On Thu, Sep 15, 2011 at 09:47:55AM -0500, Shawn Bohrer wrote:

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.

It it there to avoid pages beeing modified while they are under
writeback, which defeats various checksumming like DIF/DIX, the iscsi
CRCs, or even just the RAID parity calculations.  All of these either
failed before, or had to work around it by copying all data was
written.

If you don't use any of these you can remove the call and things
will work like they did before.

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