Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

[behlendorf]

Motivation and Context

Issue #11900. When a file is written using mmap holes will not be
correctly reported until an msync() is run and those pages go through
writeback.

Description

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored. This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing. This ensures that a clean dnode can't be dirtied before
the data/hole is located. The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

A test case was added which does some basic verification that
data/holes are reported correctly when writing a file with mmap.

How Has This Been Tested?

Added a new mmap_seek_001_pos test case which verifies
the basic data/hole reporting for files written with mmap(2).
Independently verified that with the patch applied the issue
can no longer be recreated with the original reproducer.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

@asomers I believe this is a general fix for https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256205, would you mind reviewing it. Particularly the FreeBSD specific bits to make sure we're using the right interface.

[rincebrain]

LGTM

[ahrens]

Great find! Does this supersede #12710, or do we think there may be additional problems with SEEK_HOLE?

[asomers]

@asomers I believe this is a general fix for https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256205, would you mind reviewing it. Particularly the FreeBSD specific bits to make sure we're using the right interface.

@kostikbel could you review this one?

[behlendorf]

Does this supersede #12710, or do we think there may be additional problems with SEEK_HOLE?

Yes, I believe this removes the need for #12710. After applying the patch the original reported issue is no longer reproducible, and we did confirm that it was doing its writes with mmap. So that explains it nicely.

[tonyhutter]

I don't see any surface level issues with this (although I'm unfamiliar with the dmu/dnode code).

[kostikbel]

@asomers I believe this is a general fix for https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256205, would you mind reviewing it. Particularly the FreeBSD specific bits to make sure we're using the right interface.

@kostikbel could you review this one?

It looks fine, as far as the vnode is locked around calls to vn_flush_cached_data()

[behlendorf]

According to the CI it appears vm_object_mightbedirty() wasn't yet available in FreeBSD 12. I've gone ahead and updated the code to use vp->v_object->flags & OBJ_MIGHTBEDIRTY for the older releases which is what was done before.