Re: [PATCH v7 15/17] dax: add struct iomap based DAX PMD support

[Ross Zwisler]

On Mon, Oct 17, 2016 at 11:36:55AM +0530, Aneesh Kumar K.V wrote:
> Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> writes:
> 
> > DAX PMDs have been disabled since Jan Kara introduced DAX radix tree based
> > locking.  This patch allows DAX PMDs to participate in the DAX radix tree
> > based locking scheme so that they can be re-enabled using the new struct
> > iomap based fault handlers.
> >
> > There are currently three types of DAX 4k entries: 4k zero pages, 4k DAX
> > mappings that have an associated block allocation, and 4k DAX empty
> > entries.  The empty entries exist to provide locking for the duration of a
> > given page fault.
> >
> > This patch adds three equivalent 2MiB DAX entries: Huge Zero Page (HZP)
> > entries, PMD DAX entries that have associated block allocations, and 2 MiB
> > DAX empty entries.
> >
> > Unlike the 4k case where we insert a struct page* into the radix tree for
> > 4k zero pages, for HZP we insert a DAX exceptional entry with the new
> > RADIX_DAX_HZP flag set.  This is because we use a single 2 MiB zero page in
> > every 2MiB hole mapping, and it doesn't make sense to have that same struct
> > page* with multiple entries in multiple trees.  This would cause contention
> > on the single page lock for the one Huge Zero Page, and it would break the
> > page->index and page->mapping associations that are assumed to be valid in
> > many other places in the kernel.
> >
> > One difficult use case is when one thread is trying to use 4k entries in
> > radix tree for a given offset, and another thread is using 2 MiB entries
> > for that same offset.  The current code handles this by making the 2 MiB
> > user fall back to 4k entries for most cases.  This was done because it is
> > the simplest solution, and because the use of 2MiB pages is already
> > opportunistic.
> >
> > If we were to try to upgrade from 4k pages to 2MiB pages for a given range,
> > we run into the problem of how we lock out 4k page faults for the entire
> > 2MiB range while we clean out the radix tree so we can insert the 2MiB
> > entry.  We can solve this problem if we need to, but I think that the cases
> > where both 2MiB entries and 4K entries are being used for the same range
> > will be rare enough and the gain small enough that it probably won't be
> > worth the complexity.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > ---
> >  fs/dax.c            | 378 +++++++++++++++++++++++++++++++++++++++++++++++-----
> >  include/linux/dax.h |  55 ++++++--
> >  mm/filemap.c        |   3 +-
> >  3 files changed, 386 insertions(+), 50 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 0582c7c..153cfd5 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -76,6 +76,26 @@ static void dax_unmap_atomic(struct block_device *bdev,
> >  	blk_queue_exit(bdev->bd_queue);
> >  }
> >  
> > +static int dax_is_pmd_entry(void *entry)
> > +{
> > +	return (unsigned long)entry & RADIX_DAX_PMD;
> > +}
> > +
> > +static int dax_is_pte_entry(void *entry)
> > +{
> > +	return !((unsigned long)entry & RADIX_DAX_PMD);
> > +}
> > +
> > +static int dax_is_zero_entry(void *entry)
> > +{
> > +	return (unsigned long)entry & RADIX_DAX_HZP;
> > +}
> 
> How about dax_is_pmd_zero_entry() ?

It's on my to-do list to convert the 4k DAX zero page case to use a singleton
page as well, in which case it's my plan to reuse this helper for both the 4k
and the PMD case.  Having it called dax_is_zero_entry() instead of
dax_is_pmd_zero_entry() allows for this - we'll just have to rename the
underling flag.