On Tue, Jun 21, 2016 at 05:45:13PM +0200, Jan Kara wrote:
> Currently, flushing of caches for DAX mappings was ignoring entry lock.
> So far this was ok (modulo a bug that a difference in entry lock could
> cause cache flushing to be mistakenly skipped) but in the following
> patches we will write-protect PTEs on cache flushing and clear dirty
> tags. For that we will need more exclusion. So do cache flushing under
> an entry lock. This allows us to remove one lock-unlock pair of
> mapping->tree_lock as a bonus.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/dax.c | 62 +++++++++++++++++++++++++++++++++++++++-----------------------
> 1 file changed, 39 insertions(+), 23 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 761495bf5eb9..5209f8cd0bee 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -669,35 +669,54 @@ static int dax_writeback_one(struct block_device *bdev,
> struct address_space *mapping, pgoff_t index, void *entry)
> {
> struct radix_tree_root *page_tree = &mapping->page_tree;
> - int type = RADIX_DAX_TYPE(entry);
> - struct radix_tree_node *node;
> + int type;
> struct blk_dax_ctl dax;
> - void **slot;
> int ret = 0;
> + void *entry2, **slot;
Nit: Let's retain the "reverse X-mas tree" ordering of our variable
definitions.
> - spin_lock_irq(&mapping->tree_lock);
> /*
> - * Regular page slots are stabilized by the page lock even
> - * without the tree itself locked. These unlocked entries
> - * need verification under the tree lock.
> + * A page got tagged dirty in DAX mapping? Something is seriously
> + * wrong.
> */
> - if (!__radix_tree_lookup(page_tree, index, &node, &slot))
> - goto unlock;
> - if (*slot != entry)
> - goto unlock;
> -
> - /* another fsync thread may have already written back this entry */
> - if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
> - goto unlock;
> + if (WARN_ON(!radix_tree_exceptional_entry(entry)))
> + return -EIO;
>
> + spin_lock_irq(&mapping->tree_lock);
> + entry2 = get_unlocked_mapping_entry(mapping, index, &slot);
> + /* Entry got punched out / reallocated? */
> + if (!entry2 || !radix_tree_exceptional_entry(entry2))
> + goto put_unlock;
> + /*
> + * Entry got reallocated elsewhere? No need to writeback. We have to
> + * compare sectors as we must not bail out due to difference in lockbit
> + * or entry type.
> + */
> + if (RADIX_DAX_SECTOR(entry2) != RADIX_DAX_SECTOR(entry))
> + goto put_unlock;
> + type = RADIX_DAX_TYPE(entry2);
> if (WARN_ON_ONCE(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD)) {
> ret = -EIO;
> - goto unlock;
> + goto put_unlock;
> }
> + entry = entry2;
I don't think you need to set 'entry' here - you reset it in 4 lines via
lock_slot(), and don't use it in between.
> +
> + /* Another fsync thread may have already written back this entry */
> + if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
> + goto put_unlock;
> + /* Lock the entry to serialize with page faults */
> + entry = lock_slot(mapping, slot);
As of this patch nobody unlocks the slot. :) A quick test of "write, fsync,
fsync" confirms that it deadlocks.
You introduce the proper calls to unlock the slot via
put_locked_mapping_entry() in patch 3/3 - those probably need to be in this
patch instead.
> + /*
> + * We can clear the tag now but we have to be careful so that concurrent
> + * dax_writeback_one() calls for the same index cannot finish before we
> + * actually flush the caches. This is achieved as the calls will look
> + * at the entry only under tree_lock and once they do that they will
> + * see the entry locked and wait for it to unlock.
> + */
> + radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> + spin_unlock_irq(&mapping->tree_lock);
>
> dax.sector = RADIX_DAX_SECTOR(entry);
> dax.size = (type == RADIX_DAX_PMD ? PMD_SIZE : PAGE_SIZE);
> - spin_unlock_irq(&mapping->tree_lock);
>
> /*
> * We cannot hold tree_lock while calling dax_map_atomic() because it
> @@ -713,15 +732,12 @@ static int dax_writeback_one(struct block_device *bdev,
> }
>
> wb_cache_pmem(dax.addr, dax.size);
> -
> - spin_lock_irq(&mapping->tree_lock);
> - radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> - spin_unlock_irq(&mapping->tree_lock);
> - unmap:
> +unmap:
> dax_unmap_atomic(bdev, &dax);
> return ret;
>
> - unlock:
> +put_unlock:
> + put_unlocked_mapping_entry(mapping, index, entry2);
> spin_unlock_irq(&mapping->tree_lock);
> return ret;
> }
> --
> 2.6.6
>
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