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

[Ross Zwisler]

On Mon, Dec 21, 2015 at 06:15:12PM +0100, Jan Kara wrote:
> On Fri 18-12-15 22:22:15, Ross Zwisler 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.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> 
> The patch looks good to me. Just one comment: When we have this exclusion
> between different types of exceptional entries, there is no real need to
> have separate counters of 'shadow' and 'dax' entries, is there? We can have
> one 'nrexceptional' counter and don't have to grow struct inode
> unnecessarily which would be really welcome since DAX isn't a mainstream
> feature. Could you please change the code? Thanks!

Sure, this sounds good.  Thanks!
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