On Fri, Aug 01, 2014 at 05:24:59PM +0200, Jan Kara wrote:

  OK, makes sense. But then if there's heavy IO going on, anything that has
IO pending on it is pinned and IO completion can easily take something
close to a second or more. So meeting subsecond deadlines may be tough even
for ordinary data pages under heavy load, even more so for metadata where
there are further constraints. OTOH phones aren't usually IO bound so in
practice it needn't be so bad ;). 

Yeah, typically phones are not IO bound :-)

So if it is sub-second unless someone
loads the storage, then that sounds doable even for metadata. But we'll
need to attach ->migratepage callback to blkdev pages and at least in ext4
case teach it how to move pages tracked by the journal.

Right, making it possible at all if of course much prefered over not
possible, regardless of timeliness :-)

quoted

Sadly its not only mobile devices that excel in crappy hardware, there's
plenty desktop stuff that could use this too, like some of the v4l
devices iirc.

  Yeah, but in such usecases the guarantees we can offer for completion of
migration are even more vague :(.

Yeah, lets start by making it possible, after that we can maybe look at
making it better, who knows.