Stalls during writeback for mmaped I/O on XFS in 3.0

[Shawn Bohrer <sbohrer@rgmadvisors.com>]

I've got a workload that is latency sensitive that writes data to a
memory mapped file on XFS.  With the 3.0 kernel I'm seeing stalls of
up to 100ms that occur during writeback that we did not see with older
kernels.  I've traced the stalls and it looks like they are blocking
on wait_on_page_writeback() introduced in
d76ee18a8551e33ad7dbd55cac38bc7b094f3abb "fs: block_page_mkwrite
should wait for writeback to finish"

Reading the commit description doesn't really explain to me why this
change was needed.  Can someone explain what "This is needed to
stabilize pages during writeback for those two filesystems." means in
the context of that commit?  Is this a problem for older kernels as
well?  Should this have been backported to the stable kernels? What
are the downsides of reverting this commit?

Assuming this change is required are there any alternatives solutions
to avoid these stalls with mmaped I/O on XFS?

Thanks,
Shawn


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