Hi,

On Wed, 2011-09-07 at 18:46 +0200, Christoph Hellwig wrote:

On Wed, Sep 07, 2011 at 06:42:16PM +0200, Jan Kara wrote:

quoted

  Well, it depends on what you call common - usually, ->writepage is called
from kswapd which shouldn't be common compared to writeback from a flusher
thread. But now I've realized that JBD2 also calls ->writepage to fulfill
data=ordered mode guarantees and that's what causes most of redirtying of
pages on ext4. That's going away eventually but it will take some time. So
for now writeback has to handle redirtying...

Under the "right" loads it may also happen for xfs because we can't
take lock non-blockingly in the fluser thread for example.

GFS2 uses this trick for journaled data pages - the lock ordering is
transaction lock before page lock, so we cannot handle pages which are
already locked before they are handed to the fs if a transaction is
required. So we have our own ->writepages which gets the locks in the
correct order, and ->writepage will simply redirty the page if it would
have required a transaction in order to write out the page,

Steve.


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