On Wed, Jun 28, 2017 at 04:01:47PM -0600, Ross Zwisler wrote:
When servicing mmap() reads from file holes the current DAX code
a page cache page of all zeroes and places the struct page pointer in the
mapping->page_tree radix tree. This has three major drawbacks:
1) It consumes memory unnecessarily. For every 4k page that is read via a
DAX mmap() over a hole, we allocate a new page cache page. This means that
if you read 1GiB worth of pages, you end up using 1GiB of zeroed memory.
2) It is slower than using a common zero page because each page fault has
more work to do. Instead of just inserting a common zero page we have to
allocate a page cache page, zero it, and then insert it.
3) The fact that we had to check for both DAX exceptional entries and for
page cache pages in the radix tree made the DAX code more complex.
This series solves these issues by following the lead of the DAX PMD code
and using a common 4k zero page instead. This reduces memory usage and
decreases latencies for some workloads, and it simplifies the DAX code,
removing over 100 lines in total.
Andrew, I'm still hoping to get this merged for v4.13 if possible. I I have
addressed all of Jan's feedback, but he is on vacation for the next few
weeks so he may not be able to give me Reviewed-by tags. I think this
series is relatively low risk with clear benefits, and I think we should be
able to address any issues that come up during the v4.13 RC series.
This series has passed my targeted testing and a full xfstests run on both
XFS and ext4.
This series has also passed the automated 0-day kernel builds in 168 configs.