Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1679068

Re: [PATCH v3 0/5] DAX common 4k zero page

Show all headers | View raw

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.

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

[PATCH v3 0/5] DAX common 4k zero page Ross Zwisler <ross.zwisler@linux.intel.com> - 2017-06-29 00:10 +0200
  [PATCH v3 4/5] dax: remove DAX code from page_cache_tree_insert() Ross Zwisler <ross.zwisler@linux.intel.com> - 2017-06-29 00:10 +0200
  [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite() Ross Zwisler <ross.zwisler@linux.intel.com> - 2017-06-29 00:10 +0200
  [PATCH v3 5/5] dax: move all DAX radix tree defs to fs/dax.c Ross Zwisler <ross.zwisler@linux.intel.com> - 2017-06-29 00:10 +0200
  Re: [PATCH v3 0/5] DAX common 4k zero page Ross Zwisler <ross.zwisler@linux.intel.com> - 2017-06-30 21:10 +0200

csiph-web