On Wed, Jun 28, 2017 at 04:01:47PM -0600, Ross Zwisler wrote: > When servicing mmap() reads from file holes the current DAX code allocates > 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.
