On 09/13/2011 12:02 PM, Joel Becker wrote:
On Tue, Sep 13, 2011 at 11:33:29AM -0700, Allison Henderson wrote:
Hi All,
I have been trying to find a way to synchronize punch hole with read
and write operations with out the use of i_mutex. The concern is
that after punch hole has released the pages inside the hole,
another process may remap the page to a block before punch has taken
i_data_sem. I think putting i_mutex around the punch hole operation
would fix this, but since we are trying to avoid further improper
use of i_mutex, I am trying to avoid that solution.
Hey Allison,
Actually, i_mutex is the normal way to handle this. ocfs2 takes
i_mutex down under its ->fallocate(). Truncate is in the same boat,
which is why do_truncate() takes i_mutex before calling notify_change().
The read-write paths grab i_mutex for buffered operation. They
don't for O_DIRECT, which doesn't map to the pagecache. This is where
i_data_sem should speed things up.
Joel
Hi Joel,
Well, I actually already had a patch that was trying to use i_mutex to
solve this ([PATCH 4/6 v7] ext4: Lock i_mutex for punch hole). But we
decided not to apply it because of plans to reduce the usage of i_mutex
in the ext4 code. So I've been trying to figure out a different way to
solve this, but so far I haven't had a whole lot of luck finding a
solution that doesn't involve introducing a new locking mechanism. So I
wanted to check back here for more details on what the plan for i_mutex
is so I dont conflict with anything that might already be going on. :)
Ted, would you be able to give us some more details on this topic? Thx!
Allison Henderson
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