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

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > Andrew Morton wrote:
> >
> >
> > > 2.4:
> > >
> > > 	MS_ASYNC: dirty the pagecache pages, start I/O
> > > 	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O
> > >
> > > 2.6:
> > >
> > > 	MS_ASYNC: dirty the pagecache pages
> > > 	MS_SYNC: dirty the pagecache pages, start I/O, wait on I/O.
> > >
> > > So you're saying that doing the I/O in that 25-100msec window allowed your
> > > app to do more pipelining.
> > >
> > > I think for most scenarios, what we have in 2.6 is better: it gives the app
> > > more control over when the I/O should be started. 
> >
> > How so?
>
>
> Well, for example you might want to msync a number of disjoint parts of the
> mapping, then write them all out in one hit.

That should still be pretty efficient with 2.4 like behaviour? pdflush
does write them out in file offset order doesn't it?

> Or you may not actually _want_ to start the I/O now - you just want pdflush
> to write things back in a reasonable time period, so you don't have unsynced
> data floating about in memory for eight hours.  That's a quite reasonable
> application of msync(MS_ASYNC).

I think data integrity requirements should be handled by MS_SYNC.

--
SUSE Labs, Novell Inc.
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