Re: [rfc][patch] mm: direct io less aggressive syncs and invalidates

[Nick Piggin <npiggin@suse.de>]

On Wed, Oct 29, 2008 at 10:02:43AM -0400, Mikulas Patocka wrote:
> 
> 
> On Tue, 28 Oct 2008, Nick Piggin wrote:
> 
> > Improve this by doing range syncs. Also, memory no longer has to be 
> > unmapped to catch the dirty bits for syncing, as dirty bits would remain 
> > coherent due to dirty mmap accounting.
> 
> mmaped memory doesn't have have to by synchronized at all with read/write. 
> It has to be only synchonized if msync or munmap is called --- that's what 
> Posix says.
> 
> So direct i/o implementation that ignores mmap would be correct. Direct 
> i/o implementation that is not synchronized w.r.t. nondirect i/o wouldn't.

Right, but our implementation tries not to ignore mmap.

Our mmap is coherent with buffered IO, and buffered IO (kinda) coherent
with direct IO... least surprise says direct IO should be kinda coherent
with mmap ;)

At least now there is basically no extra cost to it.