Re: [PATCH v5 2/7] dax: support dirty DAX entries in radix tree

[Ross Zwisler]

On Tue, Dec 22, 2015 at 02:46:05PM -0800, Andrew Morton wrote:
> On Fri, 18 Dec 2015 22:22:15 -0700 Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> 
> > Add support for tracking dirty DAX entries in the struct address_space
> > radix tree.  This tree is already used for dirty page writeback, and it
> > already supports the use of exceptional (non struct page*) entries.
> > 
> > In order to properly track dirty DAX pages we will insert new exceptional
> > entries into the radix tree that represent dirty DAX PTE or PMD pages.
> > These exceptional entries will also contain the writeback addresses for the
> > PTE or PMD faults that we can use at fsync/msync time.
> > 
> > There are currently two types of exceptional entries (shmem and shadow)
> > that can be placed into the radix tree, and this adds a third.  We rely on
> > the fact that only one type of exceptional entry can be found in a given
> > radix tree based on its usage.  This happens for free with DAX vs shmem but
> > we explicitly prevent shadow entries from being added to radix trees for
> > DAX mappings.
> > 
> > The only shadow entries that would be generated for DAX radix trees would
> > be to track zero page mappings that were created for holes.  These pages
> > would receive minimal benefit from having shadow entries, and the choice
> > to have only one type of exceptional entry in a given radix tree makes the
> > logic simpler both in clear_exceptional_entry() and in the rest of DAX.
> > 
> >
> > ...
> >
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -36,4 +36,9 @@ static inline bool vma_is_dax(struct vm_area_struct *vma)
> >  {
> >  	return vma->vm_file && IS_DAX(vma->vm_file->f_mapping->host);
> >  }
> > +
> > +static inline bool dax_mapping(struct address_space *mapping)
> > +{
> > +	return mapping->host && IS_DAX(mapping->host);
> > +}
> 
> Can we make this evaluate to plain old "0" when CONFIG_FS_DAX=n?  That
> way a bunch of code in callers will fall away as well.
> 
> If the compiler has any brains then a good way to do this would be to
> make IS_DAX be "0" but one would need to check that the zeroness
> properly propagated out of the inline.

Ah, it already works that way due to some magic with IS_DAX().  I believe we
already use the fact that blocks protected by IS_DAX() go away if
CONFIG_FS_DAX isn't set.

The trick is that S_DAX is defined to be 0 if CONFIG_FS_DAX isn't set.

I'm pretty sure this is working because of the code in
filemap_write_and_wait_range().  I added a block with the later "dax: add
support for fsync/msync" patch which looks like this:

@@ -482,6 +482,9 @@ int filemap_write_and_wait_range(struct address_space *mapping,
 {
        int err = 0;
 
+       if (dax_mapping(mapping) && mapping->nrdax)
+               dax_writeback_mapping_range(mapping, lstart, lend);
+

Without the dax_mapping() check there the behavior is the same, but we fail to
compile if CONFIG_FS_DAX isn't set because dax_writeback_mapping_range() isn't
defined.  (Guess how I found that out.  :)  )

> >  #endif
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 3aa5142..b9ac534 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -433,6 +433,7 @@ struct address_space {
> >  	/* Protected by tree_lock together with the radix tree */
> >  	unsigned long		nrpages;	/* number of total pages */
> >  	unsigned long		nrshadows;	/* number of shadow entries */
> > +	unsigned long		nrdax;	        /* number of DAX entries */
> 
> hm, that's unfortunate - machines commonly carry tremendous numbers of
> address_spaces in memory and adding pork to them is rather a big deal. 
> We can't avoid this somehow?  Maybe share the space with nrshadows by
> some means?  Find some other field which is unused for dax files?

Jan Kara noticed the same thing:

https://lists.01.org/pipermail/linux-nvdimm/2015-December/003626.html

It'll be fixed in the next spin of the patch set.
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