On Tue, Jun 21, 2016 at 05:45:12PM +0200, Jan Kara wrote:

Hello,

currently we never clear dirty bits in the radix tree of a DAX inode. Thus
fsync(2) or even periodical writeback flush all the dirty pfns again and
again. This patches implement clearing of the dirty tag in the radix tree
so that we issue flush only when needed.

The difficulty with clearing the dirty tag is that we have to protect against
a concurrent page fault setting the dirty tag and writing new data into the
page. So we need a lock serializing page fault and clearing of the dirty tag
and write-protecting PTEs (so that we get another pagefault when pfn is written
to again and we have to set the dirty tag again).

The effect of the patch set is easily visible:

Writing 1 GB of data via mmap, then fsync twice.

Before this patch set both fsyncs take ~205 ms on my test machine, after the
patch set the first fsync takes ~283 ms (the additional cost of walking PTEs,
clearing dirty bits etc. is very noticeable), the second fsync takes below
1 us.

As a bonus, these patches make filesystem freezing for DAX filesystems
reliable because mappings are now properly writeprotected while freezing the
fs.

Patches have passed xfstests for both xfs and ext4.

So far the patches don't work with PMD pages - that's next on my todo list.

Regarding the PMD work, I had a go at this a while ago.  You may (or may not)
find these patches useful:

mm: add follow_pte_pmd()
	https://patchwork.kernel.org/patch/7616241/
mm: add pmd_mkclean()
	https://patchwork.kernel.org/patch/7616261/
mm: add pgoff_mkclean()
	https://patchwork.kernel.org/patch/7616221/

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