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Commit c2a7d2a1 authored by Dan Williams's avatar Dan Williams Committed by Dave Jiang
Browse files

filesystem-dax: Introduce dax_lock_mapping_entry() 



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@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
Signed-off-by: default avatarDave Jiang <dave.jiang@intel.com>
parent ae1139ec

fs/dax.c

+103 −6
Original line number Diff line number Diff line
@@ -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

include/linux/dax.h

+13 −0
Original line number Diff line number Diff line
@@ -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);