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

[Ross Zwisler <ross.zwisler@linux.intel.com>]

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.

One clarification, with the code as it is in v4 we are only doing
clflush/clflushopt/clwb instructions on the kaddr we've stored in the radix
tree, so I don't think that there is actually a risk of us doing a "write into
some other random vmalloc address space"?  I think at worse we will end up
clflushing an address that either isn't mapped or has been remapped by someone
else.  Or are you worried that the clflush would trigger a cache writeback to
a memory address where writes have side effects, thus triggering the side
effect?

I definitely think it needs to be fixed, I'm just trying to make sure I
understood your comment.

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