On Mon, Jun 11, 2018 at 8:41 AM, Jan Kara <jack@xxxxxxx> wrote:
> 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()?
Ok, I'll give that a shot.
