On Sun, 25 Mar 2012 13:26:10 -0700 (PDT) Hugh Dickins [off-list ref] wrote:

Building a test kernel quickly told me that inode->i_mapping->i_mutex
doesn't exist, of course it's inode->i_mutex.

Then running the test kernel quickly told me that neither ext4 nor xfs
(I didn't try ocfs2) holds inode->i_mutex where holepunching calls
truncate_inode_pages_range().

Now, there might or might not be reasons why ext4 or xfs ought to hold
i_mutex there for its own consistency, but it's beyond me to determine
that: let's assume they're correct without evidence to the contrary.

Stabilizing i_size is not a reason: holepunching does not affect i_size
and is not affected by i_size (okay, ext4 still has the bug I reported
a couple of months ago, whereby its holepunching stops at i_size,
forgetting blocks fallocated beyond; but no doubt that will get fixed).

And nothing that truncate_pagecache_range() does needs i_mutex:
neither the unmap_mapping_range() nor the truncate_inode_pages_range()
needs i_mutex.  A year ago, yes, Miklos showed how unmap_mapping_range()
was relying on mutex serialization, and added an additional mutex for
that, which Peter was able to remove once he mutified i_mmap_lock.

truncate_pagecache_range() is just a drop-in replacement for
truncate_inode_pages_range(), and has no different locking needs.

Does anything prevent new pages from getting added to pagecache and
perhaps faulted into VMAs after or during the execution of these
functions?

Also, I wonder what prevents pages in the range from being dirtied
between ext4_ext_punch_hole()'s filemap_write_and_wait_range() and
truncate_inode_pages_range().

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