On Fri 08-06-18 16:51:14, 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_page() that uses rcu_read_lock() to
> protect against the inode being freed, and revalidates the page->mapping
> association under xa_lock().
>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Some comments below...
> diff --git a/fs/dax.c b/fs/dax.c
> index cccf6cad1a7a..b7e71b108fcf 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -361,6 +361,82 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> }
> }
>
> +struct page *dax_lock_page(unsigned long pfn)
> +{
Why do you return struct page here? Any reason behind that? Because struct
page exists and can be accessed through pfn_to_page() regardless of result
of this function so it looks a bit confusing. Also dax_lock_page() name
seems a bit confusing. Maybe dax_lock_pfn_mapping_entry()?
> + pgoff_t index;
> + struct inode *inode;
> + wait_queue_head_t *wq;
> + void *entry = NULL, **slot;
> + struct address_space *mapping;
> + struct wait_exceptional_entry_queue ewait;
> + struct page *ret = NULL, *page = pfn_to_page(pfn);
> +
> + rcu_read_lock();
> + for (;;) {
> + mapping = READ_ONCE(page->mapping);
> +
> + if (!mapping || !IS_DAX(mapping->host))
> + break;
> +
> + /*
> + * In the device-dax case there's no need to lock, a
> + * struct dev_pagemap pin is sufficient to keep the
> + * inode alive.
> + */
> + inode = mapping->host;
> + if (S_ISCHR(inode->i_mode)) {
> + ret = page;
> + break;
> + }
> +
> + xa_lock_irq(&mapping->i_pages);
> + if (mapping != page->mapping) {
> + xa_unlock_irq(&mapping->i_pages);
> + continue;
> + }
> + index = page->index;
> +
> + init_wait(&ewait.wait);
> + ewait.wait.func = wake_exceptional_entry_func;
This initialization could be before the loop.
> +
> + entry = __radix_tree_lookup(&mapping->i_pages, index, NULL,
> + &slot);
> + if (!entry ||
> + WARN_ON_ONCE(!radix_tree_exceptional_entry(entry))) {
> + xa_unlock_irq(&mapping->i_pages);
> + break;
> + } else if (!slot_locked(mapping, slot)) {
> + lock_slot(mapping, slot);
> + ret = page;
> + xa_unlock_irq(&mapping->i_pages);
> + break;
> + }
> +
> + wq = dax_entry_waitqueue(mapping, index, entry, &ewait.key);
> + prepare_to_wait_exclusive(wq, &ewait.wait,
> + TASK_UNINTERRUPTIBLE);
> + xa_unlock_irq(&mapping->i_pages);
> + rcu_read_unlock();
> + schedule();
> + finish_wait(wq, &ewait.wait);
> + rcu_read_lock();
> + }
> + rcu_read_unlock();
I don't like how this duplicates a lot of get_unlocked_mapping_entry().
Can we possibly factor this out similary as done for wait_event()?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
