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SubjectRe: [PATCH v6 09/13] filesystem-dax: Introduce dax_lock_mapping_entry()
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@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: Dan Williams <dan.j.williams@intel.com>

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

Reviewed-by: Jan Kara <jack@suse.cz>

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@suse.com>
SUSE Labs, CR

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