are RCU freed, we can rely on the mapping remaining stable
> +	 * here with no risk of a truncation or similar race.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * If no mapping can be found, this implies an anonymous or otherwise
> +	 * non-file backed folio so in this instance we permit the pin.
> +	 *
> +	 * shmem and hugetlb mappings do not require dirty-tracking so we
> +	 * explicitly whitelist these.
> +	 *
> +	 * Other non dirty-tracked folios will be picked up on the slow path.
> +	 */
> +	mapping = folio_mapping(folio);
> +	return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio);

"Folios in the swap cache return the swap mapping" -- you might disallow 
pinning anonymous pages that are in the swap cache.

I recall that there are corner cases where we can end up with an anon 
page that's mapped writable but still in the swap cache ... so you'd 
fallback to the GUP slow path (acceptable for these corner cases, I 
guess), however especially the comment is a bit misleading then.

So I'd suggest not dropping the folio_test_anon() check, or open-coding 
it ... which will make this piece of code most certainly easier to get 
when staring at folio_mapping(). Or to spell it out in the comment 
(usually I prefer code over comments).

> +}
> +
>   /**
>    * try_grab_folio() - Attempt to get or pin a folio.
>    * @page:  pointer to page to be grabbed
> @@ -123,6 +170,8 @@ 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 +185,7 @@ 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 +196,16 @@ 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;
> +		}

So we perform this change before validating whether the PTE changed.

Hmm, naturally, I would have done it afterwards.

IIRC, without IPI syncs during TLB flush (i.e., 
CONFIG_MMU_GATHER_RCU_TABLE_FREE), there is the possibility that
(1) We lookup the pte
(2) The page was unmapped and free
(3) The page gets reallocated and used
(4) We pin the page
(5) We dereference page->mapping

If we then de-reference page->mapping that gets used by whoever 
allocated it for something completely different (not a pointer to 
something reasonable), I wonder if we might be in trouble.

Checking first, whether the PTE changed makes sure that what we pinned 
and what we're looking at is what we expected.

... I can spot that the page_is_secretmem() check is also done before 
that. But it at least makes sure that it's still an LRU page before 
staring at the mapping (making it a little safer?).

BUT, I keep messing up this part of the story. Maybe it all works as 
expected because we will be synchronizing RCU somehow before actually 
freeing the page in the !IPI case. ... but I think that's only true for 
page tables with CONFIG_MMU_GATHER_RCU_TABLE_FREE.

-- 
Thanks,

David / dhildenb