Re: [PATCH v5 4/7] dax: add support for fsync/sync

Dan Williams
2015-12-21T19:27:41Z

On Mon, Dec 21, 2015 at 9:05 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Sat, Dec 19, 2015 at 10:37:46AM -0800, Dan Williams wrote:
>> On Fri, Dec 18, 2015 at 9:22 PM, Ross Zwisler
>> <ross.zwisler@linux.intel.com> wrote:
>> > To properly handle fsync/msync in an efficient way DAX needs to track dirty
>> > pages so it is able to flush them durably to media on demand.
>> >
>> > The tracking of dirty pages is done via the radix tree in struct
>> > address_space. This radix tree is already used by the page writeback
>> > infrastructure for tracking dirty pages associated with an open file, and
>> > it already has support for exceptional (non struct page*) entries. We
>> > build upon these features to add exceptional entries to the radix tree for
>> > DAX dirty PMD or PTE pages at fault time.
>> >
>> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
>> [..]
>> > +static void dax_writeback_one(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;
>> > + void **slot;
>> > +
>> > + if (type != RADIX_DAX_PTE && type != RADIX_DAX_PMD) {
>> > + WARN_ON_ONCE(1);
>> > + return;
>> > + }
>> > +
>> > + 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.
>> > + */
>> > + 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;
>> > +
>> > + radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
>> > +
>> > + if (type == RADIX_DAX_PMD)
>> > + wb_cache_pmem(RADIX_DAX_ADDR(entry), PMD_SIZE);
>> > + else
>> > + wb_cache_pmem(RADIX_DAX_ADDR(entry), PAGE_SIZE);
>>
>> Hi Ross, I should have realized this sooner, but what guarantees that
>> the address returned by RADIX_DAX_ADDR(entry) is still valid at this
>> point? I think we need to store the sector in the radix tree and then
>> perform a new dax_map_atomic() operation to either lookup a valid
>> address or fail the sync request. Otherwise, if the device is gone
>> we'll crash, or write into some other random vmalloc address space.
>
> Ah, good point, thank you. v4 of this series is based on a version of
> DAX where we aren't properly dealing with PMEM device removal. I've got an
> updated version that merges with your dax_map_atomic() changes, and I'll add
> this change into v5 which I will send out today. Thank you for the
> suggestion.

To make the merge simpler you could skip the rebase for now and just
call blk_queue_enter() / blk_queue_exit() around the calls to
wb_cache_pmem.