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

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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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