Re: msync() behaviour broken for MS_ASYNC, revert patch?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Linus Torvalds wrote:
> 
> On Sat, 11 Feb 2006, Nick Piggin wrote:
> 
>>Well in that case in your argument your FADV_WRITE_START is of
>>the "waits for writeout then starts writeout if dirty" type.
>>
>>In which case you've just made 3 consecutive  write+wait cycles
>>to the same page, so it is hardly an optimal IO pattern.
> 
> 
> The point is, this is the interface that an app would want to use if they 
> want _perfect_ IO patterns. 
> 

I'll be annoying and take you up on this again.

It is possible that my FADV_WRITE_SYNC will do an extra write of a page
if it has since become dirty again, however that would seem to be rare
for such a thing to happen (ie. because the app has asked for some previous
copy of data to be on disk).

I'm not saying it would never happen, your sub-page sized example is one
probably valid case - however in that case Andrew's wait-for-write doesn't
always do the right thing either.

But I will grant that start-writeout + wait-for-write must be at least as
expressive as write-and-wait.

*however*, it still isn't perfect and it still does things worse than my
proposal. For example, if the kernel itself actually decides to start writeout
before you call fadvise(FADV_START_WRITEOUT) then it is now going to block
and wait for the io to finish.

Anyway if we agree they are both much of a muchness, then I hope we can
go for FADV_WRITE_ASYNC, FADV_WRITE_SYNC because it is consistent with the
rest of the userspace API we expose.

-- 
SUSE Labs, Novell Inc.
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