Re: [PATCH v4 6/6] shm: wait for pins to be released when sealing

[Hugh Dickins <hughd@google.com>]

On Sun, 20 Jul 2014, David Herrmann wrote:

> If we set SEAL_WRITE on a file, we must make sure there cannot be any
> ongoing write-operations on the file. For write() calls, we simply lock
> the inode mutex, for mmap() we simply verify there're no writable
> mappings. However, there might be pages pinned by AIO, Direct-IO and
> similar operations via GUP. We must make sure those do not write to the
> memfd file after we set SEAL_WRITE.
> 
> As there is no way to notify GUP users to drop pages or to wait for them
> to be done, we implement the wait ourself: When setting SEAL_WRITE, we
> check all pages for their ref-count. If it's bigger than 1, we know
> there's some user of the page. We then mark the page and wait for up to
> 150ms for those ref-counts to be dropped. If the ref-counts are not
> dropped in time, we refuse the seal operation.
> 
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

Acked-by: Hugh Dickins <hughd@google.com>

I'd have moved this one up before the testing ones - except changing
the sequence in between postings can be confusing.  I'd be happy if
akpm happened to move it up - but unconcerned if he did not.

> ---
>  mm/shmem.c | 110 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 109 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 770e072..df1aceb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1780,9 +1780,117 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
>  	return offset;
>  }
>  
> +/*
> + * We need a tag: a new tag would expand every radix_tree_node by 8 bytes,
> + * so reuse a tag which we firmly believe is never set or cleared on shmem.
> + */
> +#define SHMEM_TAG_PINNED        PAGECACHE_TAG_TOWRITE
> +#define LAST_SCAN               4       /* about 150ms max */
> +
> +static void shmem_tag_pins(struct address_space *mapping)
> +{
> +	struct radix_tree_iter iter;
> +	void **slot;
> +	pgoff_t start;
> +	struct page *page;
> +
> +	lru_add_drain();
> +	start = 0;
> +	rcu_read_lock();
> +
> +restart:
> +	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
> +		page = radix_tree_deref_slot(slot);
> +		if (!page || radix_tree_exception(page)) {
> +			if (radix_tree_deref_retry(page))
> +				goto restart;
> +		} else if (page_count(page) - page_mapcount(page) > 1) {
> +			spin_lock_irq(&mapping->tree_lock);
> +			radix_tree_tag_set(&mapping->page_tree, iter.index,
> +					   SHMEM_TAG_PINNED);
> +			spin_unlock_irq(&mapping->tree_lock);
> +		}
> +
> +		if (need_resched()) {
> +			cond_resched_rcu();
> +			start = iter.index + 1;
> +			goto restart;
> +		}
> +	}
> +	rcu_read_unlock();
> +}
> +
> +/*
> + * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
> + * via get_user_pages(), drivers might have some pending I/O without any active
> + * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
> + * and see whether it has an elevated ref-count. If so, we tag them and wait for
> + * them to be dropped.
> + * The caller must guarantee that no new user will acquire writable references
> + * to those pages to avoid races.
> + */
>  static int shmem_wait_for_pins(struct address_space *mapping)
>  {
> -	return 0;
> +	struct radix_tree_iter iter;
> +	void **slot;
> +	pgoff_t start;
> +	struct page *page;
> +	int error, scan;
> +
> +	shmem_tag_pins(mapping);
> +
> +	error = 0;
> +	for (scan = 0; scan <= LAST_SCAN; scan++) {
> +		if (!radix_tree_tagged(&mapping->page_tree, SHMEM_TAG_PINNED))
> +			break;
> +
> +		if (!scan)
> +			lru_add_drain_all();
> +		else if (schedule_timeout_killable((HZ << scan) / 200))
> +			scan = LAST_SCAN;
> +
> +		start = 0;
> +		rcu_read_lock();
> +restart:
> +		radix_tree_for_each_tagged(slot, &mapping->page_tree, &iter,
> +					   start, SHMEM_TAG_PINNED) {
> +
> +			page = radix_tree_deref_slot(slot);
> +			if (radix_tree_exception(page)) {
> +				if (radix_tree_deref_retry(page))
> +					goto restart;
> +
> +				page = NULL;
> +			}
> +
> +			if (page &&
> +			    page_count(page) - page_mapcount(page) != 1) {
> +				if (scan < LAST_SCAN)
> +					goto continue_resched;
> +
> +				/*
> +				 * On the last scan, we clean up all those tags
> +				 * we inserted; but make a note that we still
> +				 * found pages pinned.
> +				 */
> +				error = -EBUSY;
> +			}
> +
> +			spin_lock_irq(&mapping->tree_lock);
> +			radix_tree_tag_clear(&mapping->page_tree,
> +					     iter.index, SHMEM_TAG_PINNED);
> +			spin_unlock_irq(&mapping->tree_lock);
> +continue_resched:
> +			if (need_resched()) {
> +				cond_resched_rcu();
> +				start = iter.index + 1;
> +				goto restart;
> +			}
> +		}
> +		rcu_read_unlock();
> +	}
> +
> +	return error;
>  }
>  
>  #define F_ALL_SEALS (F_SEAL_SEAL | \
> -- 
> 2.0.2
> 
> 

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