Re: [patch 03/13] io-accounting: write accounting

[Suleiman Souhlal <ssouhlal@FreeBSD.org>]

Suleiman Souhlal wrote:
> Andrew Morton wrote:
>
> > On Wed, 13 Dec 2006 00:45:50 -0800
> > Suleiman Souhlal <ssouhlal@FreeBSD.org> wrote:
> >
> >
> > > akpm@osdl.org wrote:
> > >
> > > > From: Andrew Morton <akpm@osdl.org>
> > > >
> > > > Accounting writes is fairly simple: whenever a process flips a page 
> > > > from clean
> > > > to dirty, we accuse it of having caused a write to underlying 
> > > > storage of
> > > > PAGE_CACHE_SIZE bytes.
> > >
> > >
> > > On architectures where dirtying a page doesn't cause a page fault 
> > > (like i386), couldn't you end up billing the wrong process (in fact, 
> > > I think that even on other archituctures set_page_dirty() doesn't get 
> > > called immediately in the page fault handler)? 
> >
> >
> >
> > Yes, that would be a problem in 2.6.18 and earlier.
> >
> > In 2.6.19 and later, we do take a fault when transitioning a page from
> > pte-clean to pte-dirty.  That was done to get the dirty-page accounting
> > right - to avoid the 
> > all-of-memory-is-dirty-but-the-kernel-doesn't-know-it
> > problem.
>
>
> Ah yes indeed. I'm unable to keep up with all the new developments. :-(
>
> However, if my understanding of this code is correct, it seems that the
> page fault is only done for shared writable VMAs, so you still can't
> rely on set_page_dirty() always being called by the process that
> dirtied the page in the first place.
>
> Am I wrong?

Yes I am.
The only I/O non-shared VMAs might cause is from swapping, and I'm not
sure if the io accounting patches actually care about that.
My confusion came from the term "shared": A VMA is considered shared
whenever MAP_SHARED is specified, even if it only has only one single
"user".

-- Suleiman
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