On Fri 21-07-17 16:39:53, 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.
This is easily visible by looking at the overall memory consumption of the
system or by looking at /proc/[pid]/smaps:

	7f62e72b3000-7f63272b3000 rw-s 00000000 103:00 12   /root/dax/data
	Size:            1048576 kB
	Rss:             1048576 kB
	Pss:             1048576 kB
	Shared_Clean:          0 kB
	Shared_Dirty:          0 kB
	Private_Clean:   1048576 kB
	Private_Dirty:         0 kB
	Referenced:      1048576 kB
	Anonymous:             0 kB
	LazyFree:              0 kB
	AnonHugePages:         0 kB
	ShmemPmdMapped:        0 kB
	Shared_Hugetlb:        0 kB
	Private_Hugetlb:       0 kB
	Swap:                  0 kB
	SwapPss:               0 kB
	KernelPageSize:        4 kB
	MMUPageSize:           4 kB
	Locked:                0 kB

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.  Here are the
average latencies of dax_load_hole() as measured by ftrace on a random test
box:

Old method, using zeroed page cache pages:	3.4 us
New method, using the common 4k zero page:	0.8 us

This was the average latency over 1 GiB of sequential reads done by this
simple fio script:

  [global]
  size=1G
  filename=/root/dax/data
  fallocate=none
  [io]
  rw=read
  ioengine=mmap

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.

Solve these issues by following the lead of the DAX PMD code and using a
common 4k zero page instead.  As with the PMD code we will now insert a DAX
exceptional entry into the radix tree instead of a struct page pointer
which allows us to remove all the special casing in the DAX code.

Note that we do still pretty aggressively check for regular pages in the
DAX radix tree, especially where we take action based on the bits set in
the page.  If we ever find a regular page in our radix tree now that most
likely means that someone besides DAX is inserting pages (which has
happened lots of times in the past), and we want to find that out early and
fail loudly.

This solution also removes the extra memory consumption.  Here is that same
/proc/[pid]/smaps after 1GiB of reading from a hole with the new code:

	7f2054a74000-7f2094a74000 rw-s 00000000 103:00 12   /root/dax/data
	Size:            1048576 kB
	Rss:                   0 kB
	Pss:                   0 kB
	Shared_Clean:          0 kB
	Shared_Dirty:          0 kB
	Private_Clean:         0 kB
	Private_Dirty:         0 kB
	Referenced:            0 kB
	Anonymous:             0 kB
	LazyFree:              0 kB
	AnonHugePages:         0 kB
	ShmemPmdMapped:        0 kB
	Shared_Hugetlb:        0 kB
	Private_Hugetlb:       0 kB
	Swap:                  0 kB
	SwapPss:               0 kB
	KernelPageSize:        4 kB
	MMUPageSize:           4 kB
	Locked:                0 kB

Overall system memory consumption is similarly improved.

Another major change is that we remove dax_pfn_mkwrite() from our fault
flow, and instead rely on the page fault itself to make the PTE dirty and
writeable.  The following description from the patch adding the
vm_insert_mixed_mkwrite() call explains this a little more:

***
  To be able to use the common 4k zero page in DAX we need to have our PTE
  fault path look more like our PMD fault path where a PTE entry can be
  marked as dirty and writeable as it is first inserted, rather than
  waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.

  Right now we can rely on having a dax_pfn_mkwrite() call because we can
  distinguish between these two cases in do_wp_page():

          case 1: 4k zero page => writable DAX storage
          case 2: read-only DAX storage => writeable DAX storage

  This distinction is made by via vm_normal_page().  vm_normal_page()
  returns false for the common 4k zero page, though, just as it does for
  DAX ptes.  Instead of special casing the DAX + 4k zero page case, we will
  simplify our DAX PTE page fault sequence so that it matches our DAX PMD
  sequence, and get rid of the dax_pfn_mkwrite() helper.  We will instead
  use dax_iomap_fault() to handle write-protection faults.

  This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
  and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set
  insert_pfn() will do the work that was previously done by wp_page_reuse()
  as part of the dax_pfn_mkwrite() call path.
***

Signed-off-by: Ross Zwisler <redacted>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

And I really like that we've got rid of these pagecache hole pages!

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR
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