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Message-ID: <43EC3326.4080706@yahoo.com.au>
Date: Fri, 10 Feb 2006 17:31:02 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Andrew Morton <akpm@osdl.org>
CC: linux@horizon.com, linux-kernel@vger.kernel.org, sct@redhat.com,
       torvalds@osdl.org
Subject: Re: msync() behaviour broken for MS_ASYNC, revert patch?
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Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:

>>and had no need for a MS_SYNC anywhere in the meantime.
>>If you did have the need for MS_SYNC, then kicking off the IO
>>ASAP is going to be more efficient.
> 
> 
> Of course these sorts of applications don't know what they'll be doing in
> the future.  Often the location of the next update is driven by something
> which came across the network.
> 

If there is no actual need for the application to start a write (eg
for data integrity) then why would it ever do that?

> 
> There's no need to do that.   Look:
> 
> msync(MS_ASYNC): propagate pte dirty flags into pagecache
> 
> LINUX_FADV_ASYNC_WRITE: start writeback on all pages in region which are
> dirty and which aren't presently under writeback.
> 
> LINUX_FADV_WRITE_WAIT: wait on writeback of all pages in range.
> 
> I think that covers all conceivable scenarios.  One thing per operation,
> leave the decisions and tuning up to the application.  And it gives us two
> operations which are also useful in association with regular write().
> 

Oh yeah it is easy if you want to define some more APIs and do
it in a Linux specific way.

But the main function of msync(MS_ASYNC) AFAIK is to *start* IO.
Why do we care so much if some application goes stupid with it?
Why not introduce a linux specific MS_flag to propogate pte dirty
bits?

-- 
SUSE Labs, Novell Inc.
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