Re: [PATCH v6 09/13] filesystem-dax: Introduce dax_lock_mapping_entry()

[Jan Kara]

On Fri 13-07-18 21:50:16, Dan Williams wrote:
> In preparation for implementing support for memory poison (media error)
> handling via dax mappings, implement a lock_page() equivalent. Poison
> error handling requires rmap and needs guarantees that the page->mapping
> association is maintained / valid (inode not freed) for the duration of
> the lookup.
> 
> In the device-dax case it is sufficient to simply hold a dev_pagemap
> reference. In the filesystem-dax case we need to use the entry lock.
> 
> Export the entry lock via dax_lock_mapping_entry() that uses
> rcu_read_lock() to protect against the inode being freed, and
> revalidates the page->mapping association under xa_lock().
> 
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Just got back from vacation... This patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza


> ---
>  fs/dax.c            |  109 ++++++++++++++++++++++++++++++++++++++++++++++++---
>  include/linux/dax.h |   13 ++++++
>  2 files changed, 116 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 4de11ed463ce..57ec272038da 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -226,8 +226,8 @@ static inline void *unlock_slot(struct address_space *mapping, void **slot)
>   *
>   * Must be called with the i_pages lock held.
>   */
> -static void *get_unlocked_mapping_entry(struct address_space *mapping,
> -					pgoff_t index, void ***slotp)
> +static void *__get_unlocked_mapping_entry(struct address_space *mapping,
> +		pgoff_t index, void ***slotp, bool (*wait_fn)(void))
>  {
>  	void *entry, **slot;
>  	struct wait_exceptional_entry_queue ewait;
> @@ -237,6 +237,8 @@ static void *get_unlocked_mapping_entry(struct address_space *mapping,
>  	ewait.wait.func = wake_exceptional_entry_func;
>  
>  	for (;;) {
> +		bool revalidate;
> +
>  		entry = __radix_tree_lookup(&mapping->i_pages, index, NULL,
>  					  &slot);
>  		if (!entry ||
> @@ -251,14 +253,31 @@ static void *get_unlocked_mapping_entry(struct address_space *mapping,
>  		prepare_to_wait_exclusive(wq, &ewait.wait,
>  					  TASK_UNINTERRUPTIBLE);
>  		xa_unlock_irq(&mapping->i_pages);
> -		schedule();
> +		revalidate = wait_fn();
>  		finish_wait(wq, &ewait.wait);
>  		xa_lock_irq(&mapping->i_pages);
> +		if (revalidate)
> +			return ERR_PTR(-EAGAIN);
>  	}
>  }
>  
> -static void dax_unlock_mapping_entry(struct address_space *mapping,
> -				     pgoff_t index)
> +static bool entry_wait(void)
> +{
> +	schedule();
> +	/*
> +	 * Never return an ERR_PTR() from
> +	 * __get_unlocked_mapping_entry(), just keep looping.
> +	 */
> +	return false;
> +}
> +
> +static void *get_unlocked_mapping_entry(struct address_space *mapping,
> +		pgoff_t index, void ***slotp)
> +{
> +	return __get_unlocked_mapping_entry(mapping, index, slotp, entry_wait);
> +}
> +
> +static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)
>  {
>  	void *entry, **slot;
>  
> @@ -277,7 +296,7 @@ static void dax_unlock_mapping_entry(struct address_space *mapping,
>  static void put_locked_mapping_entry(struct address_space *mapping,
>  		pgoff_t index)
>  {
> -	dax_unlock_mapping_entry(mapping, index);
> +	unlock_mapping_entry(mapping, index);
>  }
>  
>  /*
> @@ -374,6 +393,84 @@ static struct page *dax_busy_page(void *entry)
>  	return NULL;
>  }
>  
> +static bool entry_wait_revalidate(void)
> +{
> +	rcu_read_unlock();
> +	schedule();
> +	rcu_read_lock();
> +
> +	/*
> +	 * Tell __get_unlocked_mapping_entry() to take a break, we need
> +	 * to revalidate page->mapping after dropping locks
> +	 */
> +	return true;
> +}
> +
> +bool dax_lock_mapping_entry(struct page *page)
> +{
> +	pgoff_t index;
> +	struct inode *inode;
> +	bool did_lock = false;
> +	void *entry = NULL, **slot;
> +	struct address_space *mapping;
> +
> +	rcu_read_lock();
> +	for (;;) {
> +		mapping = READ_ONCE(page->mapping);
> +
> +		if (!dax_mapping(mapping))
> +			break;
> +
> +		/*
> +		 * In the device-dax case there's no need to lock, a
> +		 * struct dev_pagemap pin is sufficient to keep the
> +		 * inode alive, and we assume we have dev_pagemap pin
> +		 * otherwise we would not have a valid pfn_to_page()
> +		 * translation.
> +		 */
> +		inode = mapping->host;
> +		if (S_ISCHR(inode->i_mode)) {
> +			did_lock = true;
> +			break;
> +		}
> +
> +		xa_lock_irq(&mapping->i_pages);
> +		if (mapping != page->mapping) {
> +			xa_unlock_irq(&mapping->i_pages);
> +			continue;
> +		}
> +		index = page->index;
> +
> +		entry = __get_unlocked_mapping_entry(mapping, index, &slot,
> +				entry_wait_revalidate);
> +		if (!entry) {
> +			xa_unlock_irq(&mapping->i_pages);
> +			break;
> +		} else if (IS_ERR(entry)) {
> +			WARN_ON_ONCE(PTR_ERR(entry) != -EAGAIN);
> +			continue;
> +		}
> +		lock_slot(mapping, slot);
> +		did_lock = true;
> +		xa_unlock_irq(&mapping->i_pages);
> +		break;
> +	}
> +	rcu_read_unlock();
> +
> +	return did_lock;
> +}
> +
> +void dax_unlock_mapping_entry(struct page *page)
> +{
> +	struct address_space *mapping = page->mapping;
> +	struct inode *inode = mapping->host;
> +
> +	if (S_ISCHR(inode->i_mode))
> +		return;
> +
> +	unlock_mapping_entry(mapping, page->index);
> +}
> +
>  /*
>   * Find radix tree entry at given index. If it points to an exceptional entry,
>   * return it with the radix tree entry locked. If the radix tree doesn't
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 3855e3800f48..cf8ac51cf0d7 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -88,6 +88,8 @@ int dax_writeback_mapping_range(struct address_space *mapping,
>  		struct block_device *bdev, struct writeback_control *wbc);
>  
>  struct page *dax_layout_busy_page(struct address_space *mapping);
> +bool dax_lock_mapping_entry(struct page *page);
> +void dax_unlock_mapping_entry(struct page *page);
>  #else
>  static inline bool bdev_dax_supported(struct block_device *bdev,
>  		int blocksize)
> @@ -119,6 +121,17 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping,
>  {
>  	return -EOPNOTSUPP;
>  }
> +
> +static inline bool dax_lock_mapping_entry(struct page *page)
> +{
> +	if (IS_DAX(page->mapping->host))
> +		return true;
> +	return false;
> +}
> +
> +static inline void dax_unlock_mapping_entry(struct page *page)
> +{
> +}
>  #endif
>  
>  int dax_read_lock(void);
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR