Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area

[Peter Zijlstra <peterz@infradead.org>]

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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 :-)

> > 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.

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