Re: [PATCH v7.1] block: Coordinate flush requests

[Darrick J. Wong]

On Sat, Jan 15, 2011 at 02:33:44PM +0100, Tejun Heo wrote:
> Hello,
> 
> On Thu, Jan 13, 2011 at 10:59:46AM -0800, Darrick J. Wong wrote:
> > > But wouldn't it be better to implement this directly in the flush
> > > machinary instead of as blkdev_issue_flush() wrapper?  We have all the
> > > information at the request queue level so we can easily detect whether
> > > flushes can be merged or not and whether something is issued by
> > > blkdev_issue_flush() or by directly submitting bio wouldn't matter at
> > > all.  Am I missing something?
> > 
> > Could you clarify where exactly you meant by "in the flush
> > machinery"?  I think what you're suggesting is that
> > blk_flush_complete_seq could scan the pending flush list to find a
> > run of consecutive pure flush requests, submit the last one, and set
> > things up so that during the completion of the flush, all the
> > requests in that run are returned with the real flush's return code.
> 
> Yeah, something along that line.
> 
> > If that's what you meant, then yes, it could be done that way.
> > However, I have a few reasons for choosing the blkdev_issue_flush
> > approach.  First, as far as I could tell in the kernel, all the code
> > paths that involve upper layers issuing pure flushes go through
> > blkdev_issue_flush, so it seems like a convenient place to capture
> > all the incoming pure flushes.
> 
> That means it _can_ be done there but doesn't mean it's the best spot.
> 
> > Other parts of the kernel issue writes with the flush flag set, but
> > we want those to go through the regular queuing mechanisms anyway.
> > Second, with the proposed patch, any upper-layer code that has a
> > requirement for a pure flush to be issued without coordination can
> > do so by submitting the bio directly (or blkdev_issue_flush_now can
> > be exported).
> 
> I don't think anyone would need such capability but even if someone
> does that should be implemented as a separate explicit
> DONT_MERGE_THIS_FLUSH flag, not by exploting subtle inconsistencies on
> the wrapper interface.
> 
> > Third, baking the coordination into the flush machinery makes that
> > machinery more complicated to understand and maintain.
> 
> And the third point is completely nuts.  That's like saying putting
> things related together makes the concentrated code more complex, so
> let's scatter things all over the place.  No, it's not making the code
> more complex.  It's putting the complexity where it belongs.  It
> decreases maintenance overhead by increasing consistency.  Not only
> that it even results in visibly more consistent and sane _behavior_
> and the said complex machinary is less than 300 lines long.
> 
> > So in short, I went with the blkdev_issue_flush approach because the
> > code is easier to understand, and it's not losing any pure flushes
> > despite casting a narrower net.
> 
> No, not at all.  You're adding an unobvious logic into a wrapper
> interface creating incosistent behavior and creating artificial
> distinctions between pure and non-pure flushes and flushes issued by
> bio and the wrapper interface.  Come on.  Think about it again.  You
> can't be seriously standing by the above rationales.

Since you're the primary author of that file anyway, I'll defer to your
experience.  I can squeeze in a few test runs of whatever patches you propose
to the mailing list.

--D
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