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

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

Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> > If there is no actual need for the application to start a write (eg
> > for data integrity) then why would it ever do that?
>
>
> To get the data sent to disk in a reasonable amount of time - don't leave it
> floating about in memory for hours or days.

This is a Linux implementation detail. As such it would make sense to
introduce a new Linux specific MS_ flag for this.

> > 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?
>
>
> Because delaying the writeback to permit combining is a good optimisation.

Definitely. And when the app gives us a hint that it really wants the
data on the disk, starting it as early as possible is also a good
optimisation.


> > Why not introduce a linux specific MS_flag to propogate pte dirty
> > bits?
>
>
> That's what MS_ASYNC already does.  We're agreed that something needs to
> change and we're just discussing what that is.  I'm proposing something
> which is complete and flexible.

I don't think there's anything wrong with your fadvise additions.
I'd rather see MS_ASYNC start IO immediately and add another MS_
flag for Linux to propogate bits.

MS_ASYNC behaviour would also somewhat match your proposed FADV_ASYNC
behaviour.

> Another point here is that msync(MS_SYNC) starts writeout of _all_ dirty
> pages in the file (as MS_ASYNC used to do) and it waits upon writeback of
> the whole file.  That's quite inefficient for an app which has lots of
> threads writing to and msync()ing the same MAP_SHARED file.
>
> We could easily enough convert msync() to only operate on the affected
> region of the (non-linearly-mapped) file.  But I don't think we can do that
> now, because people might be relying upon the side-effects.

I think if the interface was always documented correctly then we should
be able to. If the app breaks it was buggy anyway.

> The fadvise() extensions allow us to fix this.  And we've needed them for
> some time for regular write()s anyway.  

Yes they'd be nice.

Instead of
LINUX_FADV_ASYNC_WRITE
LINUX_FADV_WRITE_WAIT

can we have something more consistent? Perhaps
FADV_WRITE_ASYNC
FADV_WRITE_SYNC

--
SUSE Labs, Novell Inc.
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